A&P BIO 141: Bones And Skeletal Tissues

Skeletal cartilage Is made of some variety of cartilage tissue, which consists primarily of water. The high water content of cartilage accounts for its resilience, that is, its ability to spring back to its original shape after being compressed. The perichondrium The perichondrium acts like a girdle to resist outward expansion when the cartilage is compressed. Additionally, the perichondrium contains the blood vessels from which nutrients diffuse through the matrix to reach the cartilage cells. This mode of nutrient delivery limits cartilage thickness. Three types of cartilage tissue in the body: hyaline elastic fibrocartilage All three types have the same basic components - cells called chondrocytes, encased in small cavities (lacunae) within an extracellular matrix containing a jellylike ground substance and fibers. The skeletal cartilages contain representatives from all three types. Hyaline cartilages Which look like frosted glass when freshly exposed, provide support when flexibility and resilience. They are they most abundant skeletal cartilages. 1. articular cartilages, which cover the ends of most bones at movable joints; 2. costal cartilages, which connect the ribs to the sternum (breastbone); 3. respiratory cartilages, which form the skeleton of the larynx (voicebox) and reinforce other respiratory passageways; and 4. nasal cartilages, which support the external nose. Elastic cartilages Look very much like hyaline cartilages, but they contain more stretchy elastic fibers and so are better able to stand up to repeated bending. They are found in only two skeletal locations, the external ear and the epiglottis (the flap that bends to cover the opening of the larynx each time we swallow). Fibrocartilages Are highly compressible and have great tensile strength. The perfect intermediate between hyaline and elastic cartilages, fibrocartilages consis of roughly parallel rows of chondrocytes alternating with thick collagen fibers. Fibrocartilages occur in sites that are subjected to both heavy pressure and stretch, such as the padlike cartilages (menisci) of the knee and the discs between vertebrae. Appositional growth In appositional growth, cartilage-forming cells in the surrounding perichondrium secrete new matrix against the external face of the existing cartilage tissue. Interstitial growth In interstitial growth, the lacunae-bound chondrocytes divide and secrete new matrix, expanding the cartilage from within. Typicall, cartilage growth ends during adolescence when the skeleton stops growing. Axial skeleton the axial skeleton forms the long axis of the body and includes the bones of the skull, vertebral column, and rib cage. Generally speaking these bones are most involved in protecting, supporting, or carrying other body parts. Appendicular skeleton The appendicular skeleton consists of the bones of the upper and lower limbs and the girdles (shoulder bones and hip bones) that attach the limbs to the axial skeleton. Bones of the limbs help us to get from pace to place (locomotion) and to manipulate our enviroment. Long bones As their name suggests, are considerably longer than they are wide. A long bone has a shaft plus two ends. All limb bones except the patella (kneecap) and the wrist and ankle bones are long bones. Notice they are named for their shape, not their size. The three bones in each of your fingers are long bones, even though they are very small. Short bones Are roughly cube shaped. The bones of the wrist and ankle are examples. Sesamoid bones Are a special type of short bone that form in a tendon. They vary in size and number in different individuals. some sesamoid bones clearly act to alter the direction of pull of a tendon. The function of others is not known. Flat bones Are thin, flattened, and usually a bit curved. The sternum (breastbone), scapulae (shoulder blades), ribs, and most skull bones are flat bones. Irregular bones Have complicated shapes that fit none of the preceding classes. Examples include the vertebrae and the hip bones. Functions of Bones Support Bones provide a framework that supports the body and cradles its soft organs. For example, bones of lower limbs act as pillars to support the body trunk when we stand, and the rib cage supports the thoracic wall. Functions of Bones Protection The fused bones of the skull protect the brain. The vertebrae surround the spinal cord, and the rib cage helps protect the vital organs of the thorax. Functions of Bones Movement Skeletal muscles, which attach to bones by tendons, use bones as levers to move the body and its parts. As a result, we can walk, grasp objects, and breathe. The design of joints determines the types of movement possible. Functions of Bones Mineral and growth factor storage Bone is a reservoir for minerals, most importantly calcium and phosphate. The stored minerals are released into the bloodstream as needed for distribution to all parts of the body. Additionally, mineralized bone matrix stores important growth factor, bone morphogenic proteins, and others. Functions of Bones Blood cell formation Most blood cell formation, or hematopoiesis, occurs in the marror cavities of certain bones. Functions of Bones Triglyceride (fat) storage Fat is stored in bone cavities and represents a source of stored energy for the body. Bone markings The external surfaces of bones are rarely smooth and featureless. Instead, they display projections, depressions, and openings that serve as sites of muscle, ligament, and tendon attachment, as joint surfaces, or as conduits for blood vessels and nerves. Compact bone Every bone has a dense outer layer that looks smooth and solid to the naked eye. This external layer is compact bone. Spongey bone Internal to the compact bone is the spongy bone (cancellous bone), a honeycomb of small needle-like or flat pieces called trabeculae ("little beams"). In living bones the open spaces between trabeculae are filled with red or yellow bone marrow. Structure of a Typical long Bone Diaphysis A tubular diaphysis, or shaft, forms the long axis of the bone. It is constructed of a relatively thick collar of compact bone that surroungs a central medullary cavity, or marrow cavity. in adults, the medullary cavity contains fat and is called the yellow marrow cavity. Structure of a Typical long Bone Epiphyses The epiphyses are the bone ends. In many cases, they are more expanded than the diaphysis. Compact bone forms the exterior of epiphyses, and their interior contains spongy bone. The joint surface of each epiphysis is covered with a thin layer of articular (hyaline) cartilage, which cushions the opposing bone ends during joint movement and absorbs stress. Between the diaphysis and each epiphysis of an adult long bone is an epiphyseal line, a remnant of the epiphyseal plate, a disc of hyaline cartilage that grows during childhood to lengthen the bone. The region where the diaphysis and epiphysis meet, whether it is the epiphyseal plate or line, is sometimes called the metaphysis. Structure of a Typical long Bone Membranes A third structural feature of long bones is membranes. The external surface of the entire bone except the joint surfaced is coverd by a glistening white, double0layered membrane called the periosteum. The outer fibrous layer is dense irregular connective tissue. The inner osteogenic layer, abutting the bone surface, consists primarily of bone-forming cells, called osteoblasts, which secrete bone matrix elements, and bone-destroying cells, called osteoclases. In addition there are primitive stem cells, osteogenic cells, that give rise to the osteoblasts. Bone markings Tuberosity Large rounded projection; may be roughened Bone markings Crest Narrow ridge of bone usually prominent. Bone markings Trochanter Very large, blunt, irregularly shaped process. Bone markings Line Narrow ridge of bone; less prominent than a crest. Bone markings Tubercle Small rounded projection or process. Bone markings Epicondyle Raised area on or above a condyle. Bone markings Spine Sharp, slender, often pointed projection. Bone markings Process Any bony prominence. Bone markings Head bony expansion carried on a narrow neck. Bone markings Facet Smoth, nearly flat articular surface. Bone markings Condyle Rounded articular projection. Bone markings Ramus Armlike bar of bone. Bone markings Groove Furrow Bone markings Fissure Narrow, slitlike opening. Bone markings Foramen Round or oval opening through a bone. Bone markings Notch Indentation at the edge of a structure. Bone markings Meatus Canal-like passageway. sinus Cavity within a bone, filled with air and lined with mucous membrane. Bone markings Fossa shallow, basinlike depression in a bone, often serving as an articular surface. Nutrient foramina The periosteum is richly supplied with nerve fibers, lymphatic vessels, and blood vessels, which enter the diaphysis via nutriend foramina. Perforating (Sharpey's) fibers the periosteum is secured to the underlying bone by perforating (Sharpey's) fibers, tufts of collagen fibers that extend from its fibrous layer into the bone matrix. Endosteum Internal bone surfaces are covered with a delicate connective tissue membrane called the endosteum. The endosteum covers the trabeculae of spongy bone and lines the canals that pass through the compact bone. Like the periosteum, the endosteum contains both bone-forming and bone-destroying cells. Diploe in flat bones, the spongy bone is called the diploe and the whole arrangement resembles a stiffened sandwich. Red marrow hematopoietic tissue, red marrow, is typically found within the trabecular cavities of spongy bone of long bones and in the diploe of flat bones. For this reason, both these cavities are often referred to as red marrow cavities. Osteoblasts Essentially, four major cell types populate bone tissue: osteogenic cells, osteoblasts, osteocytes, and osteoclasts. These, like other connective tissue cells, are surrounded by an extracellular matrix of their making. The osteogenic cells, also called osteoprogenitor cells, are mitotically active stem cells found in the membranous periosteum and endosteum. Some of their progeny differtiate into osteoblasts (bone-forming cells) while others persist as bone stem cells to provide osteoblasts in the future. Osteon The structural unit of compact bone is called either the osteon or the Haversian system. Each osteon is an elongated cylinder oriented parallel to the long axis of the bone. Functionally,osteons are tiny weight-bearing pillars. lamella An osteon is a group of hollow tubes of bone matrix, one placed outside the next like the growth rings of a tree trunk. Each matrix tube is a lamella, and for this reason compact bone is often called lamellar bone. Although all of the collagen fibers in a particular lamella run in a single direction, the collagen fibers in adjacent lamellae always run in different directions. this alternating pattern is beautifully designed to withstand torsion stresses - the adjacent lamellae reinforce one another to resist twisting. You can think of the osteon's design as a "twister resister." Central canal Running through the core of each osteon is the central canal, or Haversian canal, containing small blood vessels and nerve fibers that serve the needs of the osteon's cells. Perforating canals Canals of a second type called perforating canals, or Volkmann's canals, lie at right angles to the long axis of the bone and connect the bone and nerve supply of the periosteum to those in the central canals and the medullary cavity. Like all other internal bone cavities, these canals are lined with endosteum. Osteocytes Spider-shaped osteocytes occupy lacunae at the junctions of the lamellae. One function of osteocytes is to maintain the bone matrix. If they die, the surrounding matrix is resorbed. the osteocyted also act as stress or strain "sensors" in cases of bone deformation or other damaging stimuli. They communicate this information to the cells responsible for bone remodeling (osteoblasts and osteoclasts) so that sountermeasures can be taken or repairs made. Cannaliculi Hairlike canals called canaliculi connect the lacunae to each other and to the central canal. The canaliculi tie all the osteocytes in an osteon together, permitting nutrients and wastes to be relayed from one osteocyte to the next throughout the osteon. Although bone matrix is hard and impermeable to nutrients, its canaliculi and cell-to-cell relays (via gap junctions) allow bone cells to be well nourished. Interstitial lamellae Not all lamellae in compact bone are part of osteons. Lying between intact osteons are incomplete lamellae called interstitial lamellae. They either fill the gaps between forming osteons or are remnants of osteons that have been cut through by bone remodeling. Circumferential lamellae Located just deep to the periosteum and just superficial to the endosteum, extend around the entire circumference of the diaphysis and effectively resist twisting of the long bone. Osteoid Bone has bone organic and inorganic components. Its organic components include the cells (osteogenic cells, osteoblasts, osteocytes, and osteoclasts) and osteoid, the organic part of the matrix. Osteoid, which makes up approximately one-third of the matrix, includes ground substance (composed of proteoglycans and glycoproteins) and collagen fibers, both of which are made and secreted by osteoblasts. These organic substances, particularly collagen, contribute not only to a bone's structure but also to the flexibility and great tensile strength that allow the bone to resist stretch and twisting. Sacrificial bonds Bone's exceptional toughess and tensile strength has been the subject of intense research. It now appears that this resilience comes from the presence of sacrificial bonds in or between collagen molecules. These bonds break easily on impact, dissipating energy to prevent the force from rising to a fracture value. In the absence of continued or additional trauma, most of the sacrificial bonds re-form. hydroxyapatites The balance of bone tissue consists of inorganic hydroxyapatites, or mineral salts, largely calcium phosphates present in the form of tiny, tightly packed, needle-like crystals in and aroud the collagen fibers in the extracellular matrix. The crystals account for the most notable characteristic of bone - its exceptional hardness, which allows it to resist compression. Ossification and Osteogenesis Are synonyms meaning the process of bone formation. in embryos this process leads to the formation of the bony skeleton. Later another form of ossification known as bone growth goes on until early adulthood as the body continues to increase in size. Bones are capable of growing in thickness through life. However, ossification in adults serves mainly for bone remodeling and repair. membrane bone when a bone developes from a fibrous membrane, the process is intramembranous ossification, and the bone is called a membrane bone. endochondral bone bone development by replacing hyaline carilage is called endochondral ossification, and the resulting bone is called a cartilage, or endochondral, bone. Intramembranous ossification Results in the formation of cranial bones of the skull (frontal, parietal, occipital, and temporal bones) and the clavicles. Most bones formed by this process are flat bones. At about 8 weeks of development, ossification begins on fibrous connective tissue membranes formed by mesenchymal cells. Endochondral ossification (definition) Except for the clavicles, essentially all bones of the skeleton below the base of the skull form by endochondral ossification. Bedinning in the second month of development, this process uses hyaline cartilage "bones" formed earlier as models, or patterns, for bone construction. it is more complex than intramembranous ossification because the hyaline cartilage must be broken down as ossification proceeds. primary ossification center The formation of a long bone typically begins in the center of the hyaline cartilage shaft at a region called the primary ossification center. First, the perichondrium covering the hyaline cartilage "bone" is infiltrated with blood cessels, converting it to a vascularized periosteum. As a result of this change in nutrition, the underlying mesnchymal cells specialize into osteoblasts. Endochondral Ossification (Process) 1. A bone collar is laid down around the diaphysis of the hyaline cartilage model. Osteoblasts of the newly converted periosteum secrete osteoid against the hyaline cartilage diaphysis, encasing it in bone. This freshly formed layer of bone is called the periosteal bone collar. 2. Cartilage in the center of the diaphysis calcifies and then develops cavities. As the bone collar forms, chondrocytes within the shaft hypertrophy (enlarge) and signal the surrounding cartilage matrix to calcify. Then, because calcidied cartilage matrix is impermeable to diffusing nutrients, the chondrocytes die and the matrix begins to deteriorate. This deterioration opens up cavities, but the hyaline cartilage model is stabilized by the bone collar. Elsewhere, the cartilage remains healthy and continues to grow briskly, causing the cartilage model to elongate. 3. The periosteal bud invades the internal cavities and spongy bone forms. In month 3, the forming cavities are invaded by a collection of elements called the periosteal bud, which contains a nutrient artery and vein, lymphatic vessels, nerve fibers, red marrow elements, and the osteoblasts secrete osteoid around the remaining fragments of hyaline cartilage, forming bone-covered cartilage trabeculae. In this way, the earliest version of spongy bone in a developing long bone forms. 4. The diaphysis elongates and a medullary cavity forms. As the primary ossification center enlarges, osteoclasts break down the newly formed spongy bone and open up a medullary cavity in the center of the diaphysis. Throughout the fetal period (week 9 until birth), the rapidly growing epiphyses consit only of cartilage, and the hyaline cartilage models continue to elongate by division of viable cartilage cells at the epiphyses. Ossification "chases" cartilage formation along the length of the shaft as cartilage calcifies, is eroded, and then is replaced by bony spicules on the epiphyseal surfaces the medullary cavity. 5. The epiphyses ossify. At birth, most of our long bones have a bony diaphysis surrounding remnants of spongy bone, a widening medullary cavity, and two carilaginous epiphyses. secondary ossification centers Shortly before or after birth, secondary ossification centers appear in one of both epiphyses, and the epiphyses gain bony tissue (typically the large long bones form secondary centers in both epuphyses, whereas the small long bones form only one secondary ossification center.) Secondary ossification reproduces almost exactly the events of primary ossification, except that the spongy bone in the interior is retained and no medullary cavity forms in the epiphyses. When secondary ossification is complete, hyaline cartilage remains only at two places: 1. on the epiphyseal surfaces, as the articular cartilages, and 2. at the junction of the diaphysis and epiphysis, where it forms the epiphyseal plates. resting or quiescent zone Logitudinal bone growth mimics many of the events of endochondral ossification. The cartilage is relatively inactive on the side of the epiphyseal plate facing the epiphysis, a region called the resting or quiescent zone. proliferation or growth zone But the epiphyseal plate cartilage abuting the diaphysis organizes into a pattern that allows fastm efficient growth. The cartilage cells here form tall columns, like coins in a stack. The cells at the "top" abutting the resting zone comprise the proliferation or growth zone. These cells divide quickly, pushing the epiphysis away from the diaphysis, causing the entire long bone to lengthen. hypertrophic zone Meanwhile, the older chondrozytes in the stack, which are closer to the diaphysis (hypertrophic zone), hypertrophy, and their lacunae erode and enlarge, leaving large interconnecting spaces. Calcification zone subsequently, the surrounding cartilage matrix calcifies and these chondrocytes die and deteriorate, producing the calcification zone. ossification or osteogenic zone This leaves long slender spicules of calcified cartilage at the epiphysis-diaphysis junction, which looks like stalactites hanging from the roof of a cave. These calcified spicules ultimately become part of the ossicication or osteogenic zone, and are invaded by marrow elements from the medullary cavity. The cartilage spicules are partly eroded by osteoclasts, then quickly covered with new bone - called woven bone - by osteoblasts, and ultimately replaced by spongy bone. bone remodeling In the adult skeleton, bone deposit and bone resorption (removal) occur both at the surface of the periosteum and the surface of the endosteum. Together, the two processes constitute bone remodeling, and they are coupled and coordinated by "paclets" of adjacent osteoblasts and osteoclasts called remodeling units (with help from the stress-sensing osteocytes). In healthy young adults, total bone mass remains constant, an indication that the rates of bone deposit and resorption are essentially equal. Remodeling does not occur uniformly, however. Bone deposit Occurs wherever bone is injured or added bone strength is required. for optimal bone deposit, a healthy diet rich in proteins, vitamin C, vitamin D, vitamin A, and several minerals (calcium, phosphorus, magnesium, and manganese, to name a few) is essential. Bone resorption is accomplised by osteoclasts, giant multinucleate cells that arise from the same hematopoietic stem cells that differentiate into macrophages. Parathyroid hormone (PTH) The hormonal controls primarily involve parathyroid hormone (PTH), produced by the parathyroid glands. Calcitonin To a much lesser extent calcitonin, produced by parafollicular cells (C cells) of the thyroid gland, may be involved. PTH is released when blood levels of ionic calcium decline. The increased PTH level simulates osteoclasts to resorb bone, releasing calcium to the blood. Osteoclasts are no respecters of matrix age. When activated, they break down both old and new matrix. Only osteoid, which lacks calcium salts, escapes digestion. As blood concentrations of calcium rise, the stimulus for PTH release ends. The decline of PTH reverses its effects and causes blood Ca2+ levels to fall. In humans, calcitonin appears to be a hormone in search of a function because its effects on calcium homeostasis are negligible. When administered at pharmacological (abnormally high) doses, it does lower blood calcium levels temporarily. These hormonal controls act not to preserve the skeleton's strength or well-being but rather to maintain blood calcium homeostasis. in fact, if blood calcium levels are low for an extended time, the bones become so demineralized that they develop large, punched-out-looking holes. thus, the bones serve as a storehouse from which ionic calcium is drawn as needed. In addition to the hormones that regulate bone remodeling in response to blood calcium levels, it is not established that leptin, a hormone released by adipose tissue, plays a role in regulating bone density. Response to mechanical stress The second set of controls regulating bone remodeling, bone's response to mechanical stress (muscle pull) and gravity, serves the needs of the skeleton by keeping the bones strong where stressors are acting. Wolff's law holds that a bone grows or remodels in response to the demands placed on it. The first thing to understand is that a bone's anatomy reflects the common stresses it encounters. For example, a bone is loaded (stressed) whenever weight bears down on it or muscles pull on it. This loading is usually off center, however, and tends to bend the bone. Bending compresses the bone on one side and subjects it to tension (stretching) on the other. As a result of these mechanical stressors, long bones are thickest midway along the diaphysis, exactly where bending stresses are greatest. Both compression and tension are minimal toward the center of the bone, so a bone can "hollow out" for lightness without jeopardy. Wolff's Law (observations) 1. Handedness (being right or left handed) results in the bones of one upper limb being thicker than those of the less-used limb, and vigorous exercise of the most-used limb leads to large increases in bone strength. 2. Curved bones are thickest where they are most likely to buckle. 3. The trabeculae of spongy bone form trusses, or struts, along lines of compression. 4. Large, bony projections occur where heavy, active muscles attach. (The bones of weight lifters have enormous thickenings at the attachment sites of the most-used muscles.) Wolff's law also explains the featureless bones of the fetus and the atrophied bones of bedridden people - situations in which bones are not stressed. Bone Repair fractures Despite their remarkable strength, bones are susceptible to fractures, or breaks. During youth, most fractures result from exceptional trauma that twists or smashes the bones (spots injuries, automobile accidents, and falls, for example). Fractures may be classified by: 1. Position of the bone ends after fracture. In nondisplaced fractures the bone ends retain their normal position; in displaced fractures the bone ends are out of normal alignment. 2. Completeness of the break. If the bone is broken through, the fracture is a complete fracture; if not, it is an incomplete fracture. 3. Orientation of the break relative to the long axis of the bone. If the break parallels the long axis, the fracture is linear; if the break is perpendicular to the bone's long axis, it is transverse. 4. Whether the bone ends penetrate the skin. If so, the fracture is an open (compound) fracture; if not, it is a closed (simple) fracture. In addition to these four either-or classifications, all fractures can be described in terms of the location of the fracture, the external appearance of the fracture, and/or the nature of the break. How are fractures treated? A fracture is treated by reduction, the realignment of the broken bone ends. In closed (external) reduction, the bone ends are coaxed into position by the physician's hands. In open (internal) reduction, the bone ends are secured together surgically with pins or wires. After the broken bone is reduced, it is immobilized either by a cast or traction to allow the healing process to begin. For a simple fracture the healing time is six to eight weeks for small or medium-sized bones in young adults, but is much longer for large, weight-bearing bones and for bones of elderly people (because of their poorer circulation). Repair in a simple fracture involves four major stages: 1. A hematoma forms. When a bone breaks, blood vessels in the bone and periosteum, and perhaps in surrounding issues, are torn and hemorrhage. As a result, a hemotoma, a mass of clotted blood, forms at the fracture site. Soon, bone cells deprived of nutrition die, and the tissue at the site becomes swollen, painful, and inflamed. 2. Fibrocartilaginous callus forms. Within a few days, several events lead to the formation of soft granulation tissue, also called the sofr callus. Capillaries grow into the hematoma and phagocytic cells invade the area and begin cleaning up the debris. Meanwhile, fibroblasts and osteoblasts invade the fracture site from the nearby periosteum and endosteum and begin reconstructing the bone. The fibroblasts produce collagen fibers that span the break and connect the broken bone ends, and some differentiate into chondroblasts that secrete cartilage matrix. Within this mass of repair tissue, osteoblasts begin forming spongy bone, but those farthest from the capillary supply secrete an externally bulding cartilaginous matrix that later calcifies. This entire mass of repair tissue, now called the fibrocartilaginous callus, splints the broken bone. 3. Bony callus forms. Within a week, new bone trabeculae begin to appear in the fibrocartilaginous callus and gradually convert it to a boney (hard) callus of spongy bone. Bony callus formation continues until a firm union is formed about two months later. 4. Bone remodeling occurs. Beginning during bony callus formation and continuing for several months after, the bony callus is remodeled. The excess material on the diaphysis exterior and within the medullary cavity is removed, and compact bone is laid down to reconstruct the shaft walls. The final structure of the remodeled area resembles that of the original unbroken bony region because it responds to the same set of mechanical stressors. Osteomalacia Includes a number of disorders in which the bones are inadequately minderalized. Osteoid is produced, but calcium salts are not deposited, so bones soften and weaken. The main sumptom is pain when weight is put on the affected bones. Rickets Is the analogous disease in children. Because young bones are still growing rapidly, rickets is much more severe than adult osteomalacia. Bowed legs and deformities of the pelvis, skull, and rib cage are common. Because the epiphyseal plates cannot be calcified, they continue to widen, and the ends of long bones become visible enlarged and abnormally long. Common Types of Fractures Comminuted Bone fragments into three or more pieces. Particularly common in the aged, whose bones are more brittle. Common Types of Fractures Compression Bone is crushed. Common in porous bones (i.e. osteoporotic bones) subjected to extreme trauma, as in a fall. Common Types of Fracture Spiral Ragged break occurs when excessive twisting forces are applied to a bone. Common sports fracture. Common Types of Fractures Epiphyseal Epiphysis seperates from the diaphysis along the epiphyseal plate. Tends to occur where cartilage cells are dying and calcification of the matrix is occuring. Common Types of Fractures Depressed Broken bone portion is pressed inward. Typical of skull fracture. Common Types of Fractures Greenstick Bone breaks incompletely, much in the way a green twig breaks. Only one side of the shaft breaks; the other side bends. Common in children, whose bones have relatively more organic matrix and are more flexible than those of adults. Osteoporosis Refers to a group of diseases in which bone resorption outpaces bone deposit. The bones become so fragile that something as simple as a hearty sneeze or stepping off a curb can cause them to break. The composition of the matrix remains normal but bone mass is reduced, and the bones become porous and light. Even though osteoporosis affects the entire skeleton, the spongy bone of the spine is most vulnerable, and compression fractures of the vertebrae are common. The gemur, particularly its neck, is also very susceptible to fracture (called a broken hip) in people with osteoporosis. Osteoporosis occurs most often in the aged, particularly in postmenopausal women. Although men develop it to a lesser degree, 30% of American women between the ages of 60 and 70 have osteoporosis, and 70% have it by age 80. Moreover, 30% of all Caucasian women (the most susceptible group) will experience a bone fracture due to osteoporosis. Sex hormones, particularly estrogen, help to maintain the health and normal density of the skeleton by restraining osteoclast activity and by promoting deposit of new bone. Paget's disease Often discovered by accident when X rays are taken for some other reason, Paget's disease is characterized by excessive and haphazard bone deposit and resoption. The newly formed bone, called Pagetic bone, is hastily made and has an abnormally high ratio of spongy bone to compact bone. This along with reduced mineralization, caused a spotty weakening of the bones. Late in the disease, osteoclast activity wanes, but osteoblasts continue to work, often forming irregular bone thickening or filling the marrow cavity with Pagetic bone. Paget's disease may affect any part of the skeleton, but it is usually a localized condition. The spine, pelvis, femur, and skull are most often involved and become increasingly deformed and painful. it rarely occurs before the age of 40, and it affects about 3% of North American elderly people. Its cause is unknown, but it may be initiated by a virus. Drug therapies include calcitonin, and the newer bisphosphonates (eitdronate, alendronate, and others) which have shown success in preventing bone breakdown.