A&P Chapter 6 Skeletal System

What are the four steps of bone repair?

1. hematoma formation 2. callus formation 3. callus ossification 4. bone remodeling

What happens to bone that is not subject to mechanical stress?

It wont grow

What types of cells give rise to osteoclasts?

Osteoclasts are not derived from osteochondral progenitor cells but from stem cells in red bone marrow

What are trabeculae, and what is their function?

The trabeculae bear weight and help bones resist bending and twisting. Between the trabeculae are spaces, which in life are filled with bone marrow and blood vessels.

Name the four components of the skeletal system.

bones, cartilage, tendons, and ligaments

What is a lacuna?

space surrounding an osteocyte

What is the function of perforating (Sharpey) fibers?

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.

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

LONG- FLAT - SHORT- IRREGULAR Individual bones are classified according to shape: long, flat, short, or irregular . Long bones are longer than they are wide. Most of the bones of the upper and lower limbs are long bones. Flat bones have a relatively thin, flattened shape and are usually curved. Examples of flat bones include certain skull bones, the ribs, the breastbone (sternum), and the shoulder blades (scapulae). Short bones are round or nearly cube-shaped, as exemplified by the bones of the wrist (carpal bones) and ankle (tarsal bones). Irregular bones, such as the vertebrae and facial bones, have shapes that do not readily fit into the other three categories.

What is the perichondrium?

Most cartilage is covered by a protective connective tissue sheath called the perichondrium (per-i-kon′drē-ŭm). 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.

What kinds of cells are derived from osteochondral progenitor cells?

Osteochondral progenitor cells are stem cells that 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 or chondroblasts.

Explain how the epiphyseal plate remains the same thickness while the bone increases in length.

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 cartilage replacement by bone on the diaphyseal side of the plate.

Describe the formation of spongy and compact bone during intramembranous ossification.

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. Intramembranous ossification of the membrane begins at approximately the eighth week of embryonic development and is completed by approximately 2 years of age. Many skull bones, part of the mandible (lower jaw), and the diaphyses of the clavicles (collarbones) develop by intramembranous ossification

What cells give rise to osteochondral progenitor cells?

Connective tissue develops embryologically from mesenchymal cells. Some of the mesenchymal cells become stem cells, some of which replicate and become more specialized cell types. Osteochondral progenitor cells are stem cells that can become osteoblasts or chondroblasts.

Where are the periosteum and endosteum located, and what types of cells are found in each?

The periosteum (per-ē-os′tē-ŭm) 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 is a single layer of bone cells, including osteoblasts, osteoclasts, and osteochondral progenitor cells -- The endosteum (en-dos′tē-ŭm) is a single cell layer of connective tissue that lines the internal surfaces of all cavities within bones, such as the medullary cavity of the diaphysis and the smaller cavities in spongy and compact bone. The endosteum includes osteoblasts, osteoclasts, and osteochondral progenitor cells.

Explain the process of growth at the articular cartilage.

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 page 179 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.

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

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

Why is remodeling of the ossified callus necessary?

The result is a stronger external callus - If the fracture has occurred in the diaphysis of a long bone, remodeling also restores the medullary cavity.

Name the spaces that are occupied by osteocyte cell bodies and cell extensions.

The spaces without matrix but occupied by the osteocyte cell bodies are called lacunae , and the spaces occupied by the osteocyte cell extensions are called canaliculi

What are two mechanical advantages of a long bone?

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. As a long bone increases in 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.

Describe the roles of osteoblasts and osteoclasts in this process.

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.

Describe the structure of hyaline cartilage.

Hyaline cartilage chondroblasts produce a matrix surrounding themselves. By the time the matrix has surrounded a chondroblast, the cell has differentiated into a chondrocyte (kon′drō-sīt), a rounded cell that occupies a space called a lacuna (lă-koo′nă) within the matrix. The matrix contains collagen, which provides strength, and proteoglycans, which make cartilage resilient by trapping water. ---Articular (ar-tik′ū-lăr) 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.

What happens in bone when blood calcium levels decrease?

When blood Ca2+ levels are too low, osteoclast activity increases. Osteoclasts release more Ca2+ from bone into the blood than osteoblasts remove from the blood to make new bone. ---

What happens to the epiphyseal plate and the articular cartilage when bone growth ceases?

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.

Name and describe the events occurring in the four zones of the epiphyseal plate.

--zone of resting cartilage is nearest the epiphysis and contains slowly dividing chondrocytes --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. --zone of hypertrophy, the chondrocytes produced in the zone of proliferation mature and enlarge. Thus, a maturation gradient exists 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. --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.

Sketch and label the parts of a typical long bone.

A long bone is the traditional model for overall bone structure. The diaphysis (dī-af′i-sis), or shaft, is composed primarily of compact bone, but it can also contain some spongy bone. The end of a long bone is 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.

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

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. --The external callus forms a collar around the opposing ends of the bone fragments. The external callus is a bone-cartilage collar that stabilizes the ends of the broken bone

Describe the structure of spongy bone.

Appears porous; consists of thin rods or plates called trabeculae.

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 (hī-drok′sē-ap-ă-tīt), which has the molecular formula Ca10(PO4)6(OH)2. ---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.

Describe the structure of compact bone.

Compact bone 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.

How do osteocytes within trabeculae obtain nutrients?

Each osteocyte is associated with other osteocytes through canaliculi. Usually, no blood vessels penetrate the trabeculae, so osteocytes must obtain nutrients through their canaliculi. (canals)

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

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. Some of the flat and irregular bones of the skull have air-filled spaces called sinuses), which are lined by mucous membranes.

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

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 -- 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

Name the two types of cartilage cells.

Immature cell names often end with the suffix -blast, which means "seed" or "bud." Once the immature cell has grown and differentiated into its mature form, the suffix -cyte replaces -blast in the name. Hyaline cartilage chondroblasts (kon′drō-blasts; cartilage) produce a matrix surrounding themselves). By the time the matrix has surrounded a chondroblast, the cell has differentiated into a chondrocyte (kon′drō-sīt), a rounded cell that occupies a space called a lacuna (lă-koo′nă) within the matrix.

Describe the formation of new bone by appositional growth.

In appositional growth, chondroblasts in the perichondrium add new cartilage to the outside edge of the existing cartilage. The chondroblasts lay down new matrix and add new chondrocytes to the outside of the tissue.

Differentiate between appositional and interstitial growth of cartilage.

In appositional growth, chondroblasts in the perichondrium add new cartilage to the outside edge of the existing cartilage. The chondroblasts lay down new matrix and add new chondrocytes to the outside of the tissue. ----In interstitial growth, chondrocytes in the center of the tissue divide and add more matrix in-between the existing cells

Explain interstitial growth.

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

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

In long bones, the diaphysis is the primary ossification center, and additional sites of ossification, called secondary ossification centers, are created in the epiphyses by osteoblasts that migrate into the epiphysis

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

In woven bone, the collagen fibers are randomly oriented in many directions. --Lamellar bone is mature bone that is organized into thin, concentric sheets or layers approximately 3-7 micrometers (μm) thick called lamellae

How does bone adjust to mechanical stress?

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, pressure in bone causes an electrical change that increases the activity of osteoblasts; therefore, applying weight (pressure) to a broken bone can speed the healing process.

How is calcium moved into and out of bone?

OSTEOBLASTS & OSTEOCLASTS: Bone is the major storage site for calcium in the body, and movement of Ca2+ into and out of bone helps determine blood Ca2+ levels. Calcium ions move into bone as osteoblasts build new bone, and they move out of bone as osteoclasts break down bone --When osteoblast and osteoclast activities are balanced, the movements of Ca2+ into and out of a bone are equal.

Describe how PTH controls the number of osteoclasts.

Osteoclast numbers are regulated by the interactions of osteoblasts and red bone marrow stem cells of the monocyte/macrophage lineage. Osteoblasts and stem cells have receptors for PTH. When PTH binds to these receptors, osteoblasts respond by producing receptor activator of nuclear factor kappaB ligand (RANKL). RANKL is expressed on the surface of the osteoblasts and combines with receptor activator of nuclear factor kappaB (RANK) found on the cell surfaces of osteoclast precursor stem cells. In a cell-to-cell interaction, RANKL on osteoblasts binds to RANK on osteoclast precursor stem cells, stimulating them to become osteoclasts.

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

Parathyroid hormone (PTH) is the major regulator of blood Ca2+ levels.

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

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.

Explain the role of red and yellow marrow. Where are they located in a child and in an adult?

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.

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 influence bone growth. Estrogen (a class of female reproductive hormones) and testosterone (a male reproductive hormone) initially stimulate bone growth, which accounts for the burst of growth at puberty when production of these hormones increases. However, both classes of reproductive hormones also stimulate ossification of epiphyseal plates, and thus the cessation of growth. Females usually stop growing earlier than males because estrogens cause quicker closure of the epiphyseal plate than testosterone does. Because their entire growth period is somewhat shorter, females usually do not reach the same height as males.

Describe the five major functions of the skeletal system.

SUPPORT - PROTECTION - MOVEMENT - STORAGE -PRODCUTION 1. Body support. 2. Organ protection 3. Body movement 4. Mineral storage 5. Blood cell production

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

How does breaking a bone result in hematoma formation?

When a bone is fractured, the blood vessels in the bone and surrounding periosteum are damaged and a hematoma forms. A hematoma (hē-mă-tō′mă) is a localized mass of blood released from blood vessels but confined within an organ or a space. Usually, the blood in a hematoma forms a clot, which consists of fibrous proteins that stop the bleeding.

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

basic multicellular unit (BMU), a temporary assembly of osteoclasts and osteoblasts that travels through or across the surface of bone, removing old bone matrix and replacing 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 (see figure 6.7b) are remnants of osteons that were not completely removed when a BMU formed a tunnel. Blood vessels grow into the tunnel, and osteoblasts of the BMU move in and lay down a layer of bone on 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 trabecula, forming a cavity, which the BMU then fills in with new bone matrix.

Why is it important for bone remodeling to occur?

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.

What process produces woven bone?

he first type of bone that osteoblasts form during ossification is fairly weak bone called woven bone.

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

hyaline cartilage, fibrocartilage, and elastic cartilage --hyaline cartilage is most intimately associated with bone. 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 happens in bone when blood calcium levels increase?

if blood Ca2+ levels are too high, osteoclast activity decreases. Osteoclasts release fewer Ca2+ from bone into the blood than osteoblasts remove from the blood to produce new bone. As a result, a net movement of Ca2+ occurs from the blood to bone, and blood Ca2+ levels decrease.

What is an osteon?

structural unit of compact bone The functional unit of a long bone is an osteon (os′tē-on), or haversian (ha-ver′shan) 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, and 4-20 concentric lamellae form the rings. V

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

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-These lines are caused by greater calcification below the epiphyseal plate of a bone, where it has grown at a slower rate during an illness or severe nutritional deprivation --Vitamin D is necessary for the normal absorption of calcium from the intestines --Vitamin C is necessary for collagen synthesis by osteoblasts

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

--Osteoblasts (os′tē-ō-blastz), bone-forming cells, have an extensive endoplasmic reticulum and numerous ribosomes. They produce collagen and proteoglycans, which are packaged into vesicles by the Golgi apparatus and released from the cell by exocytosis. Osteoblasts also release matrix vesicles, which contain high concentrations of Ca2+ and PO43− --Once an osteoblast has secreted enough bone matrix to become surrounded by it the cell is then referred to as an osteocyte. The osteocytes retain their connections to neighboring osteocytes through their cell extensions. Osteocytes become relatively inactive, compared with most osteoblasts, but it is possible for them to produce the components needed to maintain the bone matrix. -- Osteoclasts (os′tē-ō-klastz) are bone-destroying cells (figure 6.4). These cells perform reabsorption, or breakdown, of bone that mobilizes crucial Ca2+ and phosphate ions for use in many metabolic processes. Note we use the term reabsorption here because our point of reference is the bloodstream. As bone is broken down, the Ca2+ goes "back" into the blood. These cells are massive, multinucleated 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 osteosclasts.