Bone Histology PDF

As the main constituent of the adult skeleton, bone tissue (Figure 8--1) provides solid support for the body, protects vital organs such as those in the cranial and thoracic cavities, and encloses internal (medullary) cavities containing bone marrow where blood cells are formed. Bone (or osseous) ti...

As the main constituent of the adult skeleton, bone tissue (Figure 8--1) provides solid support for the body, protects vital organs such as those in the cranial and thoracic cavities, and encloses internal (medullary) cavities containing bone marrow where blood cells are formed. Bone (or osseous) tissue also serves as a reservoir of calcium, phosphate, and other ions that can be released or stored in a controlled fashion to maintain constant concentrations in body fluids. In addition, bones form a system of levers that multiply the forces generated during skeletal muscle contraction and transform them into bodily movements. This mineralized tissue therefore confers mechanical and metabolic functions to the skeleton. Bone is a specialized connective tissue composed of calcified extracellular material, the bone matrix, and following three major cell types (Figure 8--2): Osteocytes (Gr. osteon, bone + kytos, cell), which are found in cavities (lacunae) between bone matrix layers (lamellae), with cytoplasmic processes in small canaliculi (L. canalis, canal) that extend into the matrix (Figure 8--1b) Osteoblasts (osteon + Gr. blastos, germ), growing cells which synthesize and secrete the organic components of the matrix Osteoclasts (osteon + Gr. klastos, broken), which are giant, multinucleated cells involved in removing calcified bone matrix and remodeling bone tissue Because metabolites are unable to diffuse through the calcified matrix of bone, the exchanges between osteocytes and blood capillaries depend on communication through the very thin, cylindrical spaces of the canaliculi. All bones are lined on their internal and external surfaces by layers of connective tissue containing osteogenic cells---endosteum on the internal surface surrounding the marrow cavity and periosteum on the external surface. Because of its hardness, bone cannot be sectioned routinely. Bone matrix is usually softened by immersion in a decalcifying solution before paraffin embedding, or embedded in plastic after fixation and sectioned with a specialized microtome. › BONE CELLS **Osteoblasts** Originating from mesenchymal stem cells, osteoblasts produce the organic components of bone matrix, including type I collagen fibers, proteoglycans, and matricellular glycoproteins, such as osteonectin. Deposition of the inorganic components of bone also depends on osteoblast activity. Active osteoblasts are located exclusively at the surfaces of bone matrix, to which they are bound by integrins, typically forming a single layer of cuboidal cells joined by adherent and gap junctions (Figure 8--3). When their synthetic activity is completed, some osteoblasts differentiate as osteocytes entrapped in matrixbound lacunae, some flatten and cover the matrix surface as bone lining cells, and the majority undergo apoptosis. During the processes of matrix synthesis and calcification, osteoblasts are polarized cells with ultrastructural features denoting active protein synthesis and secretion. Matrix components are secreted at the cell surface in contact with existing bone matrix, producing a layer of unique collagen-rich material called osteoid between the osteoblast layer and the preexisting bone surface (Figure 8--3). This process of bone appositional growth is completed by subsequent deposition of calcium salts into the newly formed matrix. The process of matrix mineralization is not completely understood, but basic aspects of the process are shown in Figure 8--4. Prominent among the noncollagen proteins secreted by osteoblasts is the vitamin K-dependent polypeptide osteocalcin, which together with various glycoproteins binds Ca2+ ions and concentrates this mineral locally. Osteoblasts also release membrane-enclosed matrix vesicles rich in alkaline phosphatase and other enzymes whose activity raises the local concentration of PO4 3− ions. In the microenvironment with high concentrations of both these ions, matrix vesicles serve as foci for the formation of hydroxyapatite \[Ca10(PO4)6(OH)2\] crystals, the first visible step in calcification. These crystals grow rapidly by accretion of more mineral and eventually produce a confluent mass of calcified material embedding the collagen fibers and proteoglycans (Figure 8--4). ›››MEDICAL APPLICATION Cancer originating directly from bone cells (a primary bone tumor) is fairly uncommon (0.5% of all cancer deaths), although a cancer called osteosarcoma can arise in osteoprogenitor cells. The skeleton is often the site of secondary, metastatic tumors, however, arising when cancer cells move into bones via small blood or lymphatic vessels from malignancies in other organs, most commonly the breast, lung, prostate gland, kidney, or thyroid gland. **Osteocytes** As mentioned, some osteoblasts become surrounded by the material they secrete and then differentiate as osteocytes enclosed singly within the lacunae spaced throughout the mineralized matrix. During the transition from osteoblasts to osteocytes, the cells extend many long dendritic processes, which also become surrounded by calcifying matrix. The processes thus come to occupy the many canaliculi, 250-300 nm in **d**iameter, radiating from each lacuna (Figures 8--5 and 8--1b). Diffusion of metabolites between osteocytes and blood vessels occurs through the small amount of interstitial fluid in the canaliculi between the bone matrix and the osteocytes and their processes. Osteocytes also communicate with one another and ultimately with nearby osteoblasts and bone lining cells via gap junctions at the ends of their processes. These connections between osteocyte processes and nearly all other bone cells in the extensive lacunar-canalicular network allow osteocytes to serve as mechanosensors detecting the mechanical load on the bone as well as stress- or fatigue-induced microdamage and to trigger remedial activity in osteoblasts and osteoclasts. Normally the most abundant cells in bone, osteocytes exhibit significantly less RER, smaller Golgi complexes, and more condensed nuclear chromatin than osteoblasts (Figure 8--5a). Osteocytes maintain the calcified matrix and their death is followed by rapid matrix resorption. While sharing most matrix related activities with osteoblasts, osteocytes also express many different proteins, including factors with paracrine and endocrine effects that help regulate bone remodeling. ›››MEDICAL APPLICATION The extensive network of osteocyte dendritic processes and other bone cells has been called a "mechanostat," monitoring mechanical loads within bones and signaling cells to adjust ion levels and maintain the adjacent bone matrix accordingly. Resistance exercise can produce increased bone density and thickness in affected regions, while lack of exercise (or the weightlessness experienced by astronauts) leads to decreased bone density, due in part to the lack of mechanical stimulation of the bone cells. **Osteoclasts** Osteoclasts are very large, motile cells with multiple nuclei (Figure 8--6) that are essential for matrix resorption during bone growth and remodeling. The large size and multinucleated condition of osteoclasts are due to their origin from the fusion of bone marrow-derived monocytes. Osteoclast development requires two polypeptides produced by osteoblasts: macrophage-colony-stimulating factor (M-CSF; discussed with hemopoiesis, see Chapter 13) and the receptor activator of nuclear factor-κB ligand (RANKL). In areas of bone undergoing resorption, osteoclasts on the bone surface lie within enzymatically etched depressions or cavities in the matrix known as resorption lacunae (or Howship lacunae). In an active osteoclast, the membrane domain that contacts the bone forms a circular sealing zone that binds the cell tightly to the bone matrix and surrounds an area with many surface projections, called the ruffled border. This circumferential sealing zone allows the formation of a specialized microenvironment between the osteoclast and the matrix in which bone resorption occurs (Figure 8--6b). Into this subcellular pocket, the osteoclast pumps protons to acidify and promote dissolution of the adjacent hydroxyapatite, and releases matrix metalloproteinases and other hydrolytic enzymes from lysosome-related secretory vesicles for the localized digestion of matrix proteins. Osteoclast activity is controlled by local signaling factors from other bone cells. Osteoblasts activated by parathyroid hormone produce M-CSF, RANKL, and other factors that regulate the formation and activity of osteoclasts. ›››MEDICAL APPLICATION In the genetic disease osteopetrosis, which is characterized by dense, heavy bones ("marble bones"), the osteoclasts lack ruffled borders and bone resorption is defective. This disorder results in overgrowth and thickening of bones, often with obliteration of the marrow cavities, depressing blood cell formation and causing anemia and the loss of white blood cells. The defective osteoclasts in most patients with osteopetrosis have mutations in genes for the cells' proton-ATPase pumps or chloride channels. › BONE MATRIX About 50% of the dry weight of bone matrix is inorganic materials. Calcium hydroxyapatite is most abundant, but bicarbonate, citrate, magnesium, potassium, and sodium ions are also found. Significant quantities of noncrystalline calcium phosphate are also present. The surface of hydroxyapatite crystals is hydrated, facilitating the exchange of ions between the mineral and body fluids. The organic matter embedded in the calcified matrix is 90% type I collagen, but also includes mostly small proteoglycans and multiadhesive glycoproteins such as osteonectin. Calcium-binding proteins, notably osteocalcin, and the phosphatases released from cells in matrix vesicles promote calcification of the matrix. Other tissues rich in type I collagen lack osteocalcin and matrix vesicles and therefore do not normally become calcified. The association of minerals with collagen fibers during calcification provides the hardness and resistance required for bone function. If a bone is decalcified by a histologist, its shape is preserved but it becomes soft and pliable like other connective tissues. Because of its high collagen content, decalcified bone matrix is usually acidophilic. › PERIOSTEUM & ENDOSTEUM External and internal surfaces of all bones are covered by connective tissue of the periosteum and endosteum, respectively (Figures 8--1a and 8--1c). The periosteum is organized much like the perichondrium of cartilage, with an outer fibrous layer of dense connective tissue, containing mostly bundled type I collagen, but also fibroblasts and blood vessels. Bundles of periosteal collagen, called perforating (or Sharpey) fibers, penetrate the bone matrix and bind the periosteum to the bone. Periosteal blood vessels branch and penetrate the bone, carrying metabolites to and from bone cells. The periosteum's inner layer is more cellular and includes osteoblasts, bone lining cells, and mesenchymal stem cells referred to as osteoprogenitor cells. With the potential to proliferate extensively and produce many new osteoblasts, osteoprogenitor cells play a prominent role in bone growth and repair. Internally, the very thin endosteum covers small trabeculae of bony matrix that project into the marrow cavities (Figure 8--1). The endosteum also contains osteoprogenitor cells, osteoblasts, and bone lining cells, but within a sparse, delicate matrix of collagen fibers. ›››MEDICAL APPLICATION Osteoporosis, frequently found in immobilized patients and in postmenopausal women, is an imbalance in skeletal turnover so that bone resorption exceeds bone formation. This leads to calcium loss from bones and reduced bone mineral density (BMD). Individuals at risk for osteoporosis are routinely tested for BMD by dual-energy x-ray absorptiometry (DEXA scans). › TYPES OF BONE Gross observation of a bone in cross section (Figure 8--7) shows a dense area near the surface corresponding to compact (cortical) bone, which represents 80% of the total bone mass, and deeper areas with numerous interconnecting cavities, called cancellous (trabecular) bone, constituting about 20% of total bone mass. Histological features and important locations of the major types of bone are summarized in Table 8--1. In long bones, the bulbous ends---called epiphyses (Gr. epiphysis, an excrescence)---are composed of cancellous bone covered by a thin layer of compact cortical bone. The cylindrical part---the diaphysis (Gr. diaphysis, a growing between)---is almost totally dense compact bone, with a thin region of cancellous bone on the inner surface around the central marrow cavity (Figure 8--1). Short bones such as those of the wrist and ankle usually have cores of cancellous bone surrounded completely by compact bone. The flat bones that form the calvaria (skullcap) have two layers of compact bone called plates, separated by a thicker layer of cancellous bone called the diploë. At the microscopic level, both compact and cancellous bones typically show two types of organization: mature lamellar bone, with matrix existing as discrete sheets, and woven bone, newly formed with randomly arranged components. **Lamellar Bone** Most bone in adults, compact or cancellous, is organized as lamellar bone, characterized by multiple layers or lamellae of calcified matrix, each 3-7 μm thick. The lamellae are organized as parallel sheets or concentrically around a central canal. In each lamella, type I collagen fibers are aligned, with the pitch of the fibers' orientation shifted orthogonally (by about 90 degrees) in successive lamellae (Figure 8--1a). This highly ordered organization of collagen within lamellar bone causes birefringence with polarizing light microscopy; the alternating bright and dark layers are due to the changing orientation of collagen fibers in the lamellae (Figure 8--8). Like the orientation of wood fibers in plywood, the highly ordered, alternating organization of collagen fibers in lamellae adds greatly to the strength of lamellar bone. An osteon (or Haversian system) refers to the complex of concentric lamellae, typically 100-250 μm in diameter, surrounding a central canal that contains small blood vessels, nerves, and endosteum (Figures 8--1 and 8--9). Between successive lamellae are lacunae, each with one osteocyte, all interconnected by the canaliculi containing the cells' dendritic processes (Figure 8--9). Processes of adjacent cells are in contact via gap junctions, and all cells of an osteon receive nutrients and oxygen from vessels in the central canal (Figure 8--1). The outer boundary of each osteon is a layer called the cement line that includes many more noncollagen proteins in addition to mineral and collagen. Each osteon is a long, sometimes bifurcated, cylinder generally parallel to the long axis of the diaphysis. Each has 5-20 concentric lamellae around the central canal that communicate with the marrow cavity and the periosteum. Canals also communicate with one another through transverse perforating canals (or Volkmann canals) that have few, if any, concentric lamellae (Figures 8--1 and 8--10). All central osteonic canals and perforating canals form when matrix is laid down around areas with preexisting blood vessels.Scattered among the intact osteons are numerous irregularly shaped groups of parallel lamellae called interstitial lamellae. These structures are lamellae remaining from osteons partially destroyed by osteoclasts during growth and remodeling of bone (Figure 8--10).Compact bone (eg, in the diaphysis of long bones) also includes parallel lamellae organized as multiple external circumferential lamellae immediately beneath the periosteum and fewer inner circumferential lamellae around the marrow cavity (Figure 8--1a). The lamellae of these outer and innermost areas of compact bone enclose and strengthen the middle region containing vascularized osteons.Bone remodeling occurs continuously throughout life. In compact bone, remodeling resorbs parts of old osteons and produces new ones. As shown in Figure 8--11, osteoclasts remove old bone and form small, tunnel-like cavities. Such tunnels are quickly invaded by osteoprogenitor cells from the endosteum or periosteum and sprouting loops of capillaries. Osteoblasts develop, line the wall of the tunnels, and begin to secrete osteoid in a cyclic manner, forming a new osteon with concentric lamellae of bone and trapped osteocytes ›››MEDICAL APPLICATION The antibiotic tetracycline is a fluorescent molecule that binds newly deposited osteoid matrix during mineralization with high affinity and specifically labels new bone under the UV microscope (Figure 8--12). This discovery led to methods for measuring the rate of bone growth, an important parameter in the diagnosis of certain bone disorders. In one technique, tetracycline is administered twice to patients, with an intervening interval of 11-14 days. A bone biopsy is then performed, sectioned without decalcification, and examined. Bone formed while tetracycline was present appears as fluorescent lamellae and the distance between the labeled layers is proportional to the rate of bone appositional growth. This procedure is of diagnostic importance in such diseases as osteomalacia, in which mineralization is impaired, and osteitis fibrosa cystica, in which increased osteoclast activity results in removal of bone matrix and fibrous degeneration. **Woven Bone** Woven bone is nonlamellar and characterized by random disposition of type I collagen fibers and is the first bone tissue to appear in embryonic development and in fracture repair. Woven bone is usually temporary and is replaced in adults by lamellar bone, except in a very few places in the body, for example, near the sutures of the calvaria and in the insertions of some tendons.In addition to the irregular, interwoven array of collagen fibers, woven bone typically has a lower mineral content (it is more easily penetrated by x-rays) and a higher proportion of osteocytes than mature lamellar bone. These features reflect the fact that immature woven bone forms more quickly but has less strength than lamellar bone. › OSTEOGENESIS Bone development or osteogenesis occurs by one of two processes: Intramembranous ossification, in which osteoblasts differentiate directly from mesenchyme and begin secreting osteoid Endochondral ossification, in which a preexisting matrix of hyaline cartilage is eroded and invaded by osteoblasts, which then begin osteoid productionThe names refer to the mechanisms by which the bone forms initially; in both processes, woven bone is produced first and is soon replaced by stronger lamellar bone. During growth of all bones, areas of woven bone, areas of bone resorption, and areas of lamellar bone all exist contiguous to one another. ›››MEDICAL APPLICATION Osteogenesis imperfecta, or "brittle bone disease," refers to a group of related congenital disorders in which the osteoblasts produce deficient amounts of type I collagen or defective type I collagen due to genetic mutations. Such defects lead to a spectrum of disorders, all characterized by significant fragility of the bones. The fragility reflects the deficit in normal collagen, which normally reinforces and adds a degree of resiliency to the mineralized bone matrix. **Intramembranous Ossification** Intramembranous ossification, by which most flat bones begin to form, takes place within condensed sheets ("membranes") of embryonic mesenchymal tissue. Most bones of the skull and jaws, as well as the scapula and clavicle, are formed embryonically by intramembranous ossification.Within the condensed mesenchyme, bone formation begins in ossification centers, areas in which osteoprogenitor cells arise, proliferate, and form incomplete layers of osteoblasts around a network of developing capillaries. Osteoid secreted by the osteoblasts calcifies as described earlier, forming small irregular areas of woven bone with osteocytes in lacunae and canaliculi (Figure 8--13). Continued matrix secretion and calcification enlarges these areas and leads to the fusion of neighboring ossification centers. The anatomical bone forms gradually as woven bone matrix is replaced by compact bone that encloses a region of cancellous bone with marrow and larger blood vessels. Mesenchymal regions that do not undergo ossification give rise to the endosteum and the periosteum of the new bone.In cranial flat bones, lamellar bone formation predominates over bone resorption at both the internal and external surfaces. Internal and external plates of compact bone arise, while the central portion (diploë) maintains its cancellous nature. The fontanelles or "soft spots" on the heads of newborn infants are areas of the skull in which the membranous tissue is not yet ossified. **Endochondral Ossification** Endochondral (Gr. endon, within + chondros, cartilage) ossification takes place within hyaline cartilage, shaped as a small version, or model, of the bone to be formed. This type of ossification forms most bones of the body and is especially well studied in developing long bones, where it consists of the sequence of events shown in Figure 8--14. In this process, ossification first occurs within a bone collar produced by osteoblasts that differentiate within the perichondrium (transitioning to periosteum) around the cartilage model diaphysis. The collar impedes diffusion of oxygen and nutrients into the underlying cartilage, causing local chondrocytes to swell up (hypertrophy), compress the surrounding matrix, and initiate its calcification by releasing osteocalcin and alkaline phosphatase. The hypertrophic chondrocytes eventually die, creating empty spaces within the calcified matrix. One or more blood vessels from the perichondrium (now the periosteum) penetrate the bone collar, bringing osteoprogenitor cells to the porous central region. Along with the vasculature, newly formed osteoblasts move into all available spaces and produce woven bone. The remnants of calcified cartilage at this stage are basophilic and the new bone is more acidophilic (Figure 8--15). This process in the diaphysis forms the primary ossification center (Figure 8--14), beginning in many embryonic bones as early as the first trimester. Secondary ossification centers appear later at the epiphyses of the cartilage model and develop in a similar manner. During their expansion and remodeling, both the primary and secondary ossification centers produce cavities that are gradually filled with bone marrow and trabeculae of cancellous bone. With the primary and secondary ossification centers, two regions of cartilage remain: Articular cartilage within the joints between long bones (Figure 8--14), which normally persists through adult life The specially organized epiphyseal cartilage (also called the epiphyseal plate or growth plate), which connects each epiphysis to the diaphysis and allows longitudinal bone growth (Figure 8--14) The epiphyseal cartilage is responsible for the growth in length of the bone and disappears upon completion of bone development at adulthood. Elimination of these epiphyseal plates ("epiphyseal closure") occurs at various times with different bones and by about age 20 is complete in all bones, making further growth in bone length no longer possible. In forensics or through x-ray examination of the growing skeleton, it is possible to determine the "bone age" of a young person, by noting which epiphyses have completed closure.An epiphyseal growth plate shows distinct regions of cellular activity and is often discussed in terms of overlapping but histologically distinct zones (Figures 8--16 and 8--17), starting with the cartilage farthest from the ossification center in the diaphysis: 1\. The zone of reserve (or resting) cartilage is composed of typical hyaline cartilage. 2\. In the proliferative zone, the cartilage cells divide repeatedly, enlarge and secrete more type II collagen and proteoglycans, and become organized into columns parallel to the long axis of the bone. 3\. The zone of hypertrophy contains swollen, terminally differentiated chondrocytes, which compress the matrix into aligned spicules and stiffen it by secretion of type X collagen. Unique to the hypertrophic chondrocytes in developing (or fractured) bone, type X collagen limits diffusion in the matrix and with growth factors promotes vascularization from the adjacent primary ossification center. 4\. In the zone of calcified cartilage, chondrocytes about to undergo apoptosis release matrix vesicles and osteocalcin to begin matrix calcification by the formation of hydroxyapatite crystals. 5\. In the zone of ossification, bone tissue first appears. Capillaries and osteoprogenitor cells invade the now vacant chondrocytic lacunae, many of which merge to form the initial marrow cavity. Osteoblasts settle in a layer over the spicules of calcified cartilage matrix and secrete osteoid, which becomes woven bone (Figures 8--16 and 8--17). This woven bone is then remodeled as lamellar bone. In summary, longitudinal growth of a bone occurs by cell proliferation in the epiphyseal plate cartilage. At the same time, chondrocytes in the diaphysis side of the plate undergo hypertrophy, their matrix becomes calcified, and the cells die. Osteoblasts lay down a layer of new bone on the calcified cartilage matrix. Because the rates of these two opposing events (proliferation and destruction) are approximately equal, the epiphyseal plate does not change thickness, but is instead displaced away from the center of the diaphysis as the length of the bone increases. Growth in the circumference of long bones does not involve endochondral ossification but occurs through the activity of osteoblasts developing from osteoprogenitor cells in the periosteum by a process of appositional growth, which begins with formation of the bone collar on the cartilaginous diaphysis. As shown in Figure 8--18, the increasing bone circumference is accompanied by enlargement of the central marrow cavity by the activity of osteoclasts in the endosteum. ›››MEDICAL APPLICATION Calcium deficiency in children can lead to rickets, a disease in which the bone matrix does not calcify normally and the epiphyseal plate can become distorted by the normal strains of body weight and muscular activity. Ossification processes are consequently impeded, which causes bones to grow more slowly and often become deformed. The deficiency can be due either to insufficient calcium in the diet or a failure to produce the steroid prohormone vitamin D, which is important for the absorption of Ca2+ by cells of the small intestine. In adults, calcium deficiency can give rise to osteomalacia(osteon + Gr. malakia, softness), characterized by deficient calcification of recently formed bone and partial decalcification of already calcified matrix. › BONE REMODELING & REPAIR Bone growth involves both the continuous resorption of bone tissue formed earlier and the simultaneous laying down of new bone at a rate exceeding that of bone removal. The sum of osteoblast and osteoclast activities in a growing bone constitutes osteogenesis or the process of bone modeling, which maintains each bone's general shape while increasing its mass. The rate of bone turnover is very active in young children, where it can be 200 times faster than that of adults. In adults, the skeleton is also renewed continuously in a process of bone remodeling that involves the coordinated, localized cellular activities for bone resorption and bone formation shown in the diagram of Figure 8--11. The constant remodeling of bone ensures that, despite its hardness, this tissue remains plastic and capable of adapting its internal structure in the face of changing stresses. A well known example of bone plasticity is the ability of the positions of teeth in the jawbone to be modified by the lateral pressures produced by orthodontic appliances. Bone forms on the side where traction is applied and is resorbed on the opposite side where pressure is exerted. In this way, teeth are moved within the jaw while the bone is being remodeled. Because it contains osteoprogenitor stem cells in the periosteum, endosteum, and marrow and is very well vascularized, bone normally has an excellent capacity for repair. Bone repair after a fracture or other damage uses cells, signaling molecules, and processes already active in bone remodeling. Surgically created gaps in bone can be filled with new bone, especially when periosteum is left in place. The major phases that occur typically during bone fracture repair include initial formation of fibrocartilage and its replacement with a temporary callus of woven bone, as shown in Figure 8--19. ›››MEDICAL APPLICATION Bone fractures are repaired by a developmental process involving fibrocartilage formation and osteogenic activity of the major bone cells (Figure 8--19). Bone fractures disrupt blood vessels, causing bone cells near the break to die. The damaged blood vessels produce a localized hemorrhage or hematoma. Clotted blood is removed along with tissue debris by macrophages and the matrix of damaged, cell-free bone is resorbed by osteoclasts. The periosteum and the endosteum at the fracture site respond with intense proliferation and produce a soft callus of fibrocartilage-like tissue that surrounds the fracture and covers the extremities of the fractured bone. The fibrocartilaginous callus is gradually replaced in a process that resembles a combination of endochondral and intramembranous ossification. This produces a hard callus of woven bone around the fractured ends of bone. Stresses imposed on the bone during repair and during the patient's gradual return to activity serve to remodel the bone callus. The immature, woven bone of the callus is gradually resorbed and replaced by lamellar bone, remodeling and restoring the original bone structure. › METABOLIC ROLE OF BONE Calcium ions are required for the activity of many enzymes and many proteins mediating cell adhesion, cytoskeletal movements, exocytosis, membrane permeability, and other cellular functions. The skeleton serves as the calcium reservoir, containing 99% of the body's total calcium in hydroxyapatite crystals. The concentration of calcium in the blood (9-10 mg/dL) and tissues is generally quite stable because of a continuous interchange between blood calcium and bone calcium. The principal mechanism for raising blood calcium levels is the mobilization of ions from hydroxyapatite to interstitial fluid, primarily in cancellous bone. Ca2+ mobilization is regulated mainly by paracrine interactions among bone cells, many of which are not well understood, but two polypeptide hormones also target bone cells to influence calcium homeostasis: Parathyroid hormone (PTH) from the parathyroid glands raises low blood calcium levels by stimulating osteoclasts and osteocytes to resorb bone matrix and release Ca2+. The PTH effect on osteoclasts is indirect; PTH receptors occur on osteoblasts, which respond by secreting RANKL and other paracrine factors that stimulate osteoclast formation and activity. Calcitonin, produced within the thyroid gland, can reduce elevated blood calcium levels by opposing the effects of PTH in bone. This hormone directly targets osteoclasts to slow matrix resorption and bone turnover. ›››MEDICAL APPLICATION In addition to PTH and calcitonin, several other hormones act on bone. The anterior lobe of the pituitary synthesizes growth hormone (GH or somatotropin), which stimulates the liver to produce insulin-like growth factor-1 (IGF-1 or somatomedin). IGF has an overall growth-promoting effect, especially on the epiphyseal cartilage. Consequently, lack of growth hormone during the growing years causes pituitary dwarfism; an excess of growth hormone causes excessive growth of the long bones, resulting in gigantism. Adult bones cannot increase in length even with excess IGF because they lack epiphyseal cartilage, but they do increase in width by periosteal growth. In adults, an increase in GH causes acromegaly, a disease in which the bones---mainly the long ones---become very thick. ›››MEDICAL APPLICATION In rheumatoid arthritis, chronic inflammation of the synovial membrane causes thickening of this connective tissue and stimulates the macrophages to release collagenases and other hydrolytic enzymes. Such enzymes eventually cause destruction of the articular cartilage, allowing direct contact of the bones projecting into the joint. › JOINTS Joints are regions where adjacent bones are capped and held together firmly by other connective tissues. The type of joint determines the degree of movement between the bones. Joints classified as synarthroses (Gr. syn, together + arthrosis, articulation) allow very limited or no movement and are subdivided into fibrous and cartilaginous joints, depending on the type of tissue joining the bones. Major subtypes of synarthroses include the following: Synostoses involve bones linked to other bones and allow essentially no movement. In older adults, synostoses unite the skull bones, which in children and young adults are held together by sutures, or thin layers of dense connective tissue with osteogenic cells. Syndesmoses join bones by dense connective tissue only. Examples include the interosseous ligament of the inferior tibiofibular joint and the posterior region of the sacroiliac joints. Symphyses have a thick pad of fibrocartilage between the thin articular cartilage covering the ends of the bones. All symphyses, such as the intervertebral discs and pubic symphysis, occur in the midline of the body. Intervertebral discs (Figure 8--20) are large symphyses between the articular surfaces of successive bony vertebral bodies.Held in place by ligaments these discoid components of the intervertebral joints cushion the bones and facilitate limited movements of the vertebral column. Each disc has an outer portion, the annulus fibrosus, consisting of concentric fibrocartilage laminae in which collagen bundles are arranged orthogonally in adjacent layers. The multiple lamellae of fibrocartilage produce a disc with unusual toughness able to withstand pressures and torsion generated within the vertebral column. Situated in the center of the annulus fibrosus, a gel-like body called the nucleus pulposus allows each disc to function as a shock absorber (Figure 8--20). The nucleus pulposus consists of a viscous fluid matrix rich in hyaluronan and type II collagen fibers, but also contains scattered, vacuolated cells derived from the embryonic notochord, the only cells of that structure to persist postnatally. The nucleus pulposus is large in children, but these structures gradually become smaller with age and are partially replaced by fibrocartilage. ›››MEDICAL APPLICATION Within an intervertebral disc, collagen loss or other degenerative changes in the annulus fibrosus are often accompanied by displacement of the nucleus pulposus, a condition variously called a slipped or herniated disc. This occurs most frequently on the posterior region of the intervertebral disc where there are fewer collagen bundles. The affected disc frequently dislocates or shifts slightly from its normal position. If it moves toward nerve plexuses, it can compress the nerves and result in severe pain and other neurologic disturbances. The pain accompanying a slipped disc may be perceived in areas innervated by the compressed nerve fibers---usually the lower lumbar region. Joints classified as diarthroses permit free bone movement. Diarthroses (Figure 8--21), such as the elbow and the knee, generally unite long bones and allow great mobility. In a diarthrosis ligaments and a capsule of dense connective tissue maintain proper alignment of the bones. The capsule encloses a sealed joint cavity, containing a clear, viscous liquid called synovial fluid. The joint cavity is lined, not by epithelium, but by a specialized connective tissue called the synovial membrane that extends folds and villi into the joint cavity and produces the lubricant synovial fluid.In different diarthrotic joints, the synovial membrane may have prominent regions with dense connective tissue or fat. The superficial regions of this tissue however are usually well vascularized, with many porous (fenestrated) capillaries. Besides having cells typical of connective tissue proper and a changing population of leukocytes, this area of a synovial membrane is characterized by two specialized cells with distinctly different functions and origins (Figure 8--22): Macrophage-like synovial cells, also called type A cells, are derived from blood monocytes and remove wear-and-tear debris from the synovial fluid. These modified macrophages, which represent approximately 25% of the cells lining the synovium, are important in regulating inflammatory events within diarthrotic joints. Fibroblastic synovial cells, or type B cells, produce abundant hyaluronan and smaller amounts of proteoglycans. Much of this material is transported by water from the capillaries into the joint cavity to form the synovial fluid, which lubricates the joint, reducing friction on all internal surfaces, and supplies nutrients and oxygen to the articular cartilage. The collagen fibers of the hyaline articular cartilage are disposed as arches with their tops near the exposed surface which, unlike most hyaline cartilage, is not covered by perichondrium (Figure 8--23). This arrangement of collagen helps distribute more evenly the forces generated by pressure on joints. The resilient articular cartilage efficiently absorbs the intermittent mechanical pressures to which many joints are subjected.