Chapter 6 & 8 anatomy test

Bone structure:Diaphysis,Epiphysis, And Metaphysis

Bone structure introduces the anatomy of the femur,the long bone of the thigh.This representative long bone has an extended tubular shaft,or diaphysis.At each end is an expanded area known as the epiphysis.the Diaphysis is connected to each Epiphysis at a narrow zone known as the metaphysis.The wall of Diaphysis consists of a layer of compact bone.

Bone tissue

Bone tissue is a supporting connective tissue.Like other connective tissue, it contains specialized cells and a matrix consisting of extracellular protein fibers and ground substance.

Surface Coverings of Bone

Bones have two kinds of coverings: periosteum on the outside and endosteum lining the inside. Let's look in more detail at each of these coverings.

Bone shape:flat

Flat bones have thin parallel surfaces which produce a flatten shape. They form the roof of the skull, the sternum (breastbone),the ribs, and the scapulae (shoulder blades). They provide protection for underlying soft tissues and offer an extensive surface area for the attachment of skeletal muscles.

Abnormal Bone Development:Gigantism

Gigantism results from an overproduction of growth hormone before puberty (Photo a). (The world record for height is 272 cm, or 8 ft, 11 in. It was reached by Robert Wadlow, of Alton, Illinois, who died at age 22 in 1940. Wadlow weighed 216 kg, or 475 lb.) If the growth hormone level rises abnormally after epiphyseal cartilages close, the skeleton does not grow longer. Instead, bones get thicker, especially in the face, jaw, and hands. Cartilage growth and alterations in soft-tissue structure lead to changes in physical features, such as the contours of the face. These physical changes take place in the disorder called acromegaly (ak-roh-MEG-ah-lē).

Bone shapes:Irregular

Irregular bones have complex shapes with short,flat, notched, or rigid surfaces.the vertebrae that form the spinal column,the bones of the pelvis,and several bones in the skull or are examples of irregular bones.

The skeletal systems major functions

Support,storage of minerals and lipid's,bone cell production,protection, and leverage

Bone Shapes:Sutural

Sutural bone or Wormian bones, are small,flat,irregularly shaped bones between the flat bones of the skull.there are individual variations in the number,shape,and position-of the sutural bones.They range in size from a grain of sand three-quarter their borders are like pieces of a jigsaw puzzle

Spongy bone

provides some support and stores marrow.In spongy bone, lamellae are not arranged in osteons. No osteons are present. The matrix in spongy bone forms a meshwork of supporting bundles of fibers called trabeculae (tra-BEK-ū-lē) These thin trabeculae branch, creating an open network. The trabeculae are oriented along stress lines and extensively cross-braced.Spongy bone is found where bones are not heavily stressed or where stresses originate from many directions. In addition, spongy bone is less dense than compact bone. Spongy bone reduces the weight of the skeleton, making it easier for muscles to move the bones. There are no capillaries or venules in the matrix of spongy bone Instead, spongy bone within the epiphyses of long bones, such as the femur, and the interior of other large bones such as the sternum and ilium (a large bone of the pelvis) contains red bone marrow

The Skeleton as a Calcium Reserve

A chemical analysis of bone reveals its importance as a mineral reserve (Figure 6-15). For the moment, we will focus on the homeostatic regulation of calcium ion concentration in body fluids. (We consider other minerals in later chapters.) Calcium is the most abundant mineral in the human body. The typical human body contains 1-2 kg (2.2-4.4 lb) of calcium, with nearly 99 percent of it deposited in the skeleton. By providing a calcium reserve, the skeleton plays the primary role in the homeostatic maintenance of a normal blood calcium ion concentration. This function can have a direct effect on the shape and strength of the bones in the skeleton. When large numbers of calcium ions are released from the bones, the bones become weaker. When calcium salts are deposited, the bones become denser and stronger.

Growth in Width: Appositional Growth

A superficial layer of bone, or bone collar, forms early in endochondral ossification. After that, the developing bone increases in diameter through appositional growth at the outer surface. In this process, cells of the inner layer of the periosteum differentiate into osteoblasts and deposit superficial layers of bone matrix. Eventually, these osteoblasts become surrounded by matrix and differentiate into osteocytes. Over much of the surface, appositional growth adds a series of layers that form circumferential lamellae. In time, the deepest circumferential lamellae are recycled and replaced by osteons typical of compact bone. However, blood vessels and collagen fibers of the periosteum can sometimes become enclosed within the matrix produced by osteoblasts. Osteons may then form around the smaller vessels. While bone matrix is being added to the outer surface of the growing bone, osteoclasts are removing bone matrix at the inner surface, but at a slower rate. As a result, the medullary cavity gradually enlarges as the bone gets larger in diameter.

Abnormal Bone Development

A variety of endocrine or metabolic problems can result in characteristic skeletal changes. In pituitary growth failure, inadequate production of growth hormone leads to reduced epiphyseal cartilage activity and abnormally short bones. This condition is becoming increasingly rare in the United States, because children can be treated with synthetic human growth hormone.

Compact Bone Structure & osteon

At the microscopic level, the basic functional unit of mature compact bone is the osteon (OS-tē-on), or Haversian (ha-VER-jhun) system. In an osteon, the osteocytes are arranged in concentric layers around a vascular central canal, or Haversian canal (Figure 6-6). This canal contains one or more blood vessels (normally a capillary and a venule, a very small vein) that carry blood to and from the osteon (Figure 6-7a). Central canals generally run parallel to the surface of the bone. Other passageways, known as perforating canals, or Volkmann's (FŌLK-manz) canals, extend perpendicular to the surface. Blood vessels in these canals supply blood both to osteons deeper in the bone and to tissues of the medullary cavity.

Bone cells

Bone contains four types of cells;osteogenic cells,osteoblasts,osteocytes, And osteoclasts.

Bone cells:Osteogenic cells pg.185

Bone contains small numbers of mesenchymal cells called osteogenic cells or osteoprogenitor cells.These stem cells divide two to produce daughter cells that differentiate into osteoblasts.Osteogenic cells maintain populations of osteoblasts and so are important in the repair of a fracture.

lamellae

Bone matrix forms layers called lamellae (lah-MEL-lē ; singular, lamella, a thin plate). The lamellae of each osteon form a series of nested cylinders around the central canal. In transverse section, these concentric lamellae create a targetlike pattern, with the central canal as the bull's-eye. Collagen fibers within each lamella form a spiral pattern that adds strength and resiliency

Blood and Nerve Supplies to Bone: Periosteal Vessels

Blood vessels from the periosteum provide blood to the superficial osteons of the shaft. During endochondral bone formation, branches of periosteal vessels also enter the epiphyses, providing blood to the secondary ossification centers.

Nutritional and Hormonal Effects on Bone:Calcitriol and Vitamin D3

Calcitriol and Vitamin D3. The hormone calcitriol (kal-si-TRĪ-ol), synthesized in the kidneys, is essential for normal calcium and phosphate ion absorption in the digestive tract. Calcitriol is synthesized from a related steroid, cholecalciferol (kō-lē-kal-SIF-er-ol; vitamin D3), which may be produced in the skin or absorbed from the diet

Hormones and Calcium Ion Balance

Calcium ions play a role in a variety of physiological processes, so the body must tightly control the calcium ion concentration in order to prevent damage to essential physiological systems. Even small variations from the normal concentration affect cellular operations, and larger changes can cause a clinical crisis. Calcium ions are particularly important to both the plasma membranes and the intracellular activities of neurons and muscle cells, especially cardiac muscle cells. If the calcium ion concentration of body fluids increases by 30 percent, neurons and muscle cells become unresponsive. If the calcium ion level decreases by 35 percent, neurons become so excitable that convulsions can occur. A 50 percent reduction in calcium ion concentration generally causes death. Calcium ion concentration is so closely regulated, however, that daily fluctuations of more than 10 percent are highly unusual.

calcium phosphate & Hydroxyapatite pigs.183&184

Calcium phosphate,Ca3 (PO4)2, makes up almost two-thirds of the weight of bone.calcium phosphate interacts with calcium hydroxide Ca (OH)2, to form crystals of Hydroxyapatite Ca10 (PO4)6(OH)2.As these crystals form they incorporate other calcium salts,such as calcium carbonate (CaCO3), and ions such as sodium,magnesium,and fluoride.A bone without a calcified matrix looks normal,but is very flexible.

Bone structure:compact bone and medullary cavity

Compact bone is a relatively dense and solid.compact forms a sturdy protective layer that surrounds a central space called the medullary cavity(marrow cavity)

parathyroid glands and parathyroid hormone (PTH);Decreasing the amount of calcium ions excreted by the kidneys

Decreasing the amount of calcium ions excreted by the kidneys. This causes more calcium to remain in the bloodstream.

fracture: Open and Closed

Despite its mineral strength, bone can crack or even break if it is subjected to extreme loads, sudden impacts, or stresses from unusual directions. The damage that results is called a fracture. Most fractures heal even after severe damage, if the blood supply and the cellular components of the endosteum and periosteum survive. Spotlight Figure 6-17 illustrates the different types of fractures and the repair process. Please study this figure carefully before reading the next section. Note that the two main types of fractures are open (or compound) and closed (or simple).

Endochondral Ossification

During development, most bones originate as hyaline cartilages that are miniature models of the corresponding bones of the adult skeleton. These cartilage models are gradually replaced by bone through the process of endochondral (en-dō-KON-drul) (chondros, cartilage) ossification. The steps in limb bone development provide a good example of this process (Spotlight Figure 6-11). Please study steps 1-4 of this figure thoroughly now. Note that this process includes the development of a primary ossification center inside the cartilage model.

Periosteum

Except at joints,Covers the outer surfaces of bones.It consists of an outer fibrous layer and an inner cellular layer.

The Periosteum

Except within joint cavities, the superficial layer of compact bone that covers all bones is wrapped by a periosteum (peri, around), a membrane with a fibrous outer layer and a cellular inner layer.The periosteum (1) isolates the bone from surrounding tissues, (2) provides a route for the blood vessels and nerves, and (3) takes part in bone growth and repair. Near joints, the periosteum becomes continuous with the connective tissues that hold the bones in place. At a joint that permits significant amounts of movement, called a synovial (si-NŌ-vē-ul) joint, the periosteum is continuous with the joint capsule. The fibers of the periosteum are also interwoven with those of the tendons attached to the bone. As the bone grows, these tendon fibers are cemented into the circumferential lamellae by osteoblasts from the cellular layer of the periosteum. Collagen fibers incorporated into bone tissue from tendons and ligaments, as well as from the superficial periosteum, are called perforating (Sharpey's) fibers.

Nutritional and Hormonal Effects on Bone; Growth Hormone and Thyroxine

Growth Hormone and Thyroxine. Growth hormone, produced by the pituitary gland, and thyroxine (thī-ROK-sē n), from the thyroid gland, stimulate bone growth. Growth hormone stimulates protein synthesis and the rates of cell division and cell growth throughout the body. Thyroxine stimulates cell metabolism and increases the rate of osteoblast activity. In proper balance, these hormones maintain normal activity at the epiphyseal cartilages until puberty.

Ossification occurs in two ways: endochondral and intramembranous

In endochondral ossification, bone replaces existing cartilage. Then bone growth occurs through interstitial growth (in length) and appositional growth (in width). In intramembranous ossification, bone develops directly from mesenchyme (loosely organized embryonic connective tissue) or fibrous connective tissue.

Blood and Nerve Supplies to Bone

In order for bones to grow and be maintained, they require an extensive blood supply. For this reason, osseous tissue is highly vascular. In a typical bone such as the humerus, three major sets of blood vessels develop: 1.)The Nutrient Artery and Vein 2.)Metaphyseal Vessels 3.)Periosteal Vessels Following the closure of the epiphyses, all three sets of vessels become extensively interconnected.

Skeletal system

Includes the bones of the skeletal and the Cartlidge,ligaments, and other connective tissue's that stabilize or interconnect the bones.

parathyroid glands and parathyroid hormone (PTH):Increasing the amount of calcium ions absorbed by the intestines by enhancing calcitriol secretion by the kidneys

Increasing the amount of calcium ions absorbed by the intestines by enhancing calcitriol secretion by the kidneys. Under normal circumstances, calcitriol is always present, and parathyroid hormone controls its effect on the intestinal epithelium.

Interstitial lamellae & Circumferential lamellae

Interstitial lamellae fill in the spaces between the osteons in compact bone. These lamellae are remnants of osteons whose matrix components have been almost completely recycled by osteoclasts. Circumferential lamellae (circum-, around + ferre, to bear) are found at the outer and inner surfaces of the bone, where they are covered by the periosteum and endosteum, respectively These lamellae are produced during the growth of the bone.

Intramembranous Ossification

Intramembranous (in-tra-MEM-bra-nus) ossification begins when osteoblasts differentiate within a mesenchymal or fibrous connective tissue. This type of ossification is also called dermal ossification because it normally takes place in the deeper layers of the dermis. The bones that result are called dermal bones. Examples of dermal bones are the flat bones of the skull, the mandible (lower jaw), and the clavicles (collarbones).

Bone shape:long

Long bones are relatively long and slender.consisting of a shaft with two ends that are wider than the shaft. They are located in the arm and forearm,thigh and leg,palms,soles,fingers,and toes. The femur the long bone of the thigh is the largest and heaviest bone in the body.

Collagen fibers

Make up about one-third of the weight of bone is collagen fibers.cells make up only 2% of the mass of a typical bone.

Skeletal system function:leverage

Many bones function as levers that can change the magnitude and direction of the force is generated by skeletal muscles. The movements produced range from the precise motion of a fingertip to changes in the position of the entire body.

The matrix of bone

Matrix of bone is very dense due to the deposits of calcium salts around the protein fibers.

Skeletal system function: storage of minerals and lipids

Minerals are in organic ions that contribute to the osmotic concentration of the body fluids. Minerals also take part in various physiological process,and several are important as enzyme cofactors. Calcium is the most abundant mineral in the human body the calcium salts of bone or valuable minerals reserve that maintains normal concentration of calcium and phosphate ions in body fluids in addition the bones of the skeleton store energy in the form of lipids and yellow bone marrow,An adipose tissue that is found in certain internal bone cavities.

Nutritional and Hormonal Effects on Bone:Minerals

Minerals. Normal bone growth and maintenance cannot take place without a constant dietary source of calcium and phosphorus. Lesser amounts of other minerals, such as magnesium, fluoride, iron, and manganese, are also required.

Canaliculi

Narrow passageways through the matrix,extend between the lacunae and nearby blood vessels, forming a branching network through which osteocytes exchange nutrients,wastes, and gasses.

Nutritional and Hormonal Effects on Bone

Normal bone growth and maintenance depend on a combination of nutritional and hormonal factors, such as the following: 1.)Minerals 2.)Calcitriol and Vitamin D3 3.)Vitamin C 4.)Vitamins A, K, and B12 5.)Growth Hormone and Thyroxine 6.)Sex Hormones 7.)Parathyroid Hormone and Calcitonin

Openings and depressions

Openings and depressions in bones are sites where blood vessels or nerves lie alongside or penetrate the bone.

Bone cells:Osteoblasts pg.186

Osteoblasts produce new bone matrix in a process called osteogenesis or ossification.Before calcium salts are deposited,this organic matrix is called osteiod.

widespread effects on aging bones;osteoclast-activating factor

Osteoporosis can also develop as a secondary effect of many cancers. Cancers of the bone marrow, breast, or other tissues release a chemical known as osteoclast-activating factor. It increases both the number and activity of osteoclasts and produces severe osteoporosis.

Nutritional and Hormonal Effects on Bone;Parathyroid Hormone and Calcitonin

Parathyroid Hormone and Calcitonin. Parathyroid hormone, from the parathyroid glands, has an important effect on bone, and to a much lesser extent, so does calcitonin (kal-si-TŌ-nin) from the C cells of the thyroid gland.

Projections

Projections form where muscles,tendons, and ligaments attach and where adjacent bones form joints.

Skeletal system function:Blood cell production

Red blood cells,white blood cells,and other blood almonds are produced in red bone marrow which fills the internal cavities of many bones.we describe blood cell formation when we examine the cardiovascular and lymphatic systems.

Bone shapes:Sesamoid

Sesamoid bones are usually small, round ,and flat. they are shaped somewhat like a sesame seed they tend to develop within tendons. except for the patellae (a small shallow dish) or kneecaps,there are individual variations in the location and number of sesamoid bones. These variations along with varying numbers of sutural bones,account for individual differences and the total number of bones in the skeleton. (Sesamoid bones may form in at least 26 locations)

Abnormal Bone Development: Marfan syndrome

Several inherited metabolic conditions that affect many systems influence the growth and development of the skeletal system. These conditions produce characteristic variations in body proportions. For example, many individuals with Marfan syndrome are very tall and have long, slender limbs (Photo b). The cause is excessive cartilage formation at the epiphyseal cartilages. The underlying mutation affects the structure of connective tissue throughout the body, and commonly causes cardiovascular events such as the sudden death of athletes during strenuous athletic contests.

Nutritional and Hormonal Effects on Bone;Sex Hormones

Sex Hormones. At puberty, rising levels of sex hormones (estrogen in females and testosterone in males) stimulate osteoblasts to produce bone faster than the rate at which epiphyseal cartilage expands, leading to epiphyseal closure. Differences in male and female sex hormones account for significant variations in body size and proportions. Because estrogen causes faster epiphyseal closure than does testosterone, women are generally shorter than men at maturity.

Bone shape:Short

Short bones are boxy with approximately equal dimensions. They are box like in appearance and examples of short bones include the carpal bones which is located in the wrist and the tarsal bones which is located in the ankles.

Skeletal system function:protection

Skeletal structures surround many soft tissues and organs.the ribs protect the heart and lungs, the skull encloses the brain,the vertebrae shield the spinal cord, and the pelvis cradles digestive and reproductive organs.

parathyroid glands and parathyroid hormone (PTH):Stimulating osteoclast activity (indirectly)

Stimulating osteoclast activity (indirectly). Osteoclasts do not have PTH receptors, but PTH binds to receptors on adjacent osteoblasts. This binding causes the osteoblasts to release an osteoclast differentiation factor called RANKL (receptor activator of nuclear factor-kB). RANKL activates receptors on pre-osteoclast cells, which in turn causes them to differentiate into mature osteoclasts. These mature osteoclasts release enzymes that promote bone resorption, thereby releasing calcium ions into the bloodstream.

Bone structure:spongey bone(trabecular bone)

The Epiphysis consists largely of spongy bone that consists of an open network of struts and plates that resembles a three-dimensional garden lattice.

Bone shapes

The adult skeleton System typically contains 206 major bones. We can divide these into the following six broad categories according to their individual shapes.Sutural bones,irregular bones,short bones,flat bones,long bones, and sesamoid bones.

Blood and Nerve Supplies to Bone: The Nutrient Artery and Vein

The blood vessels that supply the diaphysis form by invading the cartilage model as endochondral ossification begins. Most bones have only one nutrient artery and one nutrient vein, but a few bones, including the femur, have more than one of each. The vessels enter the bone through one or more round passageways called nutrient foramina in the diaphysis. Branches of these large vessels form smaller perforating canals and extend along the length of the shaft into the osteons of the surrounding compact bone.

widespread effects on aging bones;osteopenia

The bones of the skeleton become thinner and weaker as a normal part of the aging process. Inadequate ossification is called osteopenia (os-tē-ō-PĒ-nē-uh; penia, lacking). All of us become slightly osteopenic as we age. This reduction in bone mass begins between ages 30 and 40. At this age, osteoblast activity begins to decrease, while osteoclast activity continues at the previous level. Once the reduction begins, women lose about 8 percent of their skeletal mass every decade. Men lose less—about 3 percent per decade. Not all parts of the skeleton are equally affected. Epiphyses, vertebrae, and the jaws lose more mass than other sites, resulting in fragile limbs, reduction in height, and loss of teeth.

Growth in Length: Interstitial Growth

The completion of epiphyseal growth is called epiphyseal closure. The timing of epiphyseal closure differs from bone to bone and from individual to individual. The toes may complete ossification by age 11, but parts of the pelvis or the wrist may continue to enlarge until about age 25. The timing of endochondral ossification can be monitored by comparing the width of the epiphyseal cartilages in successive x-rays. In adults, the former location of this cartilage can often be seen in x-rays as a narrow epiphyseal line, which remains after epiphyseal closure

compact bone and spongy bone work together to support the body's weight

The distribution of forces applied to the femur at the hip joint, and illustrates the functional relationship between compact bone and spongy bone. The head of the femur articulates with a socket on the lateral surface of the pelvis. At the proximal epiphysis of the femur, trabeculae transfer forces from the pelvis to the compact bone of the femoral shaft, across the hip joint. At the distal epiphysis, trabeculae transfer weight from the shaft to the leg, across the knee joint. The femoral head projects medially, and the body weight compresses the medial side of the shaft. However, because the force is applied off center, the bone must also resist the tendency to bend into a lateral bow. So while the medial portion of the shaft is under compression, the lateral portion of the shaft, which resists this bending, is placed under a stretching load, or tension. The medullary cavity does not reduce the bone's strength because the center of the bone is not subjected to compression or tension.

The Endosteum

The endosteum (endo-, inside), an incomplete cellular layer, lines the medullary cavity. This layer is active during bone growth, repair, and remodeling. It covers the trabeculae of spongy bone and lines the inner surfaces of the central canals of compact bone. The endosteum consists of a simple flattened layer of osteogenic cells that covers the bone matrix, generally without any intervening connective tissue fibers. Where the cellular layer is not complete, the matrix is exposed. At these exposed sites, osteoclasts and osteoblasts can remove or deposit matrix components.

The framework of trabeculae

The framework of trabeculae supports and protects the cells of the bone marrow. Blood vessels within this tissue deliver nutrients to the osteocytes by diffusion along canaliculi that open onto the surfaces of trabeculae; they also remove wastes generated by the osteocytes. At other sites, spongy bone may contain yellow bone marrow—adipose tissue important as an energy reserve.

ossification,calcification, and osteogenesis

The growth of the skeleton determines the size and proportions of your body. In this section, we consider the physical processes of bone formation, or ossification, and bone growth. Ossification, or osteogenesis, refers specifically to the formation of bone. The process of calcification—the deposition of calcium salts—takes place during ossification, but it can also occur in other tissues. When calcification occurs in tissues other than bone, the result is a calcified tissue (such as calcified cartilage) that does not resemble bone.

Bone cell matrix pg.183

The matrix contains bone cells, or osteocytes within pockets called lacunae.The lacunae of bone are typically organized around blood vessels that branch through the bony matrix

Blood and Nerve Supplies to Bone: Metaphyseal Vessels

The metaphyseal (met-a-FIZ-ē-ul) vessels supply blood to the inner (diaphyseal) surface of each epiphyseal cartilage, where that cartilage is being replaced by bone.

The periosteum

The periosteum also contains a network of lymphatic vessels (lymphatics) and sensory nerves. The lymphatics collect lymph from branches that enter the bone and reach individual osteons by the perforating canals. The sensory nerves penetrate the compact bone with the nutrient artery to innervate the endosteum, medullary cavity, and epiphyses. Because of the rich sensory innervation, injuries to bones are usually very painful.

bone remodeling

The process of bone remodeling continuously recycles and renews the organic and mineral components of the bone matrix. Bone remodeling goes on throughout life, as part of normal bone maintenance. Remodeling can replace the matrix but leave the bone as a whole unchanged, or it may change the shape, internal architecture, or mineral content of the bone. Through remodeling, older mineral deposits are removed from bone and released into the circulation at the same time that circulating minerals are being absorbed and deposited. Bone remodeling involves an interplay among the activities of osteocytes, osteoblasts, and osteoclasts. In adults, osteocytes are continuously removing and replacing the surrounding calcium salts. Osteoclasts and osteoblasts also remain active even after the epiphyseal cartilages have closed. Osteoclasts are constantly removing matrix, and osteoblasts are always adding to it. Normally, their activities are balanced: As quickly as osteoblasts form one osteon, osteoclasts remove another. The homeostatic balance between the opposing activities of osteoclasts and osteoblasts is very important. When osteoclasts remove calcium salts faster than osteoblasts deposit them, bones weaken. When osteoblast activity predominates, bones become stronger and more massive. This opposition causes some interesting differences in bone structure among individuals. People who subject their bones to muscular stress through weight training or strenuous exercise develop not only stronger muscles, but also stronger bones. Alternatively, declining muscular activity due to immobility leads to reduced bone mass at sites of muscle attachment. The turnover rate of bone is quite high. In young adults, almost one-fifth of the skeleton is recycled and replaced each year. However, not every part of every bone is affected equally. The rate of turnover differs regionally and even locally. For example, the spongy bone in the head of the femur may be replaced two or three times each year, but the compact bone along the shaft remains largely unchanged. Because of their biochemical similarity to calcium, heavy-metal ions such as lead, strontium, cobalt, or radioactive uranium or plutonium can be incorporated into the matrix of bone. Osteoblasts do not differentiate between these heavy-metal ions and calcium. This means that any heavy metal ions present in the bloodstream will be deposited into the bone matrix. Some of these ions are potentially dangerous, and the turnover of bone matrix can have detrimental health effects as ions that are absorbed and accumulated are released into the circulation over a period of years. This was one of the major complications in the aftermath of the Ukrainian Chernobyl nuclear reactor incident in 1986. Radioactive compounds released in the meltdown of the reactor were deposited into the bones of exposed individuals. Over time, the radiation released by their own bones has caused thyroid cancers, leukemia (cancer of the blood cells, which starts in the red bone marrow), and other potentially fatal cancers.

Skeletal system function:Support

The skeletal system provide structural support for the entire body. individual bones or groups of bones provide a framework for the attachment of soft tissues and organs

Bone markings (surface features)

The surface of each bone in your body has characteristic bone markings.Examples include projections,openings, and depressions.

The Effects of Exercise on Bone

The turnover and recycling of minerals give each bone the ability to adapt to new stresses. The sensitivity of osteoblasts to electrical events has been theorized as the mechanism that controls the internal organization and structure of bone. Whenever a bone is stressed, the mineral crystals generate minute electrical fields. Osteoblasts are apparently attracted to these electrical fields and, once in the area, begin to produce bone. This finding has led to the successful use of small electric fields in stimulating bone healing. Because of their adaptability, bone shapes reflect the forces applied to them. For example, bumps and ridges on the surface of a bone mark the sites where tendons are attached. If muscles become more powerful, the corresponding bumps and ridges enlarge to withstand the increased forces. Heavily stressed bones become thicker and stronger, and bones that are not subjected to ordinary stresses become thin and brittle. For this reason, regular exercise is an important stimulus for maintaining normal bone structure. Champion weight lifters typically have massive bones with thick, prominent ridges. In nonathletes (especially "couch potatoes"), moderate amounts of physical activity and weight-bearing activities are essential for stimulating normal bone maintenance and maintaining adequate bone strength.

Two hormones with opposing effects maintain calcium ion homeostasis

These hormones, parathyroid hormone and calcitonin, coordinate the storage, absorption, and excretion of calcium ions. Three target sites and functions are involved: (1) bones (storage), (2) digestive tract (absorption), and (3) kidneys (excretion).

perforating (Sharpey's) fibers

This method of attachment bonds the tendons and ligaments into the general structure of the bone, providing a much stronger attachment than would otherwise be possible. An extremely powerful pull on a tendon or ligament will usually break a bone rather than snap the collagen fibers at the bone surface.

Nutritional and Hormonal Effects on Bone:Vitamin C

Vitamin C. An adequate level of vitamin C must be present in the diet. This vitamin, which is required for certain key enzymatic reactions in collagen synthesis, also stimulates osteoblast differentiation. One of the signs of vitamin C deficiency—a condition called scurvy—is a loss of bone mass and strength.

Nutritional and Hormonal Effects on Bone;Vitamins A, K, and B12

Vitamins A, K, and B12. Three other vitamins have significant effects on bone structure. Vitamin A, which stimulates osteoblast activity, is particularly important for normal bone growth in children. Vitamins K and B12 are required for the synthesis of proteins in normal bone.

widespread effects on aging bones;osteoporosis

When the reduction in bone mass is sufficient to compromise normal function, the condition is known as osteoporosis (os-tē-ō-po-RŌ-sis; porosus, porous). The brittle, fragile bones that result are likely to break when exposed to stresses that younger individuals could easily tolerate. For example, a hip fracture can occur when a 90-year-old person simply tries to stand. Any fractures in older individuals lead to a loss of independence and immobility that further weakens the skeleton. The extent of the loss of spongy bone mass due to osteoporosis is shown in Figure 6-18. The reduction in compact bone mass is equally severe.

Bone Structure:Diploe

Within the cranium the layer of spongy bone between the layers of compact bone is called the diploe.Red bone marrow is present within spongey bone ,but there is no large medullary cavity as in Diaphysis of a long bone.

yellow bone marrow

adipose tissue important as an energy reserve.

Bone Cells: Osteoclasts

are cells that absorb and remove bone matrix (Figure 6-5d). They are large cells with 50 or more nuclei. The osteoclasts generally occur in shallow depressions called osteoclastic crypts (Howship's lacunae) that they have eroded into the matrix. Osteoclasts secrete acids and protein-digesting enzymes that dissolve the matrix and release the stored minerals. During this process, fingerlike processes, called the ruffled border, increase the secretory surface area of the cell in contact with the surrounding matrix. This erosion process is called osteolysis (os-tē-OL-i-sis; osteo-, bone + lysis, a loosening), or resorption. The released products are resorbed at this border. Osteolysis is an important part of the regulation of calcium and phosphate ion concentrations in body fluids. Osteoclasts are not related to osteogenic cells or their descendants. Instead, they are derived from the same stem cells that produce monocytes and macrophages, which are cells involved in the body's defense mechanisms.

Bone cells:Osteocytes

are mature bone cells that make up most of the cell population. Each osteocyte occupies a lacuna, a pocket sandwiched between layers of matrix.Osteocytes cannot divide, and a lacuna never contains more than one osteocyte. Narrow passageways called canaliculi radiate through the matrix. Canaliculi contain cytoplasmic extensions of osteocytes, thus supporting cell-to-cell communication between osteocytes in different lacunae and access to nutrients supplied by blood vessels in the central canal. Neighboring osteocytes are linked by gap junctions, which permit the cells to rapidly exchange ions and small molecules, including nutrients and hormones. The interstitial fluid that surrounds the osteocytes and their extensions is an additional route for the diffusion of nutrients and wastes. Osteocytes have two major functions: Osteocytes maintain the protein and mineral content of the surrounding matrix. Matrix components are continually turned over. Osteocytes secrete chemicals that dissolve the adjacent matrix, and the minerals released enter the circulation. Osteocytes then rebuild the matrix, stimulating the deposition of new hydroxyapatite crystals. The turnover rate varies from bone to bone. We consider this process further in a later section. Osteocytes take part in the repair of damaged bone. If released from their lacunae, osteocytes can convert to a less specialized type of cell, such as an osteoblast or an osteogenic cell. These additional cells assist bone repair.

The bony skeleton by either endochondral or intramembranous ossification

begins to form about six weeks after fertilization, when the embryo is approximately 12 mm (0.5 in.) long. (At this stage, the existing skeletal elements are made of cartilage.) During fetal (beyond the eighth week) development, these cartilages are then replaced by bone, begins to form about six weeks after fertilization, when the embryo is approximately 12 mm (0.5 in.) long. (At this stage, the existing skeletal elements are made of cartilage.) During development after birth, the bones undergo a tremendous increase in size. Bone growth continues through adolescence, and portions of the skeleton generally do not stop growing until about age 25.

red bone marrow

forms blood cells.

parathyroid glands and parathyroid hormone (PTH)

indicates factors that increase the calcium ion level in the blood. When the calcium ion concentration in the blood falls below normal, cells of the parathyroid glands, embedded in the thyroid gland in the neck, release parathyroid hormone (PTH) into the bloodstream. Parathyroid hormone has the following three major effects, all of which increase the blood calcium ion level: 1.)Stimulating osteoclast activity (indirectly) 2.)Increasing the amount of calcium ions absorbed by the intestines by enhancing calcitriol secretion by the kidneys 3.)Decreasing the amount of calcium ions excreted by the kidneys

Compact bone functions

to protect, support, and resist stress