Chapter 6 Anatomy and Physiology

What happens once the matrix has surrounded the chondroblast?

Once the matrix has surrounded the chondroblast, it has differentiated into a chondrocyte.

What is the concentric lamellae?

Surrounding the central canal are rings of bone matrix called concentric lamellae.

What do perforating canals do?

Perforating canals deliver blood to the central canals of the osteons.

How does the skeletal system provide body movement?

Skeletal muscles attach to bones by tendons, which are strong bands of connective tissue. Contraction of the skeleton muscles moves the bones, producing body movements. Joints, which are formed where two or more bones come together, allow movement between bones. Smooth cartilage covers the ends of bones within some joints, allowing the bones to move freely. Ligaments allow some movement between bones but prevent excessive movements.

Lacuna

Small space or cavity; potential space within the matrix of bone or cartilage normally occupied by a cell that can be visualized only when the cell shrinks away from he matrix during fixation; space containing maternal blood within the placenta.

Lacunae

Small space or cavity; potential space within the matrix of bone or cartilage normally occupied by a cell that can be visualized only when the cell shrinks away from the matrix during fixation; space containing maternal blood within the placenta.

Osteomalacia

Softening of bones due to calcium depletion; adult-rickets.

Why do some fracture types occur mainly in children?

Some fracture types occur mainly in children because their bones contain more organic matrix relative to inorganic matrix.

How many zones is the epiphyseal plate organized into?

The epiphyseal plate is organized into five zones.

What are the end products of intramembranous bone formation?

Thus, the end products of intramembranous bone formation are bones with outer compact bone surfaces and spongy centers. Remodeling converts woven bone to lamellar bone and contributes to the final shape of the bone.

What else is required for normal growth of all tissues?

Thyroid hormone is also required for normal growth of all tissues, including cartilage; therefore, a decrease in this hormone can result in a smaller individual.

Direct Effects of PTH

b. Kidney tubules PTH stimulates the reabsorption of Ca2+ from urine in the kidney tubules, which reduces the amount of Ca2+ excreted in the urine.

Medullary cavity

Large cavity within the diaphysis

Osteoclasts

Large, multinucleate cell that absorbs bone.

How does the skeletal system provide blood cell production?

Many bones contain cavities filled with red bone marrow, which gives rise to blood cells and platelets.

What are the steps of intramembranous Ossification?

1. A cross section of a newly formed trabecular shows the youngest bone in this series of photomicrographs. Osteocytes are surrounded by bone matrix, and osteoblasts are forming a ring on the outer surface of the trabecula. As the osteoblasts lay down bone, the trabeculae increase in size. 2. A lower magnification shows older bone than in step 1. Spongy bone has formed as a result of the enlargement of interconnections of many trabeculae. 3. A different stain that makes the bone appear blue, shows the oldest bone in this series. Within the spongy bone are trabeculae (blue) and developing red bone marrow (pink). Beneath the periosteum is an outer layer of developing compact bone.

List the steps of bone repair.

1. Blood released from damaged blood vessels forms a hematoma. 2. The internal callus forms between the ends of the bones, and the external callus form a collar around the break. 3. Woven, spongy bone replaces the internal and external calluses. 4. Compact bone replaces woven bone, and part of the internal callus is removed, restoring the medullary cavity.

List the steps of endochondral ossification.

1. Chondroblasts produce a cartilage model that is surrounded by perichondrium, except where joints will form. 2. The perichondrium of the diaphysis becomes the periosteum, and a bone collar is produced. Internally, the chondrocytes hypertrophy, and calcified cartilage forms. 3. A primary ossification center forms as blood vessels and osteoblasts invade the calcified cartilage. The osteoblasts lay down matrix, forming spongy bone. 4. Secondary ossification centers form in the epiphyses of long bones. 5. In a mature bone, the epiphyseal plate has become the epiphyseal line, and all the cartilage in the epiphyses, except the articular cartilage, has become bone.

What are the categories that fall under fractures that are categorized by the completeness of the break and its alignment relative to the bone?

1. Incomplete Fracture-a fracture that only traverses part of the bone 2. Complete Fracture-a fracture that completely separates the bone into at least two fragments. 3. Comminuted Fracture-a fracture where the bone breaks into multiple fragments.

What are the five zones that the epiphyseal plate is organized into?

1. The zone of resting cartilage is nearest the epiphysis and contains slowly dividing chondrocytes. 2. The chondrocytes in the zone of proliferation produce new cartilage through interstitial cartilage growth. The chondrocytes divide and form columns resembling stacks of plates or coins. 3. In the zone of hypertrophy, the chondrocytes produced in the zone of proliferation mature and enlarge. Thus, a maturation gradient exists in each column: The cells near the epiphysis are younger and actively proliferating, whereas the cells progressively nearer the diaphysis are older and undergoing hypertrophy. 4. The zone of calcification is very thin and contains hypertrophied chondrocytes and calcified cartilage matrix. The hypertrophied chondrocytes die, and blood vessels form the diaphysis grow into the area. The connective tissue surrounding the blood vessels contains osteoblasts from the endosteum. 5. The osteoblasts line up on the surface of the calcified cartilage and, through appositional bone growth, deposit new bone matrix, which is later remodeled.

List the zones of the epiphyseal plate?

1. Zone of Resting Cartilage: Slowly dividing chondrocytes. 2. Zone of Proliferation: New cartilage is produced on the epiphyseal side of the plate as the chondrocytes divide and form stacks of cells. 3. Zone of Hypertrophy: Chondrocytes mature and enlarge. 4. Zone of Calcification: Matrix is calcified, and chondrocytes die. 5. Ossified bone: The cartilage on the diaphysial side of the plate is replaced by bone.

What is BMU?

A BMU is temporary assembly of osteoclasts and osteoblasts that ravels through or across the surface of bone.

What is the traditional model for overall bone structure?

A long bone is the traditional model for overall bone structure.

Compare the structure of a long bone with those of a flat, short, and irregular bones. Explain where compact and spongy bones are found in each type.

A long bone is the traditional model for overall bone structure. The diaphysis, is the center portion of the bone. It is composed primarily of compact bone, surrounding a hollow center called the medullary cavity. Some spongy bone can be found lining the medullary cavity. The ends of a long bone are called epiphyses. The epiphyses are mostly spongy bone, with an outer layer of compact bone. Within joints, the end of a long bone is covered with hyaline cartilage called articular cartilage. Flat bones contain an interior framework of spongy bone sandwiched between two layers of compact bone. Short and irregular bones have a composition similar to the epiphyses of long bones-compact bone surfaces surrounding a spongy bone center with small spaces that are usually filled with marrow. Short and irregular bones are not elongated and have no diaphyses. However, certain regions of these bones, such as the processes (projections), have epiphyseal growth plates and therefore small epiphyses.

When does a slow loss of compact bone begin?

A slow loss of compact bone begins about age 40 and increases after age 45. However, the rate of compact bone loss is approximately half the rate of spongy bone loss.

List the four basic shapes of bones, and give an example of each.

A long bone is one that is cylindrical in shape, being longer than it is wide. Keep in mind, however, that the term describes the shape of a bone, not its size. Long bones are found in the arms (humerus, ulna, radius) and legs (femur, tibia, fibula), as well as in the fingers (metacarpals, phalanges) and toes (metatarsals, phalanges). Long bones function as levers; they move when muscles contract. A short bone is one that is cube-like in shape, being approximately equal in length, width, and thickness. The only short bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles. Short bones provide stability to support as well as some limited motion. The term "flat bone" is somewhat of a misnomer because, although a flat bone is typically thin, it is also often curved. Examples include the cranial (skull) bones, the scapulae (shoulder blades), the sternum (breastbone), and the ribs. Flat bones serve as point of attachment for muscles and often protect internal organs. An irregular bone is one that does not have any easily characterized shape and therefore does not fit any other classification. These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. Many facial bones, particularly the ones containing sinuses, are classified as irregular bones. A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form in tendons (the sheaths of tissue that connect bones to muscles) where a great deal of pressure is generated in a joint. The sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and placement from person to person but are typically found in tendons associated with the feet, hands, and knees. The patellae (singular = patella) are the only sesamoid bones found in common with every person.

What are the four major steps bone can undergo to repair if it is damaged?

Bone is a living tissue that can undergo repair if it is damaged. This process has four major steps: 1. Hematoma formation. A hematoma is a localized mass of blood released from blood vessels but confined within an organ or space. When a bone is fractured, the blood vessels in the bone and surrounding periosteum are damaged and a hematoma forms. Usually, the blood in a hematoma forms a lot, which consists of fibrous proteins that stop bleeding. Disruptions of blood vessels in the central canals result in inadequate blood delivery to osteocytes, and bone tissue adjacent to the fracture site dies. Tissues around the bone often become inflamed and swollen following the injury. 2. Callus formation. A callus is a mss of bone tissue that forms at a fracture site. An external callus encircles the break and connects the broken ends of the bone. An internal callus forms between the ends of the broken bone, as well as in the marrow cavity if the fracture occurs in the diaphysis of a long bone. Several days after the fracture, blood vessels grow into the clot. As the clot dissolves, (1) macrophages clean up cell debris, (2) osteoclasts break down dead bone tissue, and (3) fibroblasts produce collagen fibers, a denser fibrous network, which helps hold the bone together, is produced. Chondroblasts derived from osteochondral progenitor cells of the periosteum and endosteum begin to produce cartilage in the fibrous network. As these events are occurring, osteochondral progenitor cells in the endosteum become osteoblasts and produce new bone, which contributes to the internal callus. If the formation of the internal callus is prevented by infection, bone movements, or the nature of the bone. This condition can be treated surgically by implanting an appropriate substrate, such as living bone from another site in the body or dead bone from a cadaver. Other substrates have also been used. For example, a specific marine coral calcium phosphate is converted into a predominantly hydroxyapatite bio matrix that is very much like spongy bone. The external callus forms a collar around the opposing ends of the bone fragments. Osteocondral progenitor cells from the periosteum become osteoblasts, which produce bone, and chondroblasts, which produce cartilage. Cartilage production is more rapid than bone production, and the cartilage from each side of the break eventually grows together. The external callus is a bone-cartilage collar that stabilizes the ends of the broken bone. In modern medical practice, applying a cast or surgically implanting metal supports can help stabilize the bone. 3. Callus ossification. Like the cartilage models formed during fetal development, the cartilage in the external callus is replaced by woven spongy bone through endochondral ossification. the result is a stronger external callus. Even as the internal callus is forming and replacing the hematoma, osteoblasts from the periosteum and endosteum enter the internal callus and begin to produce bone. eventually, the fibers and cartilage of the internal callus are replaced by woven spongy bone, which further stabilizes the broken bone. 4. Bone remodeling. Filling the gap between bone fragments with an internal callus of woven bone is not the end of the repair process because woven bone is not as structurally strong as the original lamellar bone. Repair is complete only when the woven bone of the internal callus and the dead bone adjacent to the fracture site have been replaced by compact bone. In this compact bone, osteons from both sides of the break extend across the fracture line to "peg" the bone fragments together. This remodeling process takes time-as much as a year or more. As the internal callus is remodeled and becomes stronger, the external callus is reduced in size by osteoclast activity. Eventually, repair may be so complete that no evidence of the break remains; however, the repaired zone usually remains slightly thicker than the adjacent bone. if the fracture has occurred in the diaphysis of a long bone, remodeling also restores the medullary cavity.

Where are additional lamellae produced?

Additional concentric lamellae are produced, filling in the tunnel from the outside to the inside, until an osteon is formed, with the center of the tunnel becoming a central canal containing blood vessels.

Why would treatment with medications used to prevent osteoporosis also be effective for treatment of Paget disease?

After reading this chapter and Systems Pathology 6.1, you learned that osteoporosis and Paget disease both result in overly weak bones due to unchecked bone reabsorption. Paget diseases is different form osteoporosis in that bone turnover is greatly accelerated, resulting in bone haphazardly constructed. Because osteoclasts are the primary affected cells in Paget diseases, its principal manifestation is weakend bone. Some osteoporosis treatment medications are called bispohophonates. Bisphosphonates are osteoclast inhibitors. In Amir's case, his bones were becoming excessively demineralized and disproportionately composed of spongy bone. By inhibiting bone reabsorption, the excessive demineralization can be slowed, and bones remain able to support the mechanical stress of everyday movements.

What happens after woven bone formation?

After woven bone formation, osteoclasts break down the woven bone and osteoblasts build the stronger, more permanent lamellar bone.

Which cartilage is most associated with bone?

Although each type of cartilage can provide support, hyaline cartilage is most intimately associated with bone.

Distinguish between the location and composition of the internal callus and those of the external callus.

An external callus encircles the break and connects the broken ends of the bone. An internal callus forms between the ends of the broken bone, as well as in the marrow cavity if the fracture occurs in the diaphysis of a long bone.

What is an osteon composed of?

An osteon is composed of concentric rings of matrix, which surround a central tunnel and contain osteocytes.

Relate the importance of cartilage to the structure of the skeletal system.

An understanding of hyaline cartilage structure is important because most bones in the body start out as a hyaline cartilage model. In addition, growth in bone length and bone repair often involve making hyaline cartilage first, then replacing it with bone.

Why is understanding hyaline cartilage important?

An understanding of hyaline cartilage structure is important because most bones in the body start out as a hyaline cartilage model. In addition, growth in bone length and bone repair often involve making hyaline cartilage first, then replacing it with bone.

What is articular cartilage?

Articular cartilage, which is hyaline cartilage that covers the ends of bones where they come together to from joints, has no perichondrium, blood vessels, or nerves.

What happens when a long bone increases length and diameter?

As a long bone increases length and diameter, the size of the medullary cavity also increases, keeping the bone from becoming very heavy. In addition, as the bone grows in diameter, the relative thickness of compact bone is maintained as osteoclasts remove bone on the inside and osteoblasts add bone to the outside.

What happens when bone formation progresses?

As bone formation progresses, the newly formed bone goes through several stages.

What happens to bone as it is broken down?

As bone is broken down, the Ca2+ goes "back" into the blood.

How does the overall length of the diaphysis increase?

As new cartilage cells form in the zone of proliferation, and as these cells enlarge in the zone of hypertrophy, the overall length of the diaphysis increases.

What happens as ossification continues?

As ossification continues, bone is further molded and shaped into its final form.

Why do adult bones stop growing in length?

As the bones achieve normal adult size, they stop growing in length because the epiphyseal plate has ossified and become the epihphyseal line.

When do men and women experience bone loss?

At age 35, both men and women experience bone loss at a rate of 0.3-0.5% a year. This loss can increase by 10 times in women after menopause, when they can lose bone mass at a rate of 3-5% a year for approximately 5-7 years.

How is spongy bone lost?

At first, spongy bone is lost as the trabecular become thinner and weaker.

What is the breakdown of bone called?

Breakdown of bone is called bone reabsorption.

What do chondrocytes occupy?

Chondrocytes are rounded cells that occupy a space called lacuna, within the matrix.

What affects bone growth?

Because bone growth requires chondroblast and osteoblast proliferation, any metabolic disorder that affects the rate of cell proliferation or the production of collagen and other matrix components affects bone growth, as does the availability of calcium or other minerals needed in the mineralization process.

What are targets for Ca2+

Because bone is the major storage site for Ca2+ in the body, the cells governing bone reabsorption and bone deposition are targets for Ca2+-regulation hormones.

How are nutrients passed from osteocyte to osteocyte?

Because the osteocytes are in contact with each other through the canaliculi, nutrients are passed from one osteocyte to another osteocyte within each osteon. Waste products are removed in the reverse direction.

In addition to targeting the bacteria for death, what is the one theoretical medical approach that could help reduce the amount of bone loss during osteomyelitis?

Because the presence of the osteomyelitis-causing bacteria throws the RANK, RANKL, and OPG system into disarray, any osteoporosis medication that inhibits osteoclast formation could potentially slow the rate of bone loss. In addition, since osteoblasts increase inflammation in the site of infection, anti-inflammatory medication would perhaps slow bone loss that is due to excessive immune system function. However, there is still much work to be done to develop reliable treatments for bone loss prevention during osteomyelitis.

Where are interstitial lamellae?

Between the osteons are interstitial lamellae, which are remnants of concentric or circumferential lamellae that were partially removed during bone remodeling.

What is between trabecular spaces?

Between the trabeculae are spaces, which in life are filled with bone marrow and blood vessels.

What two patterns do bone formation in the fetus follow?

Bone formation in the fetus follows two patterns--(1) intremembranous ossification and (2) endochondral ossification.

Ossification

Bone formation; also called osteogenesis.

Describe the five major functions of the skeletal system.

Body support. Rigid, strong bone is well suited for bearing weight and is the major supporting tissue of the body. Organ protection. Bone is hard and protects organs it surrounds. Body movement. Skeletal muscles attach to bones by tendons, which are strong bands of connective tissue. Contraction of the skeletal muscles moves the bones, producing body movements. Mineral storage. Some minerals in the blood are stored in bone. If blood levels of these minerals decrease, the minerals in the blood are stored in bone. Blood cell production. Many bones contain cavities filled with red bone marrow, which gives rise to blood cells and platelets.

Spongy bone

Bone having many small spaces; found mainly in the epiphysis; arranged in trabeculae

What are the four steps of bone repair?

Bone is a living tissue that can undergo repair if it is damaged. This process has four major steps: 1. Hematoma formation. A hematoma is a localized mass of blood released from blood vessels but confined within an organ or space. When a bone is fractured, the blood vessels in the bone and surrounding periosteum are damaged and a hematoma forms. Usually, the blood in a hematoma forms a clot, which consists of fibrous proteins that stop bleeding. Disruptions of blood vessels in the central canals result in inadequate blood delivery to osteocytes, and bone tissue adjacent to the fracture site dies. Tissues around the bone often become inflamed and swollen following the injury. 2. Callus formation. A callus is a mss of bone tissue that forms at a fracture site. An external callus encircles the break and connects the broken ends of the bone. An internal callus forms between the ends of the broken bone, as well as in the marrow cavity if the fracture occurs in the diaphysis of a long bone. Several days after the fracture, blood vessels grow into the clot. As the clot dissolves, (1) macrophages clean up cell debris, (2) osteoclasts break down dead bone tissue, and (3) fibroblasts produce collagen fibers, a denser fibrous network, which helps hold the bone together, is produced. Chondroblasts derived from osteochondral progenitor cells of the periosteum and endosteum begin to produce cartilage in the fibrous network. As these events are occurring, osteochondral progenitor cells in the endosteum become osteoblasts and produce new bone, which contributes to the internal callus. If the formation of the internal callus is prevented by infection, bone movements, or the nature of the bone. This condition can be treated surgically by implanting an appropriate substrate, such as living bone from another site in the body or dead bone from a cadaver. Other substrates have also been used. For example, a specific marine coral calcium phosphate is converted into a predominantly hydroxyapatite bio matrix that is very much like spongy bone. The external callus forms a collar around the opposing ends of the bone fragments. Osteocondral progenitor cells from the periosteum become osteoblasts, which produce bone, and chondroblasts, which produce cartilage. Cartilage production is more rapid than bone production, and the cartilage from each side of the break eventually grows together. The external callus is a bone-cartilage collar that stabilizes the ends of the broken bone. In modern medical practice, applying a cast or surgically implanting metal supports can help stabilize the bone. 3. Callus ossification. Like the cartilage models formed during fetal development, the cartilage in the external callus is replaced by woven spongy bone through endochondral ossification. the result is a stronger external callus. Even as the internal callus is forming and replacing the hematoma, osteoblasts from the periosteum and endosteum enter the internal callus and begin to produce bone. eventually, the fibers and cartilage of the internal callus are replaced by woven spongy bone, which further stabilizes the broken bone. 4. Bone remodeling. Filling the gap between bone fragments with an internal callus of woven bone is not the end of the repair process because woven bone is not as structurally strong as the original lamellar bone. Repair is complete only when the woven bone of the internal callus and the dead bone adjacent to the fracture site have been replaced by compact bone. In this compact bone, osteons from both sides of the break extend across the fracture line to "peg" the bone fragments together. This remodeling process takes time-as much as a year or more. As the internal callus is remodeled and becomes stronger, the external callus is reduced in size by osteoclast activity. Eventually, repair may be so complete that no evidence of the break remains; however, the repaired zone usually remains slightly thicker than the adjacent bone. if the fracture has occurred in the diaphysis of a long bone, remodeling also restores the medullary cavity.

How does nutrients reach the chondrocytes in the perichondrium?

Blood vessels and nerves penetrate the outer layer of the perichondrium but do not enter the cartilage matrix, so nutrients must diffuse through the cartilage matrix to reach the chondrocytes.

What enters the substance of the bone itself?

Blood vessels enter the substance of the bone itself, and the lamellae of compact bone are primarily oriented around those blood vessels.

Explain the functions of the skeletal system.

Body support. Cartilage provides a firm yet flexible support within certain structures, such as the nose, external ear, thoracic cage, and trachea. Organ protection. For example, the skull encloses and protects the brain, and the vertebrae surround the spinal cord. The rib cage protects the heart, lungs, and other organs of the thorax. Body movement. Joints, which are formed where two or more bones come together, allow movement between bones. Smooth cartilage covers the ends of the bones within some joints, allowing bones to move freely. Mineral storage. The principal minerals stored are calcium and phosphorus, two minerals essential for many physiological processes. Blood cell production. Many bones contain cavities filled with red bone marrow, which gives rise to blood cells and platelets.

How does bone differ from cartilage?

Bone differs from cartilage in that the extensions of bone cells are in contact with one another through the canaliculi. Instead of diffusing through the mineralized matrix, nutrients and gases can pass through the small amount of fluid surrounding the cells in the canaliculi and lacunae or pass from cell to cell through the gap junctions connecting the cell extensions.

How does the skeletal system provide organ protection?

Bone is hard and protects the organs it surrounds. For example, the skull encloses and protects the brain, and the vertebrae surround the spinal cord. The rib cage protects the heart, lungs, and other organs of the thorax.

What can bone loss lead to?

Bone loss can lead to deformity, loss of height, pain, and stiffness.

When is bone mass at its highest?

Bone mass is at its highest around age 30 and men generally have denser bones than women because of the effects of testosterone and greater body weight.

What can the matrix of bone be compared to?

Bone matrix can be compared to reinforced concrete. Like reinforcing steel bars, the collagen fibers lend flexible strength to the matrix; like concrete, the mineral components give the matrix weight-bearing strength.

What is bone matrix composition responsible for?

Bone matrix composition is responsible for the characteristics of bone.

What does bone remodeling do?

Bone remodeling converts woven bone into lamellar bone and is involved in several important functions, including bone growth, changes in bone shape, adjustment of the bone to stress, bone repair, and calcium ion (Ca2+) regulation in the body.

What does bone remodeling involve?

Bone remodeling involves a basic multicellular unit (BMU).

Osteoblasts

Bone-forming cell.

PROCESS FIGURE 6.16 Bone Growth in Width

Bones can increase in width by the formation of new osteons beneath the periosteum.

What is the weight of mature bone matrix?

By weight, mature bone matrix is normally about 35% organic and 65% inorganic material.

Name the components of bone matrix, and explain their contribution to bone flexibility and bones' ability to bear weight.

By weight, mature bone matrix is normally about 35% organic and 65% inorganic material. The organic material consists primarily of collagen and proteoglycans. The inorganic material consists primarily of a calcium phosphate crystal called hydroxyapatite, which has the molecular formula. The collagen and mineral components are responsible for the major functional characteristics of bone. Bone matrix can be compared to reinforced concrete. Like reinforcing steel bars, the collagen fibers lend flexible strength to the matrix; like concrete, the mineral components give the matrix weight-bearing strength. If the mineral in a long bone is reduced, collagen becomes the primary constituent and the bone is overly flexible. On the other hand, if the amount of collagen is reduced in the bone, the mineral component becomes the primary constituent and the bone is very brittle.

What process do cartilage calcification and ossification occur by?

Cartilage calcification and ossification in the epiphyseal plate occur by the same basic process as calcification and ossification of the cartilage model during endochondral bone formation.

Comminuted Fracture

Characteristics: -Fracture results in at least three bone fragments -Occurs in patients with weakened bones or from a severe accident -Often necessary to surgically place a pin in the bone

Where is calcitonin secreted from?

Calcitonin is secreted from C cells in the thyroid gland when blood Ca2+ levels are too high.

What stimulates calcitonin secretion? How does calcitonin affect osteoclast activity?

Calcitonin is secreted from C cells in the thyroid gland when blood Ca2+ levels are too high. Calcitonin rapidly lowers blood Ca2+ levels by inhibiting osteoclast activity. However, the exact role of calcitonin, especially in adult bone remodeling, is still under investigation. PTH and calcitonin are describe more fully in chapters 18 and 27.

What does calcitonin do to blood Ca2+ levels

Calcitonin rapidly lowers blood Ca2+ levels by inhibiting osteoclast activity. However, the exact role of calcitonin, especially in adult bone remodeling, is still under investigation. PTH and calcitonin are describe more fully in chapter 18 and 27

What does calcitriol assist?

Calcitriol assists PTH in the kidney tubules by preventing Ca2+ removal through urine. These actions of calcitriol increase blood Ca2+ levels.

What does calcitriol increase?

Calcitriol increases blood Ca2+ levels.

What is calcitriol?

Calcitriol is a steroid hormone derived from vitamin D3.

What hormones regulate calcium homeostasis?

Calcium homeostasis is regulated by three hormones (chemical messengers delivered via the blood): (1) parathyroid hormone (PTH); (2) calcitriol, a biologically active form of vitamin D3; and (3) calcitonin.

What is a critical physiological regulator of many processes required to achieve and maintain homeostasis?

Calcium is a critical physiological regulator of many processes required to achieve and maintain homeostasis.

Preforating canals

Canal containing blood vessels and nerves running through bone perpendicular to the Haversian canals; also called Volkmanns' canal.

What does cartilage provide?

Cartilage provides a firm yet flexible support within certain structures, such as the nose, external ear, thoracic cage, and trachea.

Chondroblast

Cartilage-producing cell.

Osteon

Central canal containing blood capillaries and the concentric lamellae around it; occurs in compact bone; also called haversian system.

What do central canals contain?

Central canals are lined with endosperm and contain blood vessels, nerves, and loose connective tissue.

What is important to bone growth?

Certain vitamins are important to bone growth in very specific ways.

Explain how illness or malnutrition can affect bone growth. How do vitamins D and C affect bone growth?

Certain vitamins are important to bone growth in very specific ways. Vitamin D is necessary for the normal absorption of calcium from the intestines. The body can either synthesize or ingest vitamin D. Its rate of synthesis increases when the skin is exposed to sunlight. Insufficient vitamin D in children causes rickets, a disease resulting from reduced mineralization of the bone matrix. Children with rickets may have bowed bones and inflamed joints. During the winter in northern climates, children who are not exposed to sufficient sunlight can take supplementary vitamin D to prevent rickets. The body's inability to absorb lipids in which vitamin D is soluble can also result in vitamin D deficiency. This condition sometimes occurs in adults who have digestive disorders. Low vitamin D levels can be one cause of "adult rickets," or osteomalacia, a softening of the bones due to calcium depletion. Vitamin C is necessary for osteoblasts to synthesize collagen. normally, as old collagen breaks down, new collagen is synthesize to replace it. Vitamin C deficiency results in bones and cartilage with fewer collagen fibers because of collagen synthesis is impaired. In children, vitamin C deficiency can retard growth. In both children and adults, vitamin C deficiency can result in scurvy, which is marked by ulceration and hemorrhage in almost any area of the body because normal collagen synthesis is not occurring in connective tissues. Wound healing, which requires collagen synthesis, is hindered in patients with vitamin C deficiency. In extreme cases ,the teeth fall out because the ligaments that hold them in place break down.

Greenstick Fracture

Characteristic -Fracture with break on one side and bend on the other side of bone -Typical in young children due to increased collagen percentage in bone

Spiral Fracture

Characteristic: -A helical fracture from twisting of the bone -Common in snow skiers whose feet and ankles are locked into ski boots.

Open Fracture

Characteristics: -End of fractured bone produces through open wound in skin. -Increases risk of infection

What do circumferential lamellae form?

Circumferential lamellae form the outer surfaces of compact bone, which are thin plates that extend around the bone. In some bones, such as certain bones of the face, the layer of compact bone can be so thin that no osteons exist and the compact bone is composed of only circumferential lamellae.

How many categories of bone fractures are discussed in this text?

Classification of bone fractures is surprisingly complex. There are five main categories of bone fractures discussed in this text: (1) mechanism of fracture, (2) soft-tissue damage, (3) displacement vs. non displacement, (4) fracture pattern, and (5) number of fragments in the fracture bone.

What does compact bone have?

Compact bone has more bone matrix and less space than spongy bone.

What is compact bone?

Compact bone is the solid, outer layer surrounding each bone. It has more matrix and is denser with fewer pores than spongy bone.

What is the name of the tunnel in the new osteon (step 4) now occupied by a blood vessel?

Concentric lamellae.

Where does connective tissue develop from?

Connective tissue develops embryologically from mesenchymal cells (see chapter 4).

What cells give rise to osteochondral progenitor cells? What kinds of cells are derived from osteochondral progenitor cells? What types of cells give rise to osteoclasts?

Connective tissue develops embyroligcally from mesenchymal cells. Some of the mesenchymal cells becomes stem cells, a number of which replicate and become more specialized cell types. Stem cells called osteochondral progenitor cells can become osteoblasts or chondroblasts. Osteochondral progenitor cells are located in the inner layer of the perichondrium and in layers of connective tissue that cover bone (periosteum and endosteum). From these locations, they are a potential source of new osteoblasts and chondrblasts. Osteoblasts are derived from osteochondral progenitor cells, and osteocytes are derived from osteoblasts. Whether or not osteocytes freed from their surrounding bone matrix bone reabsorption can revert to become active osteoclasts are not derived from osteochondral progenitor cells but from stem cells in red bone marrow.

Red marrow

Connective tissue in the spaces of spongy bone or in the medullary cavity; the site of blood cell production

What difference in growth occurs in bones?

Consequently, growth in length of the long bones of the arm, forearm, thigh, and leg occurs at both ends of the diaphysis whereas growth in length of the hand and foot bones occurs at one end of the diaphysis.

What happens to osteoclast activity if blood Ca2+ levels are high?

Conversely, if blood Ca2+ levels are high, osteoclast activity decrease. Osteoclasts release less Ca2+ form the bone into the blood than osteoblasts remove from the blood for bone deposition. As a result, net movement of Ca2+ occurs fromm the blood to bone, and blood Ca2+ levels decrease.

What prolongs the growth phase of the epiphyseal plates?

Decreased levels of testosterone or estrogen can prolong the growth phase of the epiphyseal plates, even though the bones grow more slowly.

Compact bone

Dense bone with few internal spaces organized into osteons; forms the diaphysis and covers the spongy bone of the epiphyses.

Why does epiphyses increase in size?

Epiphyses increase in size because of growth at the articular cartilage.

Growth and Developmental Disorders: Gigantism

Description: Abnormally increased body size due to excessive growth at the epiphyseal plates.

Growth and Developmental Disorders: Dwarfism

Description: Abnormally small body size due to improper growth at the epiphyseal plates.

Growth and Developmental Disorders: Rickets

Description: Growth retardation due to nutritional deficiencies in minerals (Ca2+) or vitamin D; results in bones that are soft, weak and easily broken.

Condtion: Tumors

Description: May be malignant or benign and cause a range of bone defects

Bacterial infections: Osteomyelitis

Description: See Microbes in Your Body 6.1

Decalcification: Osteoporosis

Description: See Systems Pathology 6.1

Decaclicfication: Osteomalacia

Description: Softening of adult bones due to calcium depletion; often cause by vitamin D deficiency.

Bacterial Infections: Tuberculosis

Description: Typically, a lung bacterium that can also affect bone

What can disruptions in normal growth hormone cause?

Disruptions in normal growth hormone can cause dramatic changes in an individual's height.

Perichondrium

Double-layered connective tissue sheath surrounding cartilage.

When do bones develop from centers of ossification?

During bone formation and growth, bones develop from centers of ossification.

What happens to hormones during puberty?

During puberty, the levels of these hormones increase dramatically. The increased hormone levels stimulate significant growth spurts in the beginning.

What is lamellar bone?

Lamellar bone is mature bone.

What can prevent rickets?

During the winter in northern climates, children who are not exposed to sufficient sunlight can take supplementary vitamin D to prevent rickets.

What bones have one or more epiphyses on each end of the bone?

Each long bone of the arm, forearm, thigh, and leg has one or more epiphyses on each end of the bone. Each long bone of the hand and foot has one epiphysis, which is located on the proximal or distal end of the bone.

What is each osteocyte associated with?

Each osteocyte is associated with other osteocytes through canaliculi.

What does each osteon have?

Each osteon has from 4 to 20 concentric lamellae.

What advice should Betty give her granddaughter, so that the granddaughter will be less likely to develop osteoporosis when she is Betty's age?

Eat a healthier diet, don't smoke cigarettes and exercise frequently so your bones can become stronger.

FIGURE 6.12 Bone Formation in a Fetus

Eigtheen-week-old fetus showing intramembranous and endochondral ossification. Intramembranous ossification occurs at ossification centers in the flat bones of the skull. Endochondroal ossification has formed bones in the diaphyses of long bones. The ends of the long bones are still cartilage at this stage of development.

Epiphysis

End of the bone; develops from its own center of ossification

When does some of the cartilage from endochondral ossification start?

Endochondral ossification of some of this cartilage starts at approximately the eighth week of embryonic development, but this process might not begin in other cartilage until as late as 18-20 years of age.

What does endochondral ossification start with?

Endochondral ossification starts with a cartilage model.

Explain the process of growth at the articular cartilage. What happens to the epiphyseal plate and the articular cartilage when bone growth ceases?

Epiphyses increase in size because of growth at the articular cartilage. In addition, growth at the articular cartilage increases the size of bones that do not have an epiphysis, such as short bones. The process of growth in articular cartilage is similar to that occurring in the epiphyseal plate, except that the chondrocyte columns are not as obvious. The chondrocytes near the surface of the articular cartilage are similar to those in the zone of resting cartilage of the epiphyseal plate. In the deepest part of the articular cartilage, nearer bone tissue, the cartilage is calcified and ossified to form new bone. When the epiphyses reach their full size, the growth of cartilage and its replacement by bone cease. The articular cartilage, however, persists throughout life and does not become ossified as the epiphyseal plate does.

What is the primary female reproductive hormone?

Estrogen is the primary female reproductive hormone.

What is more effective at closing the epiphyseal plate?

Estrogens are more effective at closing the epiphyseal plate than testosterone is.

What does excessive growth hormone secretion result in?

Excessive growth hormone secretion results in pituitary gigantism.

Yellow marrow

Fat stored within the medullary cavity or in the space of spongy bone.

When do females usually stop growing?

Females usually stop growing earlier than males, around age 18.

Explain why damage cartilage takes a long time to heal. What are the advantages of articular cartilage having no perichondrium, blood vessels, or nerves?

First, let's consider the structure of cartilage. The book tells us that the perichondrium, which surrounds the cartilage, contains blood vessels, but the blood vessels do not enter the cartilage. The book also states that nutrients must diffuse through the matrix before reaching the chondrocytes. Logically, cells that aid in tissue repair would also enter cartilage more slowly than if blood vessels penetrated the cartilage. The question next asks whether the lack of perichondrium, blood vessels, and nerves would be advantageous for articular cartilage. When considering the function of articular cartilage, the absence of such structures makes sense. Articular cartilage provides a smooth, low-friction surface for two bones to move past each other easily. If there were solid structures within the joint, the effect of smooth movement would be lost. Imagine trying to ice skate on a rink whose ice had garden hoses frozen just under and along the surface. We would spend more time trying to get back to our feet than skating.

What do flat bones contain?

Flat bones contain an interior framework of spongy bone sandwiched between two layers of compact bone.

Give an example of bone loss increasing the likelihood of bone fractures.

For example, the loss of trabecular greatly increases the risk of compression fractures of the vertebrae (backbones) because the weight-bearing body of a vertebra consists mostly of spongy bone.

What does growth hormone do?

Growth hormone from the anterior pituitary increases general tissue growth, including overall bone growth, by stimulating interstitial cartilage growth and appositional bone growth.

What occurs at the epiphyseal plate?

Growth in bone length occurs at the epiphyseal plate.

How does breaking a bone result in hematoma formation?

Hematoma formation. A hematoma is a localized mass of blood released from blood vessels but confined with an organ or a space. When a bone is fractured, the blood vessels in the bone and surrounding periosteum are damaged and a hematoma forms. Usually, the blood in a hematoma forms a clot, which consists of fibrous proteins that stop the bleeding. Disruption of blood vessels in the central canals results in inadequate blood delivery to osteocytes, and bone tissue adjacent to the fracture site dies. tissues around the bone often become inflamed and swollen following the injury.

Sinus

Hollow in a bone or other tissue; enlarged channel for blood or lymph.

Calcitonin

Hormone released from parafollicular cells that acts on tissues to cause a decrease in blood levels of calcium ions.

What is important to bone growth?

Hormones are very important in bone growth.

Does the thickness of the epiphyseal plate increase?

However, the thickness of the epiphyseal plate does not increase because the rate of cartilage growth on the epiphyseal side of the plate is equal to the rate of bone formation on the diaphyseal side of the plate.

Epiphyseal plate

Hyaline cartilage between the diaphysis and epiphysis its growth allows for growth in bone length

Describe the structure of hyaline cartilage. Name the two types of cartilage cells. What is a lacuna?

Hyaline chondroblasts secrete a matrix, which surrounds the chondroblasts. Once the matrix has surrounded the chondroblast, it has differentiated into a chondrocyte. Chondrocytes are rounded cells that occupy a space called lacuna within the matrix. The matrix contains collagen, which provides strength, and proteoglycans, which make cartilage resilient by trapping water. Most cartilage is covered by a protective connective tissue sheath called the perichondrium. The perichondrium is a double-layered outer layer of dense irregular connective tissue containing fibroblasts. The inner, more delicate layer has fewer fibers and contains chondroblasts. Blood vessels and nerves penetrate the outer layer of the perichondrium but do not enter the cartilage matrix, so nutrients must diffuse through the cartilage matrix to reach the chondrocytes. Articular cartilage, which is a hyaline cartilage that covers the ends of bones where they come together to form joints, has no perichondrium, blood vessels, or nerves.

How can blood Ca2+ levels cause the bone to become weak and demineralized?

If blood Ca2+ levels are abnormally depleted for too long, excess Ca2+ is released from bone, causing it to become overly demineralized and weak.

In bone, what do bone cells produce?

In bone, as in other connective tissues, the bone cells produce the bone matrix and become entrapped within it. The cells also break down old matrix so that new matrix can replace it.

What does vitamin C deficiency in children and adults cause?

In both children and adults, vitamin C deficiency can result in scurvy, which is marked by ulceration and hemorrhage in almost any area of the body because normal collagen synthesis is not occurring in connective tissues.

what is bone?

In chapter 4, you learned that bone is a type of connective tissue.

What does vitamin C deficiency in children cause?

In children, vitamin C deficiency can retard growth.

What is the central canal?

In cross section, an osteon resembles a circular target; the bull's-eye of the target is the central canal.

What happens in interstitial growth?

In interstitial growth, chondrocytes in the center of the tissue divide and add more matrix in between the existing cells.

What does BMU do in spongy bone?

In spongy bone, the BMU removes bone matrix from the surface of a trabecula, forming a cavity, which the BMU then fills in with new bone matrix.

In the fetus what is filled with red marrow?

In the fetus, the spaces within bones are filled with red marrow.

When does intramembranous ossification begin?

Intramembranous ossification of the membrane begins at approximately the eighth week of embryonic development and is completed by approximately 2 years of age.

Characteristic: Starting Material

Intramembranous: Embryonic mesenchyme membrane Endochondral: Hyaline cartilage model surrounded by a perichondrium

Characteristic: Centers of Ossification

Intramembranous: Ossification begins in centers of ossification Spongy bone formed by internal osteoblasts Compact bone formed by periosteal osteoblasts. Endochondral: Osteoblasts invade the calcified cartilage in centers of ossification Primary ossification centers form spongy bone in diaphysis Secondary ossification centers form spongy bone in epiphyses Compact bone formed by periosteal osteoblasts

Characteristic: Periosteum Formation

Intramembranous: Osteochondral progenitor cells differentiate into osteoblasts Osteochondral progenitor cells surrounding developing bone form the eperiosteum Endochondral: Chondrocytes hypertrophy, the cartilage calcifies and is removed Osteochondral progenitor cells differentiate into osteoblasts and secrete bony matrix Perichondrium now called periosteum.

What is diaphysis composed of?

It is composed primarily of compact bone, surrounding a hollow center called the medullary cavity.

Why is woven bone weak?

It is fairly weak bone because the collagen fibers are randomly oriented in many directions.

What is lamellar bone organized into?

It is organized into thin, concentric sheets or layers approximately 3-7 micrometers thick called lamellae. In general, the collagen fibers of one lamella lie parallel to one another, but at an angle to the collagen fibers in the adjacent lamellae. Osteocytes, within their lacunae, are arranged in layers sandwiched between lamellae.

Why is remodeling of the ossified callus necessary?

Like the cartilage formed during fetal development, the cartilage in the external callus is replaced by woven spongy bone through endochondral ossification. The result is a stronger external callus. Even as the internal callus is forming and replacing the hematoma, osteoblasts from the periosteum and endosteum enter the internal callus and begin to produce bone. Eventually, the fibers and cartilage of the internal callus are replaced by woven spongy bone, which further stabilizes the broken one. Bone remodeling. Filling the gap between bone fragments with an internal callus of woven bone is not the end of the repair process because woven bone is not as structurally strong as the original lamellar bone. Repair is complete only when the woven bone of the internal callus and the dead bone adjacent to the fracture site have been replaced by compact bone. In this compact bone, osteons from both sides of the break extend across the fracture line to "peg" the bone fragments together. This remodeling process takes time-as much as a year or more. As the internal callus is remodeled and becomes stronger, the external callus is reduced in size by osteoclast activity. Eventually, repair may be so complete that no evidence of the break remains; however, the repaired zone usually remains slightly thicker than the adjacent bone. If the fracture has occurred in the diaphysis of a long bone, remodeling also restores the medullary cavity.

Hematoma

Localized mass of blood released form blood vessels but confined within an organ or a space; the blood is usually clotted.

What increases in length because of growth at the epiphyseal plate?

Long bones and bony projections increase in length because of growth at the epiphyseal plate.

What bones, or parts of bones, are formed from each type of ossification?

Many skull bones, part of the mandible (lower jaw), and the diaphyses of the clavicles (collar bones) develop by intramembranous ossification. Bones of the base of the skull, part of the mandible, the epiphyses of the clavicles, and most of the remaining skeletal system develop through endochondral ossification.

What bones develop by intramembranous ossification?

Many skull bones, part of the mandible (lower jaw), and the diaphyses of the clavicles (collarbones) develop by intramembranous ossification.

What do matrix vesicles contain?

Matrix vesicles contain high concentrations of Ca2+ and PO4^3-.

Osteocytes

Mature bone cell surrounded by bone matrix.

Chondrocyte

Mature cartilage cell.

How do mature osteoclasts carry out bone reabsorption?

Mature osteoclasts carry out bone reabsorption through a multistep process.

What does mechanical stress applied to bone increase?

Mechanical stress applied to bone increases osteoblast activity in bone tissue, and the removal of mechanical stress decreases osteoblast activity.

Endosteum

Membranous lining of the medullary cavity and the cavities of spongy bone.

What is most cartilage covered by?

Most cartilage is covered by a protective connective tissue sheath called the perichondrium.

What size are trabeculae and what do they consist of?

Most trabeculae are thin (50-400 micrometers) and consist of several lamellae with osteocytes located in lacunae between the lamellae.

How does new cartilage form?

New cartilage forms through interstitial cartilage growth followed by appositional bone growth on the surface of the existing cartilage.

What is bone first formed into?

No matter the type of bone formation, bone is first formed into woven bone, and is then remodeled into lamellar bone.

Growth hormone is the principal hormone controlling bone growth during childhood. Given the process of endochondral bone growth, how do you think growth hormone stimulates growth in height?

Not everyone has the same genes so when growth hormones are produced they affect the rate of bone growth differently for everyone.

Why do we use the term reabsorption?

Note we use the term reabsorption here because our point of reference is the bloodstream.

What is delivered to the osteocytes?

Nutrients in the blood vessels are delivered to the osteocytes.

What does ossification occur by?

Ossification occurs by appositional growth on the surface of previously existing material, either bone or cartilage.

What do osteoblasts secrete?

Osteoblasts also secrete matrix vesicles.

What are osteoblasts?

Osteoblasts are bone-building cells.

Differentiate among the characteristics and functions of osteoblasts, osteocytes, and osteoclasts.

Osteoblasts are bone-building cells. These cells have an extensive endoplasmic reticulum and numerous ribosomes. Osteoblasts produce collagen and proteoglycans, which are packaged into vesicles by the Golgi apparatus and secreted by exocytosis. Just as with the chondroblast conversion to chondrocyte, osteoblasts become osteocytes once the osteoblasts have secreted sufficient bone matrix. Osteocytes account for 90-95% of bone cells and are very long-lived, with a lifespan of up to 25 years. Osteoclasts are bone-destroying cells. These cells break down bone. breakdown of bone is called bone reabsorption. This breakdown is important for mobilizing crucial Ca2+ and phosphate ions for use in many metabolic processes.

What are osteoblasts and osteochondral progenitor cells derived from?

Osteoblasts are derived from osteochondral progenitor cells and osteocytes are derived from osteoblasts.

What do osteoblasts produce?

Osteoblasts produce collagen and protetoglycans, which are packaged into vesicles by the Golgi apparatus and secreted by exocytosis.

Where are osteochondral progenitor cells located?

Osteochondral progenitor cells are located in the inner layer of the perichondrium and in layers of connective tissue that cover bone (periosteum and endosperm). From these locations, they are a potential source of new osteoblasts or chondroblasts.

FIGURE 6.4 Osteoclast Structure

Osteoclast are massive, multinucleate cells that secrete acid and protein-digesting enzymes, which degrade bone. These cells then transport the digested matrix from the bone into the extracellular fluid.

What are osteoclasts?

Osteoclasts are massive, multinucleate cells whose differentiation follows a complex pathway. They are derived from the red bone marrow cells that also differentiate into monocytes (called macrophages in the tissues). These precursors attach to the bone matrix where direct contact with osteoblasts is required to allow eventual maturation into functional osteoclasts.

Where are osteocyte cell bodies housed?

Osteocyte cell bodies are housed within the bone matrix in spaces called lacunae.

Where are osteocyte cell extensions housed?

Osteocyte cell extensions are housed in narrow, long spaces called canaliculi; little canals.

How much do osteocytes account for?

Osteocytes account for 90-95% of bone cells and are very long-lived, with a lifespan of up to 25 years.

What are osteocytes connected to?

Osteocytes are connected to neighboring osteocytes through their cell extensions.

Where are osteocytes located?

Osteocytes are located in lacunae between the lamellar rings, and canaliculi radiate between lacunae across the lamellae, looking like minute cracks across the rings of the target.

FIGURE 6.10 Structure of a Flat Bone

Outer layers of compact bone surround spongy bone.

Trace the pathway nutrients must follow from blood vessels in the periosteum to osteocytes within lacunae in osteons.

Perforating canals deliver blood to the central canals of the osteons. The perforating canals run perpendicular to the length of the bone. They contain blood vessels from the periosteum or medullary cavity. Nutrients in the blood vessels are delivered to the osteocytes. Because the osteocytes are in contact with each other through the canaliculi, nutrients are passed from osteocyte to osteocyte of each osteon. Because the osteocytes are in contact with each other through the canaliculi, nutrients are passed from one osteocyte to another osteocyte within each osteon. Waste products are removed in the reverse direction.

FIGURE 6.1 Hyaline Cartilage

Photomicrograph of hyaline cartilage covered by perichondrium. Chondrocytes within lacunae are surrounded by a cartilage matrix. Appositional growth occurs when new cartilage is added to the surface of the cartilage by chondroblasts from the inner layer of the perichondrium. Interstitial growth occurs when new cartilage is formed within the cartilage by chondrocytes that divide and produce additional matrix.

Epiphysis

Portion of a bone developed from a secondary ossification center and separated from the remainder of the bone by the epiphyseal plate.

When do primary and secondary ossification centers appear during endochondral ossification?

Primary ossification center formation: Blood vessels grow into the enlarged lacunae of the calcified cartilage. Osteoblasts and osteoclasts migrate into the calcified cartilage are from the periosteum by way of the connective tissue surrounding the outside of the blood vessels. The primary ossification center forms as osteoblasts produce bone on the surface of the calcified cartilage. The osteoblasts transform the calcified cartilage of the diaphysis into spongy bone. As bone development proceeds, the cartilage model continues to grow, the bone collar thickens. Remodeling converts woven bone to lamellar bone and contributes to the final shape of the bone. Osteoclasts remove bone from the center of the diaphysis to form the medullary cavity, and cells within the medullary cavity specialize to form red bone marrow. Secondary ossification center formation: Secondary ossification centers are created in the epiphyses by osteoblasts that migrate into the epiphysis. The events occurring at the secondary ossification centers are the same as those at the primary ossification centers, except that the spaces in the epiphyses do not enlarge to form a medullary cavity in the diaphysis. Primary ossification centers appear during early fetal development, whereas secondary ossification centers appear in the proximal epiphysis of the femur, humerus, and tibia about 1 month before birth. A baby is considered full-term if one of these three ossification centers can be seen on radiographs at the time of birth. At about 18-20 years of age, the last secondary ossification center appears in the medial epiphysis of the clavicle. Replacement of cartilage by bone continues in the cartilage model until all the cartilage, except that in the epiphyseal plate and on articular surfaces, has been replaced by bone. The epiphyseal plate, which exists during the time a person's bones are actively growing, and the articular cartilage, which is a permanent structure, are derived from the original embryonic cartilage model. After a person's bones have stopped growing, the epiphyseal plate regresses into a "scar," called the epiphyseal line.

What affects bone mass?

Race and ethinicity also affect bone mass. African-Americans and Latinos have higher bone masses than Caucasians and Asians.

FIGURE 6.15 Fracture of the Epiphyseal Plate

Radiograph of an adolescent's knee. The femur (thighbone) is separated from the tibia (leg one) by a joint cavity. The epiphyseal plate of the femur is fractured, thereby separating the diaphysis from the epiphysis.

What do most human cells start out in?

Recall from chapter 1 that most human cells start out in an undeveloped state and then differentiate into their final form.

Where is vitamin D3 product initiated?

Recall from chapter 5 that vitamin D3 production is initiated in the skin and continued in the liver and then in the kidney where calcitriol activation occurs.

How is connective tissue characterized?

Recall that connective tissue is characterized by having spread-out cells separated by a nonliving material called matrix.

When can ossification occur?

Recall that ossification can occur multiple times during life: as a fetus, when growing larger, and anytime in life a fracture is repaired.

What is red marrow?

Red marrow is the site of blood cell formation.

What also regulates bone growth?

Reproductive hormones also regulate bone growth.

What are sinuses?

Sinuses within some of the flat and irregular bones of the skull are air-filled spaces. Sinuses are lined by mucous membranes.

How do estrogen and testosterone affect bone growth? How do these effects account for the average height difference observed in men and women?

Reproductive hormones also regulate bone growth. Estrogen is the primary female reproductive hormone and testosterone is the primary male reproductive hormone. During puberty, the levels of these hormones increase dramatically. The increased hormone levels stimulate significant growth spurts in the beginning. Toward the end of puberty, both estrogen and testosterone stop bone growth. They both cause ossification of epiphyseal plates. Females usually stop growing earlier than males, around age 18. Estrogens are more effective at closing the epiphyseal plate than testosterone is. Males can potentially continue growing until age 25. Because their entire growth period is somewhat shorter, females usually do not reach the same height as males. Decreased levels of testosterone or estrogen can prolong the growth phase of the epiphyseal plates, even though the bones grow more slowly. Overall, growth is very complex and is influenced by many factors besides reproductive hormones, such as other hormones, genetics, and nutrition.

How does the skeleton system provide body support?

Rigid, strong bone is well suited for bearing weight and is the major supporting tissue of the body. Cartilage provide a firm yet flexible support within certain structures, such as the nose, external ear, thoracic cage, and trachea. Ligaments, strong bands of fibrous connective tissue, hold bones together.

Growth and Developmental Disorders: Osteogenesis imperfacta

See Clinical Genetics 6.1

diaphysis

Shaft of a long bone.

Diaphysis

Shaft of the bone

Are short and irregular bones elongated?

Short and irregular bones are not elongated and have no diaphyses. However, certain region of these bones, such as the process (projections), have epiphyseal growth plates and therefore small epiphyses

What do short and irregular bones have?

Short and irregular bones have a composition similar to the epiphyses of longes bones-compact bone surfaces surrounding a spongy bone center with small spaces that are usually filled with marrow.

What increases the likelihood of bone fractures?

Significant bone loss increases the likelihood of bone fractures.

Epiphyseal plate

Site at which bone growth in length occurs; located between the epiphysis and diaphysis of a long bone; area of hyaline cartilage where cartilage growth is followed by endochondral ossification; also called metaphysis or growth plate.

Hormone: Calcitonin

Site of Action: Bone cells Hormonal Action: decreases Blood Ca2+ levels.

Hormone: PTH

Site of Action: Kidney tubules Bone cells Hormonal Action: Activate calcitriol Increases Blood Ca2+ levels

Hormone: Calcitriol

Site of Action: Kidney tubules Small intestine Bone cells Hormonal Action: Increases Blood Ca2+ levels.

What is the primary male reproductive hormone?

Testosterone is the primary male reproductive hormone.

What happens to the trabeculae as they become disconnected from each other.

The ability of the trabecular to provide support also decreases as they become disconnected from each other. Eventually, some of the trabeculae completely disappear.

What can modify the bone's strength?

The amount of mechanical stress, such as weightlifting, applied to a bone can modify the bone's strength.

What does the modification of bone strength occur through?

The amount of mechanical stress, such as when weightlifting, applied to bone can modify the bone's strength. This modification occurs through several mechanisms including (1) remodeling, (2) the formation of additional one, or (3) alteration in trabecular alignment to reinforce the scaffolding.

What is the endosteum?

The endosteum is a single cell layer of connective tissue that lines the internal surfaces of all cavities within bones, such as the medullary cavity or the diaphysis and the smaller cavities in spongy and compact bone.

What are epiphyses?

The ends of a long bone are called epiphyses.

Where are perforating canals?

The perforating canals run perpendicular to the length of the bone.

What determines the size and shape of a bone?

The potential shape and size of a bone and an individual's final adult height are determined genetically, but factors such as nutrition and hormones can greatly modify the expression of those genetic factors.

Where is the primary ossification center?

The primary ossification center is in the diaphysis.

What two mechanical advantages do the structure of a long bone have?

The structure of a long bone-a hollow cylinder with a medullary cavity in the center-has two mechanical advantages: (1) A hollow cylinder is lighter in weight than a solid rod, and (2) a hollow cylinder with the same height, weight, and composition as a solid rod, but with a greater diameter, can support much more weight without bending.

What are the surfaces of trabeculae covered with?

The surfaces of trabeculae are covered with a single layer of cells consisting mostly of osteoblasts with a few osteoclasts.

FIGURE 6.18 Effect of Growth Hormone on Stature

The taller man (giant) has excessive growth hormone secretion, whereas the shorter man (dwarf) has insufficient growth hormone secretion.

What is arrested growth caused by?

These lines are caused by greater calcification below the epiphyseal plate of bone, where it has grown at a slower rate during illness or severe nutritional deprivation.

What do perforating canals contain?

They contain blood vessels from the periosteum or medullary cavity.

What can arrested growth cause?

They demonstrate that illness or malnutrition during the time of bone growth can cause a person to be shorter than he or she would have been otherwise.

Periosteum

Thick, double-layered connective tissue sheath covering the entire surface of a bone, except the articular surface, which is covered with cartilage.

Endosteum

Thin connective tissue membrane lining the inner cavities of bone

Articular cartialge

Thin layer of hyaline cartilage covering a bone where it forms a joint (articulation) with another bone

Lamellae

Thin sheet or layer of bone.

Why is the breakdown of bone important?

This breakdown is important for mobilizing crucial Ca 2+ and phosphate ions for use in many metabolic processes.

What is the fracture pattern classified on?

This classification is based on the pattern of the fracture on the bone.

Can adults develop rickets?

This condition sometimes occurs in adults who have digestive disorders.

What is the event called closure of the epiphyseal plate?

This event, called closure of the epiphyseal plate, occurs between approximately 12 and 25 years of age, depending on the bone and the individual.

During endochondral ossification, calcification of cartilage results in the death of chondrocytes. However, ossification of the bone matrix does not result in the death of osteocytes. Explain.

This question requires knowledge of cartilage histology, bone formation, and bone histology. Recall that, to acquire O2 and nutrients, cartilage relies on diffusion though the matrix. As osteoclasts migrate into the developing bone structure and start to remove the cartilage, the cartilage becomes calcified, no more diffusion is possible, and the chondrocytes die. However, when we examine bone histology, we see that adjacent osteocytes are connected via cell processes. As the matrix is laid down, the area where the cell processes meet does not become covered in ossified matrix, forming canaliculi. Therefore, osteocytes continue receiving oxygen and nutrients through the canaliculi or from one osteocyte to another through cell processes.

Explain why it is advantageous for the articular cartilage never to become ossified.

To answer this question, first describe the physical difference between cartilage and bone: cartilage matrix is semisolid (pliable) when compared with bone matrix, which is solid (rigid). Next address what the advantage is of having cartilage covering articular surfaces. The advantage of covering articulations (joints) with cartilage is that it provides a flexible, smooth surface for bone movement. A major impetus for many joint replacement surgeries is degeneration of the articular cartilage, as movement of a joint without articular cartilage is quite painful (see chapter 8 Clinical Impact, "Joint Replacement").

Explain why bones cannot undergo interstitial growth as cartilage does.

To begin, define interstitial growth (from section 6.2): growth of cartilage through division of chondrocytes within the existing matrix. Because cartilage matrix is pliable, expansion of the existing matrix occurs as new matrix is added by the new chondrocytes. Remember that new bone growth is added to existing bone by appositional growth only. The definition of appositional growth is the addition of new osteoblasts and bone matrix on the surface of existing bone or cartilage. Next, consider the composition of bone matrix. It contains an inorganic component, calcium phosphate, which is not pliable. Therefore, if an existing osteocyte were to divide within its lacuna, the existing matrix would physically impede deposition of more matrix.

What happens toward the end of puberty?

Toward the end of puberty, both estrogen and testosterone stop bone growth. They both cause ossification of epiphyseal plates.

What trabeculae oriented with?

Trabeculae are oriented along the lines of mechanical stress within a bone. If the force of a bone is changed slightly (e.g., because of a fracture that heals improperly), the trabecular pattern realigns with the new lines of stress.

What happens as the bone grows in width?

When a bone grows in width slowly, the surface of the bone becomes smooth as osteoblasts from the periosteum lay down even layers of bone to form circumefential lamellae.

What are the two possibilities that exist for the position of fractured bones?

Two possibilities exist for the position of the fracture bone ends after the fracture has occurred: (1) displaced, where the ends of the bones are offset from each other and are not aligned anatomically; and (2) non displaced, where the ends of the bone remain in anatomical alignment.

What happens to osteoclast activity under conditions of reduced stress?

Under conditions of reduced stress, as when. person is bedridden or paralyzed, osteoclast activity continues at a nearly normal rate but osteoblast activity decreases, resulting in less bone density.

What do undifferentiated cell names end in?

Undifferentiated cell has grow n and developed into its final form, the suffix -cyte replaces -blast in the name.

Can bones grow by interstitial growth?

Unlike cartilage, bones cannot grow by interstitial growth.

How do osteocytes obtain nutrients?

Usually, no blood vessels penetrate the trabeculae, so osteocytes must obtain nutrients through their canaliculi.

Where are vessels of the bone contained?

Vessels that run parallel to the long axis of the bone are contained within the central canals.

What does vitamin C deficiency result in?

Vitamin C deficiency results in bones and cartilage with fewer collagen fibers because collagen synthesis is impaired.

What is vitamin C necessary for?

Vitamin C is necessary for osteoblasts to synthesize collagen. Normally, as old collagen breaks down, new collagen is synthesized to replace it.

What is vitamin D necessary for?

Vitamin D is necessary for the normal absorption of calcium from the intestines.

What happens in step 1 of bone growth in width?

When a bone rapidly grows in width, as occurs in young bones or during puberty, osteoblasts from the periosteum lay down bone to form a series of ridges with grooves between them.

What happens to osteoclast activity when blood Ca2+ levels are low?

When blood Ca2+ levels are low, osteoclast activity increases and bone reabsorption occurs. Osteoclasts release more Ca2+ from bone into the blood than osteoblasts remove from the blood to make new bone. Consequently, a net movement of Ca2+ occurs from bone into blood, and blood Ca2+ levels increase.

What is the epiphyseal line?

When bone stops growing in length, the epiphyseal plate becomes ossified and is called the epiphyseal line.

When is the movement of Ca2+ into and out of bone equal?

When osteoblast and osteoclast activities are balanced, the movements of Ca2+ into and out of bone are equal.

How are hydorxyapatite crystals formed?

When the concentrations of Ca2+ and PO4^3- reach a certain level, hydroxyapatite crystals form.

List the steps of regulation of blood ca2+ levels.

(1) Blood Ca2+ levels are in the normal range. (2) Blood Ca2+ levels increase outside their normal range, which causes homeostasis to be disturbed. (3) The control centers respond to the change in blood levels. (4) The control center releases calcitonin, which inhibits osteoclasts. (5) Reduced osteoclast activity causes blood Ca2+ levels to decrease. (6) Ca2+ levels return to their normal range and homeostasis is restored.

HOMEOSTASIS FIGURE 6.22 Regulation of Blood Ca2+ Levels

(1) Blood Ca2+ levels are in the normal range. (2) Blood Ca2+ levels increase outside their normal range, which causes homeostasis to be disturbed. (3) The control centers respond to the change in blood levels. (4) The control center release calcitonin, which inhibits osteoclasts. (5) Reduced osteoclast activity causes blood Ca2+ levels to decrease. (6) Ca2+ levels return to their normal range and homeostasis is restored. Observe response to a decrease in blood Ca2+ levels outside the normal range by following the red arrows.

FIGURE 6.6 Spongy Bone

(a) Beams of bone, the trabeculae, surround spaces in the bone. In life, the spaces are filled with red or yellow bone marrow and blood vessels. (b) Transverse section of a trabecula.

FIGURE 6.8 Compact Bone

(a) Compact bone consists mainly of osteons, which are concentric lamellae surrounding blood vessels within central canals. The outer surface of the bone is formed by circumferential lamellae, and bone between the osteons consists of interstitial lamellae. (b) Photomicrograph of an osteon.

FIGURE 6.2 Effects of Changing the Bone Matrix.

(a) Normal bone. (b) Demineralized bone, soaked in acid, in which collagen is the primary remaining component, can be bent without breaking. (c) When collagen is denatured, mineral is the primary remaining component, making the bone so brittle that it is easily shattered.

FIGURE 6.3 Ossification

(a) On a preexisting surface, such as cartilage or bone, the cell extensions of different osteoblasts join together. (b) Osteoblast have produced bone matrix and are now osteocytes. (c) Photomicrograph of an osteocyte in a lacuna with cell extensions in the canaliculi.

PROCESS FIGURE 6.21 Bone Repair

(a) On the top is a radiograph of the broken humerus of author. A. Russo's granddaughter, Viviana. On the bottom is the same humerus a few weeks later, with a callus now formed around the break. (b) The steps in bone repair.

PROCESS FIGURE 6.14 Epiphyseal Plate

(a) Radiograph and drawing on the knee, showing the epiphyseal plate of the tibia (shinbone). Because cartilage does not appear readily on x-ray film, the epiphyseal plate appears as a black area between the white diaphysis and the epiphyses. (b) Zones of the epiphyseal plate, including newly ossified bone. (c) New cartilage forms on the epiphyseal side of the plate at the same rate that new bone forms on the diaphysial side of the plate. Consequently, the epiphyseal plate remains the same thickness but the diaphysis increases in length.

PROCESS FIGURE 6.11 Intramembranous Ossification

(a) This panel shows the skull of a 12-week-old fetus. Bones formed by intramembranous ossification are yellow, and bones formed by endochondral ossification are blue. Intramembranous ossification starts at a center of ossification and expands outward. Therefore, the youngest bone is at the edge of the expanding bone and the oldest bone is at the center of ossification. (b) Steps 1-3 show progressively older bone as the skull develops.

List the steps of how osteoclasts carry out bone reabsorption.

1. First, the. osteoclasts must access the bone matrix. The current model proposes that osteoblasts lining the connective tissue around bone regulate the movement of mature osteoclasts into a bone remodeling area. 2. Once the osteoclasts have come in contact with the bone surface, they form attachment structures via interactions with cell-surface proteins called integrins. 3. Soon after, structures called podosomes develop and form a sealed compartment under the osteoclast. The osteoclast plasma membrane then further differentiates into a highly folded form called the ruffled border. The ruffled border is a specialized reabsorption-specific area of the membrane. 4. Acidic vesicles fuse with the membrane of the ruffled border, while ATP-powered H+ pumps and protein-digesting enzymes are inserted into the membrane of the ruffled border. 5. Secretion of H+ creates an acidic environment within the sealed compartment, which causes decalcification of the bone matrix. 6. The protein-digestion enzymes are secreted into the sealed compartment and digest the organic, protein component of the bone matrix. 7. After breakdown of the matrix, the degradation products are removed by a transcytosis mechanism, whereby the products enter the osteoclast and move across the cytoplasm to the other side of the osteoclast. 8. There, the degradation products are secreted into the extracellular space, enter the blood, and are used elsewhere in the body.

List the steps of bone growth in width.

1. Osteoblasts beneath the periosteum lay down ( dark brown) to form ridges separated by grooves. Blood vessels of the periosteum lie in the grooves. 2. The groove is transformed into a tunnel when the bone built on adjacent ridges meets. The periosteum of the groove becomes the endosteum of the tunnel. 3. Appositional growth by osteoblasts from the endosteum results in the formation of a new concentric lamellae. 4. The production of additional concentric lamellae fills in the tunnel and completes the formation of the osteon.

When does embryonic mesenchyme condense around the developing brain to form a membrane of connective tissue?

At approximately the fifth week of development in an embryo, embryonic mesenchyme condenses around the developing brain to form a membrane of connective tissue with delicate, randomly oriented collagen fibers.

What is a basic multicellular unit (BMU)? Explain how a BMU directs remodeling in compact bone and in spongy bone.

Bone remodeling involves a basic multicellular unit (BMU). A BMU is a temporary assembly of osteoclasts and osteoblasts that travels through or across the surface of bone. These cells remove old bone matrix and replace it with new bone matrix. The average life span of a BMU is approximately 6 months, and BMU activity renews the entire skeleton every 10 years. In compact bone, the osteoclasts of a BMU break down bone matrix, forming a tunnel. Interstitial lamellae are remnants of osteons that were not completely removed when a BMU formed a tunnel. Blood vessels grow into the tunnel wall, forming a concentric lamella. Additional concentric lamellae are produced, filling in the tunnel from the outside to the inside, until an osteon is formed, with the center of the tunnel becoming a central canal containing blood vessels. In spongy bone, the BMU removes bone matrix from the surface of a trabecular, forming a cavity, which the BMU then fills in with new bone matrix.

What happens to the bone after 40 ?

Bones become thinner, but their outer dimensions change little, because most compact bone is lost under the endosteum on the inner surface of bones. In addition, the remaining compact bone weakens as a result of incomplete bone remodeling.

How do bones increase in size?

Bones increase in size only by appositional growth, the formation of new bone on the surface of older bone cartilage. For example, trabeculae grow in size when osteoblasts deposit new bone matrix onto the surface of the trabeculae.

What bones develop through ossification?

Bones of the base of the skull, part of the mandible, the epiphyses of the clavicles, and most of the remaining skeletal system develop through endocondral ossification.

Displaced Fracture

Characteristics: -Ends of fractured bone move so that they are no longer aligned. -More painful than other fracture types and may cause internal damage from the bone end

Describe the structure of compact bone. What is an osteon? Name three types of lamellae found in compact bone.

Compact bone is the solid, outer layer surrounding each bone. It has more matrix and is denser with fewer pores than spongy bone. Blood vessels enter the substance of the bone itself, and the lamellae of compact bone are primarily oriented around those blood vessels. The functional unit of a long bone is an osteon, or haversian system. An osteon is composed of concentric rings of matrix, which surround a central tunnel and contain osteocytes. In cross section, an osteon resembles a circular target; the bull's-eye of the target is the central canal. Surrounding the central canal are rings of bone matrix called concentric lamellae. Each osteon has from 4 to 20 concentric lamellae. Central canals are lined with endosperm and contain blood vessels, nerves, and loose connective tissue. Vessels that run parallel to the long axis of the bone are contained within the central canals. Osteocytes are located in lacunae between the lamellar rings, and canaliculi radiate between lacunae across the lamellae, looking like minute cracks across the rings of the target. Circumferential lamellae form the outer surfaces of compact bone, which are thin plates that extend around the bone. In some bones, such as certain bones of the face, the layer of compact bone can be so thin that no osteons exist and the compact bone is composed of only circumferential lamellae. Between the osteons are interstitial lamellae, which are remnants of concentric or circumferential lamellae that were partially removed during bone remodeling.

Give an example of bone loss leading to deformity, loss of height, pain, and stiffness.

For example, compression fractures of the vertebrae can cause an exaggerated curvature of the spine, resulting in a bent-forward, stooped posture. Loss of bone from the jaws can lead to tooth loss.

Give an example of bone formation.

For example, osteoblasts beneath the periosteum cover the surface of preexisting bone. Elongated cell extensions from osteoblasts connect to the cell extensions of other osteoblasts through gap junctions. Bone matrix produced by the osteoblasts covers the older bone surface and surrounds the osteoblasts cell bodies and extensions. The result is a new layer of bone.

Give an example of places red marrow and yellow marrow are located.

For example, part of the hipbone (ilium) may contain 50% red marrow and 50% yellow marrow. The hipbone is used as a source of donated red bone marrow because it is a large bone with marrow than smaller bones and it can be accessed relatively easily.

What are fontanels?

Fotanels, or soft spots, are the larger, membrane-covered spaces between the developing skull bones that have not yet been ossified.

Whats another way fractures are categorized?

Fractures are categorized by the completeness of the break and its alignment relative to the bone.

Bone growth is greatly affected by growth hormone and thyroid hormone. Explain these effects.

Hormones are very important in bone growth. Growth hormone from the anterior pituitary increases general tissue growth, including overall bone growth, by stimulating interstitial cartilage growth and appositional bone growth. Disruptions in normal growth hormone can cause dramatic changes in an individual's height. Excessive growth hormone secretion results in pituitary gigantism, whereas insufficient growth hormone secretion results in pituitary dwarfism. Thyroid hormone is also required for normal growth of all tissues, including cartilage; therefore, a decrease in this hormone can result in a smaller individual.

Articular cartilage

Hyaline cartilage covering the ends of bones within a synovial joint.

Suppose an astronaut who currently lives in the international space station fracture a bone. The doctor in the space station told her to expect bone repair to take 8 weeks. If the astronaut experienced the same bone fracture while living on Earth, do you think the expected bone repair time would be shorter or longer than 8 weeks?

I think it would be longer because of the difference in weight due to gravity.

What happens if the mineral in bone is reduced?

If mineral in a long bone is reduced, collagen becomes the primary constituent and the bone is overly flexible.

What happens if the collagen in bone is reduced?

If the amount of collagen is reduced in the bone, the mineral component becomes the primary constituent and the bone is very brittle.

What separates the epiphysis from the diaphysis in a long bone?

In a long bone, the epiphyseal plate separates the epiphysis from the diaphysis.

What do osteocytes and their extensions form?

In a sense, the cells and their extensions form a "mold" around which the matrix is formed.

What happens to osteons as you age from young to adult?

In a young bone, when osteons are removed, the resulting spaces are filled with new stenos. With aging, the new osteons fail to completely fill in the spaces produced when the older osteons are removed.

What is one of the most potent stimulators of RANKL?

In addition of calcitriol stimulating intestinal Ca2+ absorption, it is one of the most potent stimulators of RANKL production by osteoblasts. Thus, calcitriol and PTH work together to increase osteoclast activity for bone reabsorption.

What does regulating blood Ca2+ levels do?

In addition to regulating blood Ca2+ levels, interactions between osteoclasts, osteoblasts, and Ca2+-regulating hormones also contribute to bone growth, remodeling, and repair.

What else does articular cartilage increase the size of?

In addition, growth at the articular cartilage increases the size of bones that do not have an epiphysis, such as short bones.

What initiates electrical charges that stimulate osteoblast activity?

In addition, increased physical pressure in bone initiates electrical changes that stimulate osteoblast activity. Therefore, applying weight (pressure) to a broken bone can speed the healing process. Weak pulses of electric current are sometimes applied to a broken bone to speed healing.

What are perforating fibers?

In addition, some of the collagen fibers of the tendons or ligaments fibers penetrate the periosteum into the outer part of the bone. These bundles of collagen fibers are called perforating fibers, or Sharpey fibers, and they strengthen the attachment of the tendons or ligaments to the bone.

What happens in appositional growth?

In appositional growth, chondroblasts in the perichondrium add new cartilage to the outside edge of the existing cartilage.

What happens in the deepest part of the articular cartilage?

In the deepest part of the articular cartilage, nearer bone tissue, the cartilage is calcified and ossified to form new bone.

What happens in bone remodeling?

In this process, osteoclasts remove old bone and osteoblasts deposit new bone.

FIGURE 6.17 Arrested Growth Lines

In this radiograph, the horizontal dark lines are lines of arrested growth.

Hypertrophy

Increase in bulk or size; not due to an increase in number of individual elements.

What does insufficient growth hormone result in?

Insufficient growth hormone secretion results in pituitary dwarfism.

What does insufficient vitamin D cause?

Insufficient vitamin D in children causes rickets, a disease resulting from reduced mineralization of the bone matrix. Children with rickets may have bowed bones and inflamed joints.

What are interstitial lamellae?

Interstitial lamellae are remnants of osteons that were not completely removed when a BMU formed a tunnel. Blood vessels grow into the tunnel wall, forming a concentric lamella.

Characteristic: Remodeling

Intramembranous: Woven bone is remodeled into lamellar bone and is indistinguishable from endochondral bone Endochondral: Woven bone is remodeled into lamellar bone and is indistinguishable from intramembranous bone.

Where does intramembranous ossification start?

Intremmebranous ossification starts within embryonic connective tissue membranes.

An x-ray reveled that Jill had suffered a fracture of her left femur while playing soccer in junior high school. The bone was set, without surgery, and she had to have a cast on her leg for longer than the 6 weeks normally required for a fracture. A year after the accident, her left femur is shorter than her right femur. Explain how this occurred.

It is likely that Jill is still growing. Consequently, the epiphyseal plates in her long bones have not yet been converted to epiphyseal lines. If a break occurs in an epiphyseal lines. If a break occurs in an epiphyseal plate, it can slow bone growth and interfere with bone elongation. As a result, the femur, and therefore her left leg, will be shorter than her right leg. Recovery is difficult because cartilage repairs slowly due to the fact that cartilage is much less vascular than bone.

What is bone remodeling?

Just as our homes must be remodeled when they fall into disrepair, bone that becomes old is replaced with new bone in a process called bone remodeling.

Why is it important for bone remodeling to occur?

Just as our homes must be remodeled when they fall into disrepair, bone that becomes old is replaced with new bone in a process called bone remodeling. In this process, osteoclasts remove old bone and osteoblasts deposit new bone. Bone remodeling converts woven bone into lamellar bone and is involved in several important functions, including bone growth, changes in bone shape, adjustment of the bone to stress, bone repair, and calcium ion (Ca2+) regulation in the body.

How do osteoblasts become osteocytes?

Just as with the chondroblast conversion to chondrocyte, osteoblasts become osteocytes once the osteoblasts have secreted sufficient bone matrix.

What do ligaments do?

Ligaments, strong bands of fibrous connective tissue, hold bones together.

Explain the differences in structure between long bones and flat, short, and irregular bones.

Long bone The diaphysis is the center portion of the bone. It is composed primarily of compact bone, surrounding a hollow center called the medullary cavity. Flat bone Flat bones contain an inferior framework of spongy bone sandwiched between two layers of compact Bone. Short bone Short and irregular bones have composition similar to the epiphyses of long bones-compact bone surface surrounding a spongy bone center with small spaces that are usually filled with marrow. Irregular bones Have unusual shapes.

How do long bones grow?

Long bones grow by creating new cartilage in the epiphyseal plate.

Name and describe the events occurring in the four zones of the epiphyseal plate. Explain how the epiphyseal plate remains the same thickness while the bone increases in length.

Long bones grow by creating new cartilage in the epiphyseal plate. Cartilage calcification and ossification in the epiphyseal plate occur by the same basic process as calcification and ossification of the cartilage model during endochondral bone formation. New cartilage forms through interstitial cartilage growth followed by appositional bone growth on the surface of the existing cartilage. The epiphyseal plate is organized into five zones. 1. The zone of resting cartilage is nearest the epiphysis and contains slowly dividing chondrocytes. 2. The chondrocytes in the zone of proliferation produce new cartilage through interstitial cartilage growth. The chondrocytes divide and form columns resembling stacks of plates or coins. 3. In the zone of hypertrophy, the chondrocytes produced in the zone of proliferation mature and enlarge. Thus, a maturation gradient exist in each column: The cells nearer the epiphysis are younger and actively proliferating, whereas the cells progressively nearer the diaphysis are older and undergoing hypertrophy. 4. The zone of calcification is very thin and contains hypertrophied chondrocytes and calcified cartilage matrix. The hypertrophied chondrocytes die, and blood vessels from the diaphysis grow into the area. The connective tissue surrounding the blood vessels contains osteoblasts from the endosteum. 5. The osteoblasts line up on the surface of the calcified cartilage and , through appositional bone growth, deposit new bone matrix, which is later remodeled. As new cartilage cells form in the zone of proliferation, and as these cells enlarge in the zone of hypertrophy, the overall length of the diaphysis increases. However, the thickness of the epiphyseal plate does not increase because the rate of cartilage growth on the epiphyseal side of the plate is equal to the rate of bone formation on the diaphyseal side of the plate. As the ones achieve normal adult size, they stop growing in length because the epiphyseal plate has ossified and become the epiphyseal line. This event, called closure of the epiphyseal plate, occurs between approximately 12 and 25 years of age, depending on the bone and the individual.

What does appositional bone growth cause?

Long bones increase in width (diameter) and other bones increase in size or thickness because of appositional bone growth beneath the periosteum.

Describe how new osteons are produced as a bone increases in width.

Long bones increase in width (diameter) and other bones increase in size or thickness because of appositional bone growth beneath the periosteum. When a bone rapidly grows in width, as occurs in young bones or doing puberty, osteoblasts from the periosteum lay down bone to form a series of ridges with grooves between tem. The periosteum covers the bone ridges and extend down into the bottom of the grooves, and one or more blood vessels of the periosteum lie within each groove. As the osteoblasts continue to produce one, the ridges increase in size, extend toward each other, and meet to change the groove into a tunnel. The membrane lining the tunnel is called the endosteum. This endosteum had preiviously been the periosteum before the growth in bone width and formation of grooves along the outer surface the bone occurred. Osteoblasts from the endosteum lay down bone to form a concentric lamella. The production of additional lamellae fills in the tunnel, encloses the blood vessel, and produces an osteon.

What does low vitamin D levels cause in adults?

Low vitamin D levels can be one cause of "adult rickets," or osteomalacia, a softening of the bones due to calcium deletion.

When do males stop growing?

Males can potentially continue growing until age 25. Because their entire growth period is somewhat shorter, females usually do not reach the same height as males.

Trabeculae

One of the supporting bundles of fibers traversing the substance of a structure, usually derived from the capsule or one of the fibrous septa, such as trabeculae of lymph nodes, testes; a beam or plate of cancellous bone.

What are osteoclasts?

Osteoclasts are bone-destroying cells. These cells break down bone.

Can osteocytes become relatively inactive?

Osteocytes become relatively inactive, compared with most osteoblasts, but it is possible for them to produce the components needed to maintain the bone matrix.

Periosteum

Outer, double-layered connective tissue membrane with ligaments and tendons attached to bone through the periosteum; blood vessels and nerve pathways; the periosteum is where bone grows in diameter.

What else should we know about growth?

Overall, growth is very complex and is influenced by many factors besides reproductive hormones, such as other hormones, genetics, and nutrition.

What is PTH production and secretion controlled by?

PTH production and secretion are controlled by Ca2+ - sensing receptors in parathyroid gland plasma membranes.

Indirect Effects of PTH in the Small Intestine.

PTH regulates blood Ca2+ levels by indirectly increasing Ca2+ uptake form the small intestine. Increased PTH promotes the activation of calcitriol in the kidneys. Calcitriol increases absorption of Ca2+ in the small intestine.

What does PTH stimulation to calcitriol do?

PTH stimulates calcitriol activation in the kidney , which contributes to PTH-induced increases in blood Ca2+ levels.

What two general mechanisms does PTH work through?

PTH works through two general mechanisms: (1) direct effects on bone cells and in the kidney and (2) indirect effects on the small intestine

Name the hormone that is the major regulator of Ca2+ levels in the body. What stimulates the secretion of this hormone?

Parathyroid hormone is secreted by cells in the parathyroid gland and is essential for the maintenance of blood Ca2+ levels within the homeostatic limits. PTH production and secretion are controlled by Ca2+ -sensing receptors in parathyroid gland plasma membranes. The key signal for PTH secretion is a reduction in blood Ca2+ levels.

What is essential for blood Ca2+ levels within the homeostatic limits?

Parathyroid hormone is secreted by cells in the parthyroid gland and is essential for the maintenance of blood Ca2+ levels within the homeostatic limits.

How does the skeletal system provide mineral storage?

Some minerals in the blood are stored in bone. If blood levels of these mineral decrease, the minerals are released from bone into the blood. The principal minerals stored the calcium and phosphorus, two minerals essential for many physiological processes. Adipose tissue is also stored within bone cavities. If needed, the lipids are released into the blood and used by other tissues as a source of energy.

What can mesenchymal cells become?

Some of the mesenchymal cells become stem cells, a number of which replicate and become more specialized cell types.

What can be found lining the medullary cavity?

Some spongy bone can be found lining the medullary cavity.

What do hyaline cartilage chondroblast secrete?

Specifically, hyaline cartilage chondroblasts secrete a matrix, which surrounds the chondroblasts.

Describe the structure of hyaline cartilage.

Specifically, hyaline cartilage chondroblasts secrete a matrix, which surrounds the chondroblasts. Once the matrix has surrounded the chondroblast, it has differentiated into a chondrocyte. Chondrocyte are rounded cells that occupy a space called lacuna within the matrix. The matrix contains collagen, which provides strength, and proteoglycans, which make cartilage resilient by trapping water. Most cartilage is covered by a protective connective tissue sheath called the perichondrium. The perichondrium is a double-layered outer layer of dense irregular connective tissue containing fibroblasts. The inner, more delicate layer has fewer fibers and contains chondroblasts. Blood vessels and nerves penetrate the outer layer of the perichondrium but do not enter the cartilage matrix, so nutrients must diffuse through the cartilage matrix to reach the chondrocytes. Articular cartilage, which is hyaline cartilage that covers the ends of bones where they come together to form joints, has no perichondrium, blood vessels, or nerves.

Describe the structure of spongy bone. What are trabeculae, and what is their function? How do osteocytes within trabeculae obtain nutrients?

Spongy bone consists of interconnecting rods or plates of bone called trabeculae beam. Between the trabeculae are spaces, which in life are filled with bone marrow and blood vessels. Most trabeculae are thin (50-400 micrometers) and consist of several lamellae with osteocytes located in lacunae between the lamellae. Each osteocyte is associated with other osteocytes through canaliculi. Usually, no blood vessels penetrate the trabeculae, so osteocytes must obtain nutrients through their canaliculi. The surfaces of trabeculae are covered with a single layer of cells consisting mostly of osteoblasts with a few osteoclasts.

What does spongy bone consist of?

Spongy bone consists of interconnecting rods or plates of bone called trabeculae.

Where is spongy bone loss greatest?

Spongy bone loss is greatest in the trabeculae that are under least stress. In other words, more sedentary individuals experience greater bone loss.

What does spongy bone have?

Spongy bone, which appears porous, has less bone matrix and more space than compact bone.

What can stem cells called osteochondral progenitor cells become?

Stem cells called osteochondral progenitor cells can become osteoblasts or chondroblasts.

How does bone adjust to mechanical stress? Describe the roles of osteoblasts and osteoclasts in this process. What happens to bone that is not subject to mechanical stress?

The amount of mechanical stress, such as when weightlifting, applied to a bone can modify the bone's strength. This modification occurs through several mechanisms including (1) remodeling, (2) the formation of additional bone, or (3) alteration in trabecular alignment to reinforce the scaffolding. Mechanical stress applied to bone increases osteoblast activity in bone tissue, and the removal of mechanical stress decreases osteoblast activity. Under conditions of reduced stress, as when a person is bedridden or paralyzed, osteoclast activity continues at a nearly normal rate but osteoblast activity decreases, resulting in less bone density. In addition, increased physical pressure in bone initiates electrical changes that stimulate osteoblast activity. Therefore, applying weight (pressure) to a broke bone can speed the healing process. Weak pulses of electric current are sometimes applied to a broken bone to speed healing.

What is the average life span of a BMU?

The average life span of a BMU is approximately 6 months, and BMU activity renews the entire skeleton very 10 years.

What can the body do to vitamin D?

The body can either synthesize or ingest vitamin D. Its rate of synthesis increases when the skin is exposed to sunlight.

What can cause vitamin D deficiency?

The body's inability to absorb lipids in which vitamin D is soluble can also result in vitamin D deficiency.

Why is the bone matrix of an older bone more brittle than a younger bone?

The bone matrix in an older bone is more brittle than a younger bone because decreased collagen production results in relatively more mineral and fewer collagen fibers. With aging, the amount matrix also decreases because the rate of matrix formation by osteoblasts becomes slower than the rate of matrix breakdown by osteoclasts.

When do fontanels close?

The bones eventually grow together, and all the fontanels have usually closed by 2 years of age.

What do the bones of an individual skeleton usually do?

The bones of an individual's skeleton usually reach a certain length, thickness, and shape through the processes described in the previous sections.

What is filled with marrow?

The cavities of spongy bone and the medullary cavity are filled with marrow.

What are red and yellow bone marrows? Where are they located in a child and in an adult?

The cavities of spongy bone and the medullary cavity are filled with marrow. Red marrow is the site of blood cell formation, and yellow marrow is mostly adipose tissue. In the fetus, the spaces within bones are filled with red marrow. The conversion of red marrow to yellow marrow begins just before birth and continues well into adulthood. Yellow marrow completely replaces the red marrow in the long bones of the limbs, except for some red marrow in the proximal part of the arm bones and thighbones. Elsewhere, varying proportions of yellow and red marrow are found. For example, part of the hipbone (ilium) may contain 50% red marrow and 50% yellow marrow. The hipbone is used as a source of donated red bone marrow because it is a large bone with more marrow than smaller bones and it can be accessed relatively easily.

What do the centers of ossification expand to form?

The centers of ossification expand to form a bone by gradually ossifying the membrane. Thus, the centers have the oldest bone, and the expanding spaces between the developing skull bones that have not yet been ossified.

What are the chondrocytes near the surface of articular cartilage similar to?

The chondrocytes near the surface of the articular cartilage are similar to those in the zone of resting cartilage of the epiphyseal plate.

What do the circumferential lamallae break down to form?

The circumferential lamellae break down during remodeling to form osteons.

What is collagen and mineral components of bones responsible for?

The collagen and mineral components are responsible for the major functional characteristics of bone.

When does the conversion of red marrow to yellow marrow begin?

The conversion of red marrow to yellow marrow begins just before birth and continues well into adulthood.

What is diaphysis?

The diaphysis, is the center portion of the bone.

What does the endosteum include?

The endosteum includes osteoblasts, osteoclasts, and osteochondral progenitor cells.

Where is the epiphyseal plate located?

The epiphyseal plate, or growth plate, is located between the epiphysis and the diaphysis.

What are epiphysis composed of?

The epiphyses are mostly spongy bone, with an outer layer of compact bone.

Where does the epiphysis develop from?

The epiphysis develops from different centers of ossification from the diaphysis.

FIGURE 6.19 Remodeling of a Long Bone

The epiphysis enlarges and the diaphysis increases in length as new cartilage forms and is replaced by bone during remodeling. The diameter of the bone increases as a result of bone growth on the outside of the bone, and the size of the medullary cavity increases because of bone reabsorption.

FIGURE 6.9 Long Bone

The femur (thighbone) serves as a model of the parts of a long bone. (a) The presence of epiphyseal plates, as well as the condition of the diaphysis and epiphyses, shows that this is a young bone. The femur is unusual in that it has two epiphyses at its proximal end. (b) Adult long bone with epiphyseal lines. (c) Internal features of a portion of the diaphysis in (a).

Some patients take ATP-powered H+ pump inhibitors, also known as proton pump inhibitors (PPIs), to control acid reflux. Predict the effect, if any, of long-term, high-dose use of PPIs on blood Ca2+ levels.

The first concept you're presented with in this question is that proton pumps can be artificially "turned off" by medications. As you've learned in previous chapters, certain chemicals, such as medicines, may affect transport proteins anywhere in the body. So, PPIs taken for stomach acid inhibition could also inhibit proton pumps in osteoclasts. Presumably, if osteoclasts were unable to create an acidic environment to dissolve bone, blood Ca2+ levels could decrease too much. In addition, lowering stomach acid production may lead to poor Ca2+ absorption and consequently, osteoporosis. This would be especially true if the patient took a high dose of a PPI for an extended time. In fact, the U.S. Food and Drug Administration has previously issued warnings about this very topic. However, there has not been overwhelming evidence either way, although there has been enough convincing evidence to warrant continued research and caution on the part of physicians. If a patient has other risk factors for osteoporosis, a PPI may not be the best course of treatment.

How do the first criterion for bone fracture fall under categories?

The first criterion for bone fracture type falls under the category of the mechanism by which the fracture occurred.

When does the formation of cartilage begin?

The formation of cartilage begins at approximately the end of the fourth week of embryonic development.

What is the formation of new bone called?

The formation of new bone by osteoblasts is called ossification, or osteogenesis.

Describe the formation of new bone by appositional growth. Name the spaces that are occupied by osteocyte cell bodies and cell extensions.

The formation of new bone by osteoblasts is called ossification, or osteogenesis. Ossification occurs by appositional growth on the surface of previously existing material, either bone or cartilage. For example, osteoblasts beneath the periosteum cover the surface of preexisting bone. Elongated cell extensions from osteoblasts connect to the cell extensions of other osteoblasts through gap junctions. Bone matrix produced by the osteoblasts cover the older bone surface and surrounds the osteoblast cell bodies and extensions. The result is a new layer of bone. Osteocyte cell bodies are housed within the bone matrix in spaces called lacunae. Osteocyte cell extensions are housed in narrow, long spaces called canaliculi. In a sense, the cells and their extensions form a "mold" around which the matrix is formed.

What is the functional unit of a long bone?

The functional unit of a long bone is an osteon, or Haversian system.

What do hydroxyapatite crystals act as?

The hydroxyapatite crystals act as templates that stimulate further hydroxyapatite formation and mineralization of the matrix.

What is the inner layer?

The inner layer is a single layer of bone cells, including osteoblasts, osteoclasts, and osteochondral progenitor cells.

What does the inner layer of the perichondrium contain?

The inner, more delicate layer has fewer fibers and contains chondroblasts.

What does the inorganic material of bone consist of?

The inorganic material consists primarily of a calcium phosphate crystal called hydroxyapatite, which is the molecular formula Ca10(PO4)6(OH)2.

What is the key signal for PTH secretion?

The key signal for PTH secretion is a reduction in blood Ca2+ levels.

What are the locations where ossification begins called?

The locations in the membrane where ossification begins are called centers of ossification.

Where are the periosteum and endosperm located, and what types of cells are found in each? What is the function of perforating (Sharpey) fibers?

The periosteum is a connective tissue membrane covering the outer surface of a bone. The outer fibrous layer is dense irregular collagenous connective tissue that contains blood vessels and nerves. The inner layer in a single layer of bone cells, including osteoblasts, osteoclasts, and osteochondral progenitor cells. where tendons and ligaments attach to bone, the collagen fibers of the tendon or ligament become continuous with those of the periosteum. In addition, some of the collagen fibers of the tendons or ligaments penetrate the periosteum into the outer part of the bone. These bundles of collagen fibers are called perforating fibers, or Sharpey fibers, and they strengthen the attachment of the tendons or ligaments to the bone.

Describe the formation of spongy and compact bone during intramembranous ossification. What are centers of ossification? What are fontanels?

The locations in the membrane where ossification begins are called centers of ossification. The centers of ossification expand to form a bone by gradually ossifying the membrane. Thus, the centers have the oldest bone, and the expanding edges the youngest bone. Fontanels, or soft spots, are the larger, membrane-covered spaces between the developing skull bones that have not yet been ossified. The bones eventually grow together, and all the fontanels have usually closed by 2 years of age. The steps in intramembranous ossification are as follows: 1. Osteoblast formation: Some embryonic mesenchymal cells in the connective tissue membrane differentiate into osteochondral progenitor cells. The osteochondral progenitor cells then form osteoblasts. The osteoblasts produce bone matrix. The bone matrix will surround the collagen fibers of the connective tissue membrane. Once they are embedded in bone matrix, the osteoblasts become osteocytes. As a result of this process, many tiny trabeculae of woven bone develop. 2. Spongy bone formation: Additional osteoblasts gather on the surfaces of the trabeculae and produce more bone, thereby causing the the trabeculae to become larger and longer. Spongy bone forms as the trabeculae join together, resulting in an interconnected network of trabeculae separated by spaces. 3. Compact bone formation: Cells within the spaces of the spongy bone specialize to form red bone marrow, and cells surrounding the developing bone specialize to form the periosteum. Osteoblasts from the periosteum lay down bone matrix to form an outer surface of compact bone.

What do long bones of a child sometimes exhibit?

The long bones of a child sometimes exhibit lines of arrested growth, which are transverse regions of greater bone density crossing an otherwise normal bone.

What does the space called lacuna within the matrix contain?

The matrix contains collagen, which provides strength, and proteoglycans, which make cartilage resilient by trapping water.

What happens in step 3 of bone growth in width?

The membrane lining the tunnel is called the endosteum. This endosteum had previously been the periosteum before the growth in bone width and formation of grooves along the outer surface of the bone occurred. Osteoblast from the endosteum lay down bone to form a concentric lamella.

What is the most effective preventive measure against the effects of aging?

The most effective preventive measure against the effects of aging on the skeletal system is the combination of increasing physical activity and taking dietary calcium and vitamin D supplements. Intensive exercise, especially weight-bearing exercise, can even reverse the loss of bone matrix.

How can a person be protected from the effects of aging on the skeletal system?

The most effective preventive measure against the effects of aging on the skeletal system is the combination of increasing physical activity and taking dietary calcium and vitamin D supplements. Intensive exercise, especially weight-bearing exercise, can even reverse the loss of bone matrix.

What effect does aging have on the quality and quantity of bone matrix?

The most siginificant age-related changes in the skeletal system affect the quality and quantity of bone matrix. Recall that a mineral (hydroxyapatite) in the bone matrix gives bone compression (weight-bearing) strength, but collagen fibers make the bone flexible. The bone matrix in an older bone is more brittle than a younger bone because decreased collagen production results in relatively more mineral and fewer collagen fibers. With aging, the amount of matrix also decreases because the rate of matrix formation by osteoblasts becomes slower than the rate of matrix breakdown by osteoclasts.

What do the most significant age-related changes in the skeletal system affect?

The most significant age-related changes in the skeletal system affect the quality and quantity of bone matrix. Recall that a mineral (hydroxyapatite) in the bone matrix gives compression (weight-bearing) strength, but collagen fibers make the bone flexible.

What is another way bone fractures are classified?

The next criterion by which bone fractures are classified is by the amount of soft-tissue damage.

What does the organic material of bone consist of?

The organic material consists primarily of collagen and proteoglycans.

What is the outer fibrous layer?

The outer fibrous layer is dense irregular collagenous connective tissue that contains blood vessels and nerves.

What is the perichondrium?

The perichondrium is a double-layered outer layer of dense irregular connective tissue containing fibroblasts.

What happens in step 2 of bone growth in width?

The periosteum covers the bone ridges and extends down into the bottom of the groves, and one or more blood vessels of the periosteum lie within each groove. As osteoblasts continue to produce bone, the ridges increase in size, extend toward each other, and meet to change the groove into a tunnel.

What is the periosteum?

The periosteum is a connective tissue membrane covering the outer surface of a bone.

What is the process of growth in articular cartilage similar too?

The process of growth in articular cartilage is similar to that occurring in the epiphyseal plate, except that the chondrocyte columns are not as obvious.

List the steps to achieve and maintain homeostasis to regulate calcium.

The processes include: (1) stimulation of skeletal muscle contraction, (2) stimulation and regulation of cardiac muscle contraction, and (3) exocytosis of cellular molecules, including those important for neural signaling.

What happens in step 4 of bone growth in width?

The production of additional lamellae fills in the tunnel, encloses the blood vessel, and produces an osteon.

FIGURE 6.7 Trabeculae Oriented Along Lines of Stress

The proximal end of a long bone (femur) showing trabeculae oriented along lines of stress (red lines). The trabeculae bear weight and help bones resist bending and twisting.

Nellie is a 12 year-old female who has an adrenal tumor that is producing a large amount of estrogen. If untreated, what effect will this condition have on her growth for approximately the next 6 months? How will her height have been affected by the time she is 18?

The question tells us that Nellie's blood levels of estrogen are much higher than normal for a 12-year-old girl. The text explained that important growth stimulators are reproductive hormones, which usually promote a burst of growth at puberty (approximately 12 years for girls). Because Nellie's estrogen levels are higher than normal, she will most likely grow at a faster rate than she normally would have over the next 6 months. However, if the estrogen levels are not lowered back to normal, Nellie will probably be shorter at 18 than expected. Recall that, in addition, to stimulating a burst of growth at puberty, estrogen causes a closure of the epiphyseal plate and growth in bone length stops. Also, estrogen is more effective at this than testosterone, so Nellie may stop growing years before she would have with normal estrogen levels.

List the components of the skeletal system.

The skeletal system has four components: bones, cartilage, tendons, and ligaments.

Name the four components of the skeletal system.

The skeletal system has four components: bones, cartilage, tendons, and ligaments.

What are the four components of the skeletal system?

The skeletal system has four components: bones, cartilage, tendons, and ligaments.

What other functions does the skeletal system include?

The skeleton is usually thought of as the framework of the body, but the skeletal system has many other functions as well, including the following: 1. Body support 2. Organ protection 3. Body movement 4. Mineral storage 5. Blood cell production.

For the process of endochondral ossification, describe the formation of the following structures: cartilage model, bone collar, calcified cartilage, primary ossification center, medullary cavity, secondary ossification center, epiphyseal plate, epiphyseal line, and articular cartilage.

The steps in endochondral ossification are as follows: 1. Cartilage model formation: Embryonic mesenchymal cells aggregate in regions of future bone formation. The mesenchymal cells differentiate into osteochondral progenitor cells that become chondroblasts. The chondroblasts produce a hyaline cartilage model having the approximate shape of the bone that will later be formed. As the chondroblasts are surrounded by cartilage matrix, they become chondrocytes. The cartilage model is surrounded by perichondrium, except where a joint will form connecting one bone to another bone. The perichondrium is continuous with tissue that will become the joint capsule later in development. 2. Bones collar formation: When blood vessels invade the perichondrium surrounding the cartilage model, osteochondral progenitor cells within the perichondrium become osteoblasts. Once the osteoblasts begin to produce bone, the perichondrium becomes the periosteum. The osteoblasts produce compact bone on the surface of the cartilage model, forming a bone collar. Two other events occur at the same time that the bone collar is forming. First, the cartilage model increases in size as a result of interstitial and appositional cartilage growth. Second, the chondrocytes in the center of the cartilage model absorb some of the cartilage matrix and hypertrophy, or enlarge. The chondrocytes also release matrix vesicles, which initiate the formation of hydroxyapatite crystals in the cartilage matrix. At this point, the cartilage is called calcified cartilage. The chondrocytes in this calcified area eventually die, leaving enlarged lacunae with thin walls of calcified matrix. 3. Primary ossification center formation: Blood vessels grow into the enlarged lacunae of the calcified cartilage. Osteoblasts and osteoclasts migrate into the calcified cartilage area from the periosteum by way of the connective tissue surrounding the outside of the blood vessels. The primary ossification center forms as osteoblasts produce bone on the surface of the calcified cartilage. The osteoblasts transform the calcified cartilage of the diaphysis into spongy bone. As one development proceeds, the cartilage model continues to grow, the bone collar thickens. Remodeling converts woven bone to lamellar bone and contributes to the final shape of the bone. Osteoclasts remove bone from the center of the diaphysis to form the medullary cavity, and cells within the medullary cavity specialize to form red bone marrow. 4. Secondary ossification center formation: Secondary ossification centers are created in the epiphyses by osteoblasts that migrate into the epiphysis. The events occurring at the secondary ossification centers are the same as those at the primary ossification centers, except that in spaces in the epiphyses do not enlarge to form a medullary cavity as in the diaphysis. Primary ossification centers appear during early fetal development whereas secondary ossification centers appear in the proximal epiphysis of the femur, humerus, and tibia about 1 month before birth. A baby is considered full-term if one of these three ossification centers can be seen on radiographs at the time of birth. At about 18-20 years of age, the last secondary ossification center appears in the medial epiphysis of the clavicle. Replacement of cartilage by bone continues in the cartilage model until all the cartilage, except that in the epiphyseal plate and on articular surfaces, has been replaced by bone. The epiphyseal plate, which exists during the time a person's bones are actively growing, and the articular cartilage, which is permanent structure, are derived from the original embryonic cartilage model. After a person's bones have stopped growing, the epiphyseal plate regresses into a "scar," called the epiphyseal line. 5. Adult bone: In mature bone, spongy and compact bone are fully developed, and the epiphyseal plate has become the epiphyseal line. The only cartilage present is the articular cartilage at the ends of the bone. All the original perichondrium that surrounded the cartilage model has become periosteum.

What are the steps of endochondral ossification?

The steps in endochondral ossification are as follows: 1. Cartilage model formation: Embryonic mesenchymal cells aggregate in regions of future bone formation. The mesenchymal cells differentiate into osteochondral progenitor cells that become chondroblasts. The chondroblasts produce a hyaline cartilage model having the approximate shape of the bone that will later be formed. As the chondroblasts are surrounded by cartilage matrix, they become chondrocytes. The cartilage model is surrounded by perichondrium, except where a joint will form connecting one bone to another bone. The perichondrium is continuous with tissue that will become the joint capsule later in development. 2. Bones collar formation: When blood vessels invade the perichondrium surrounding the cartilage model, osteochondral progenitor cells within the perichondrium become osteoblasts. Once the osteoblasts begin to produce bone, the perichondrium becomes the periosteum. The osteoblasts produce compact bone on the surface of the cartilage model, forming a bone collar. Two other events occur at the same time that the bone collar is forming. First, the cartilage model increases in size as a result of interstitial and appositional cartilage growth. Second, the chondrocytes in the center of the cartilage model absorb some of the cartilage matrix and hypertrophy, or enlarge. The chondrocytes also release matrix vesicles, which initiate the formation of hydroxyapatite crystals in the cartilage matrix. At this point, the cartilage is called calcified cartilage. The chondrocytes in this calcified area eventually die, leaving enlarged lacunae with thin walls of calcified matrix. 3. Primary ossification center formation: Blood vessels grow into the enlarged lacunae of the calcified cartilage. Osteoblasts and osteoclasts migrate into the calcified cartilage area from the periosteum by way of the connective tissue surrounding the outside of the blood vessels. The primary ossification center forms as osteoblasts produce bone on the surface of the calcified cartilage. The osteoblasts transform the calcified cartilage of the diaphysis into spongy bone. As one development proceeds, the cartilage model continues to grow, the bone collar thickens. Remodeling converts woven bone to lamellar bone and contributes to the final shape of the bone. Osteoclasts remove bone from the center of the diaphysis to form the medullary cavity, and cells within the medullary cavity specialize to form red bone marrow. 4. Secondary ossification center formation: Secondary ossification centers are created in the epiphyses by osteoblasts that migrate into the epiphysis. The events occurring at the secondary ossification centers are the same as those at the primary ossification centers, except that in spaces in the epiphyses do not enlarge to form a medullary cavity as in the diaphysis. Primary ossification centers appear during early fetal development whereas secondary ossification centers appear in the proximal epiphysis of the femur, humerus, and tibia about 1 month before birth. A baby is considered full-term if one of these three ossification centers can be seen on radiographs at the time of birth. At about 18-20 years of age, the last secondary ossification center appears in the medial epiphysis of the clavicle. Replacement of cartilage by bone continues in the cartilage model until all the cartilage, except that in the epiphyseal plate and on articular surfaces, has been replaced by bone. The epiphyseal plate, which exists during the time a person's bones are actively growing, and the articular cartilage, which is permanent structure, are derived from the original embryonic cartilage model. After a person's bones have stopped growing, the epiphyseal plate regresses into a "scar," called the epiphyseal line. 5. Adult bone: In mature bone, spongy and compact bone are fully developed, and the epiphyseal plate has become the epiphyseal line. The only cartilage present is the articular cartilage at the ends of the bone. All the original perichondrium that surrounded the cartilage model has become periosteum.

What are the steps in intramembranous ossification?

The steps in intramembranous ossification are as follows: 1. Osteoblast formation: Some embryonic mesenchymal cells in the connective tissue membrane differentiate into osteochondral progenitor cells in the connective tissue membrane differentiate into osteochondral progenitor cells. The osteochondral progenitor cells then form osteoblasts. The osteoblasts produce bone matrix. The bone matrix will surround the collagen fibers of the connective tissue membrane. Once they are embedded in bone matrix, the osteoblasts become osteocytes. As a result of this process, many tiny trabeculae of woven bone develop. 2. Spongy bone formation: Additional osteoblasts gather on the surfaces of the trabeculae and produce more bone, thereby causing the trabeculae to become larger and longer. Spongy bone forms as the trabeculae join together, resulting in an interconnected network of trabeculae separated by spaces. 3. Compact bone formation: Cells within the spaces of the spongy bone specialize to form red bone marrow, and cells surrounding the developing bone specialize to form the periosteum. Osteoblasts from the periosteum lay down bone matrix to form an outer surface of compact bone.

What are the three primary ways fractures can occur?

There are three primary ways fractures can occur: (1) through a trauma to the bone (traumatic), (2) through a pathology (disease) of the bone (pathologic), or (3) at a location of an implant on the bone (periprosthetic).

What are the three types of bone cells?

There are three types of bone cells-osteoblasts, osteocytes, and osteoclasts-each with different functions and a unique origin.

What are the three types of cartilage?

There are three types of cartilage: hyaline cartilage, fibrocartilage, and elastic cartilage.

What are the three types of cartilage? Which type is more closely associated with bone?

There are three types of cartilage: hyaline cartilage, fibrocartilage, and elastic cartilage. Although each type of cartilage can provide support, hyaline cartilage is most intimately associated with bone.

What are the two types of bone fracture by the amount of soft tissue damage?

There are two main types: (1) Closed (stable or simple): In a closed fracture, there is no visible damage to the skin at the injury site; (2) Open (compound): In an open fracture, there is visible damage to the skin at the trauma site, possibly including a fragment of the fracture bone protruding from the skin.

What are the two types of bone formed during ossification?

There are two types of bone formed during ossification: (1) woven bone and (2) lamellar bone.

What are the two types of cartilage growth?

There are two types of cartilage growth: (1) appositional growth and (2) interstitial growth.

Differentiate between appositional and interstitial growth of cartilage.

There are two types of cartilage growth: (1) appositional growth and (2) interstitial growth. In appositional growth, chondroblasts in the perichondrium add new cartilage to the outside edge of the existing cartilage. In interstitial growth, chondrocytes in the center of the tissue divide and add more matrix in between the existing cells.

Explain the types of cartilage growth.

There are two types of cartilage growth: (1) appositional growth and (2) interstitial growth. In appositional growth, chondroblasts in the perichondrium add new cartilage to the outside edge of the existing cartilage. In interstitial growth, chondrocytes in the center of the tissue divide and add more matrix in between the existing cells.

What do osteoblasts have?

These cells have an extensive endoplasmic reticulum and numerous ribosomes.

What do BMU do?

These cells remove old bone matrix and replace it with new bone matrix

What fractures occur mainly in children?

These include (1) greenstick fractures where only one side of the bone breaks and the opposite side just bonds (such as when trying to break a green branch from a tree), and (2) epiphyseal fractures where the epiphysis separates from the diaphysis within the epiphyseal plate. These can result in differing lengths of adult bones due to uneven growth of the two limbs.

What are the 5 fractures that are classified under fracture pattern?

These include: 1. Linear Fracture-runs parallel to the length of the bone 2. Spiral Fracture-results from twisting of one part of the bone 3. Avulsion Fracture-separation of a bone fragment from the rest of the bone 4. Stress (hairline) Fracture-incomplete fracture resulting from overuse of the bone 5. Compression Fracture-the bone collapses; common in spongy bone, often due to weakening of the bone such as in osteoporosis.

How is calcium move into and out of bone? What happens in the bone when blood calcium levels decrease? When blood calcium levels increase?

When blood Ca2+ levels are low, osteoclast activity increases and bone reabsorption occurs. Osteoclasts release more Ca2+ from bone into the blood than osteoblast remove from the blood to make new bone. Consequently, a net movement of Ca2+ occurs from bone into blood, and blood Ca2+ levels increase. Conversely, if blood Ca2+ levels are high, osteoclast activity decreases. Osteoclasts release less Ca2+ from bone into the blood than osteoblasts remove from the blood for bone deposition. As a result, net movement of Ca2+ occurs from the blood to bone, and blood Ca2+ levels decrease.

What happens when the epiphyses reach their full size?

When the epiphyses reach their full size, the growth of cartilage and its replacement by bone cease. The articular cartilage, however, persists throughout life and does not become ossified as the epiphyseal plate does.

What becomes continuous with those of the periosteum?

Where tendons and ligaments attach to bone, the collagen fibers of the tendon or ligament become continuous with those of the periosteum.

How is bone classified?

Whether bone is woven or lamellar, it can be classified according to the amount of bone matrix relative to the amount of space within the bone.

What is a debated issue about osteocytes?

Whether or not osteocytes freed from their surrounding bone matrix by reabsorption can revert to become active osteoblasts is a debated issue. As discussed in the previous section, osteoclasts are not derived from osteochondral progenitor cells but from stem cells in red bone marrow.

What is the end of long bone covered with?

Within joints, the end of a long bone is covered with hyaline cartilage called articular cartilage.

What is hindered in patients with vitamin C deficiency?

Wound healing, which requires collagen synthesis, is hindered in patients with vitamin C deficiency. In extreme cases, the teeth fall out because the ligaments that hold them in place break down.

What is woven bone?

Woven bone is the first type of bone that osteoblasts form during ossification.

How is the organization of collagen fibers different in woven and lamellar bone? What process produces woven bone?

Woven bone is the first type of bone that osteoblasts form during ossification. It is fairly weak bone because the collagen fibers are randomly oriented in many directions. Lamellar bone is mature bone. It is organized into thin, concentric sheets or layers approximately 3-7 micrometers thick called lamellae. In general, the collagen fibers of one lamella lie parallel to one another, but at an angle to the collagen fibers in the adjacent lamellae.

Does yellow marrow completely replace red marrow in all bones?

Yellow marrow completely replaces the red marrow in the long bones of the limbs, except for some red marrow in the proximal part of the arm bones and thighbones. Elsewhere, varying proportions of yellow and red marrow are found.

What is yellow marrow?

Yellow marrow is mostly adipose tissue.

Direct effects of PTH

a. Bone Cells PTH increases blood Ca2+ levels by exerting direct regulatory control of osteoblasts and osteocytes to increase formation an activation of osteoclasts, the principal bone-reabsorbing cells. Recall from section 6.3 that osteoblasts produce factors that induce development of osteoclasts. Specifically, osteoblast and osteocyte membranes express a cell-to-cell communication molecule called receptor activator of nuclear factor kappaB legion (RANKL). RANKL binds to its receptor, called receptor activator of nuclear factor kappa B (RANK), which is expressed in the plasma membrane of both undifferentiated and differentiated osteoclasts. This cell-to-cell binding allows RANKL to activate osteoclast precursor cells to differentiate into active osteoclasts. In addition, RANKL initiates already formed osteoclasts to become activated. Binding of PTH to its receptor in osteoblast and osteocyte cell membranes stimulate production of RANKL and thus an increased number of active osteoclasts. Further, PTH increases blood Ca2+ levels by preventing inhibition of osteoclast formation. A regulatory molecule, called osteoprotegerin, prevents osteoclast differentiation by preventing RNAKL binding to RANK. When PTH binds to its receptor on osteoblasts and osteocytes, it decreases their secretion of OPG. Lower OPG levels also allow for increased activate osteoclasts. Active osteoclast formation is thus regulated by the balance between levels of RANKL and OPG. It is interesting to note that PTH has been found to also promote one deposition. Although contradictory to PTH's known bone reabsorption function, giving patients PTH intermittently rather than continuously helps prevent osteoporosis (bone demineralization) from progressing. However, the exact mechanism for PTH stimulation of bone formation is yet unclear.

Describe how PTH controls the number of osteoclasts. What are the effects of PTH on the formation of calcitriol, Ca2+ uptake in the small intestine, and reabsorption of Ca2+ from the urine?

a. Bone Cells PTH increases blood Ca2+ levels by exerting direct regulatory control of osteoblasts and osteocytes to increase formation an activation of osteoclasts, the principal bone-reabsorbing cells. Recall from section 6.3 that osteoblasts produce factors that induce development of osteoclasts. Specifically, osteoblast and osteocyte membranes express a cell-to-cell communication molecule called receptor activator of nuclear factor kappaB legion (RANKL). RANKL binds to its receptor, called receptor activator of nuclear factor kappa B (RANK), which is expressed in the plasma membrane of both undifferentiated and differentiated osteoclasts. This cell-to-cell binding allows RANKL to activate osteoclast precursor cells to differentiate into active osteoclasts. In addition, RANKL initiates already formed osteoclasts to become activated. Binding of PTH to its receptor in osteoblast and osteocyte cell membranes stimulate production of RANKL and thus an increased number of active osteoclasts. Further, PTH increases blood Ca2+ levels by preventing inhibition of osteoclast formation. A regulatory molecule, called osteoprotegerin, prevents osteoclast differentiation by preventing RNAKL binding to RANK. When PTH binds to its receptor on osteoblasts and osteocytes, it decreases their secretion of OPG. Lower OPG levels also allow for increased activate osteoclasts. Active osteoclast formation is thus regulated by the balance between levels of RANKL and OPG. It is interesting to note that PTH has been found to also promote one deposition. Although contradictory to PTH's known bone reabsorption function, giving patients PTH intermittently rather than continuously helps prevent osteoporosis (bone demineralization) from progressing. However, the exact mechanism for PTH stimulation of bone formation is yet unclear. b. Kidney tubules PTH stimulates the reabsorption of Ca2+ from urine in the kidney tubules, which reduces the amount of Ca2+ excreted in the urine.

Use your knowledge of bone physiology and figure 6.22 to answer the following questions. A. Why is Henry more likely to break a bone than are most men his age? B. How have Henry's eating habits contributed to his low bone density? C. Would Henry's PTH levels be lower than normal, normal, or higher than normal? D. What effect has Henry's nocturnal lifestyle had on his bone density? E. How has lack of exercise affected his bone density?

a.Henry's bone density is less than normal for a man his age. Less dense bone is more likely to break. b.Henry's eating habits have resulted in insufficient dietary intake of Ca2+ and vitamin D. Therefore, the absorption of Ca2+ from his intestine into his blood has been inadequate. c.We might expect Henry's blood Ca2+ to be low because of his diet. However, low blood Ca2+levels stimulate increased PTH secretion. An increase in PTH maintains normal blood Ca2+ by increasing the number of osteoclasts, which break down bone and release Ca2+ into the blood. Thus, Henry's blood Ca2+ levels are maintained at the expense of his bones, which become less dense as more matrix than usual is broken down. Increased PTH levels also promote more Ca2+ reabsorption from the urine. d.Normally, exposure to sunlight activates a precursor molecule in the skin that eventually becomes activated vitamin D in the kidneys (see chapter 5). Henry produces few, if any, precursor molecules because of his nocturnal lifestyle. Therefore, Henry has low vitamin D levels and, so, has reduced absorption of Ca2+from his small intestine. e.Exercise is a major source of mechanical stress on bones, which increases osteoblast activity. Because Henry does not exercise, his osteoblasts have not been as active as they might have otherwise been, which has allowed the osteoclasts to dissolve his bone to a greater degree than normal. The overactivity of the osteoclasts partially accounts for Henry's lower bone density.

What type of cellular process is most likely the mechanism employed by osteoblasts to release bone matrix until they are surrounded by it?

gene expression