8. Bone

Tartrate-resistant acid phosphatase (TRAP)

the 35 kDa iron-containing_________, is used clinically as a marker of osteoclast activity and diff erentiation.

bone lining cells

flat cells found on bone surfaces where bone remodeling is not going on. Bone-lining cells on external bone surfaces are called periosteal cells, and those lining internal bone surfaces are often called endosteal cells

Acromegaly

results from an excess of pituitary growth hormone in adults. It is characterized by very thick bones in the extremities and in portions of the facial skeleton.

Osteopetrosis

unlike osteoporosis, is a genetic disorder affecting osteoclasts so that they do not possess ruffled borders; therefore, these osteoclasts cannot resorb bone, which creates an imbalance between bone formation and bone resorption. Thus, persons with osteopetrosis display increased bone density. This condition leads to anemia because of decreased marrow space, blindness, deafness, and damage to the c ranial nerves as the foramina of the skull become narrow and impinge on the nerves

Secondary centers of ossification develop at the epiphyses in a sequence of events similar to that described for the primary center, except a bone collar is not formed.

(1 ) Development of these centers begins when osteoprogenitor cells invade the epiphysis and differentiate into osteoblasts, which elaborate bone matrix to replace the disintegrating cartilage. When the epiphyses are filled with bone tissue, cartilage remains in two areas, the articular surfaces and the epiphyseal plates. (2) Articular cartilage persists and does not contribute to bone formation. (3) Epiphyseal plates continue to grow by adding new cartilage at the epiphyseal end while it is being replaced with bone at the diaphyseal end (lengthening the bone). (4) Ossification of the epiphyseal plates and cessation of growth occur at about 20 years of age.

Bone resorption (Figure 7.4) involves the following events:

(1 ) Osteoclasts secrete acid, which decalcifies the surface layer of bone. (2) Acid hydrolases, collagenases, and proteolytic enzymes, and the acid environment within Howship lacuna release the mineral content of bone. Cathepsin K secreted by the osteoclasts is released into Howship lacuna to degrade the organic portion of the bone. (3) Osteoclasts resorb the organic and inorganic residues of the bone matrix and release them into connective tissue capillaries. (4) At the conclusion of the bone resorption function, osteoclasts undergo apoptosis. (5) Increases in PTH stimulate osteoclastic activity, thereby promoting bone resorption, whereas calcitonin, secreted by C cells (parafollicular cells) in the thyroid gland, inhibits osteoclastic activity. However, evidence indicates that PTH, under certain conditions, may stimulate bone formation although the mechanism for this is not known.

Three functional states, each with a characteristic morphology, have been identified based on the appearance of osteocytes in electron micrographs:

• Quiescent osteocytes exhibit a paucity of rER and a markedly diminished Golgi apparatus (Fig. 8.12a). An osmiophilic lamina representing mature calcified matrix is seen in close apposition to the cell membrane. • Formative osteocytes show evidence of matrix deposition and exhibit certain characteristics similar to those of osteoblasts. Thus, the rER and Golgi apparatus are more abundant, and there is evidence of osteoid in the pericellular space within the lacuna (Fig. 8.12b). • Resorptive osteocytes, like formative osteocytes, contain numerous profiles of endoplasmic reticulum and a well-developed Golgi apparatus. Moreover, lysosomes are conspicuous (Fig. 8.12c). Degradation of bone by MMPs secreted by the resorptive osteocytes previously was called osteocytic osteolysis. The current concept of osteocytic remodeling is that the lytic role of osteocytes is responsible for calcium and phosphate ion homeostasis.

The four main groups of noncollagenous proteins found in the bone matrix are the following:

• Proteoglycan macromolecules contain a core protein with various numbers of covalently attached side chains of glycosaminoglycans (hyaluronan, chondroitin sulfate, and keratan sulfate). They contribute to the compressive strength of bone. They are also responsible for binding growth factors and may inhibit mineralization. Proteoglycans are described in detail in Chapter 6 (Table 6.3, page 172). • Multiadhesive glycoproteins are responsible for attachment of bone cells and collagen fibers to the mineralized ground substance. Some of the more important glycoproteins are osteonectin, which serves as a glue between the collagen and hydroxyapatite crystals; podoplanin (E11), which is produced exclusively by osteocytes in response to mechanical stress; dentin matrix protein (DMP), which is critical for bone matrix mineralization; and sialoproteins such as osteopontin (known as BSP-1), which mediates attachment of cells to bone matrix, and BSP-2, which mediates cell attachment and initiates calcium phosphate formation during the mineralization process. • Bone-specific, vitamin K-dependent proteins, including osteocalcin, which captures calcium from the circulation and attracts and stimulates osteoclasts in bone remodeling; protein S, which assists in the removal of cells undergoing apoptosis; and matrix Gla-protein (MGP), which participates in the development of vascular calcifi cations. • Growth factors and cytokines, which are small regulatory proteins such as insulin-like growth factors (IGFs), tumor necrosis factor a (TNF-a), transforming growth factor b (TGF-b), platelet-derived growth factors (PDGFs), bone morphogenic proteins (BMPs), sclerostin (BMP antagonist), and interleukins (IL-1, IL-6). The most unique members of this group are BMPs because they induce the differentiation of mesenchymal cells into osteoblasts, the bone-producing cells. Recombinant human BMP-7, also known as osteogenic protein-1 (OP-1), is now used clinically to induce bone growth after bone surgery involving large bone defects, spinal fusions, or implantation of graft materials.

The primary center of ossification develops at the midriff of the diaphysis of the hyaline cartilage model, containing type II collagen, by the following sequence of events:

(1 ) Vascularization of the perichondrium at this site causes the transformation of chondrogenic cells to osteoprogenitor cells, which differentiate into osteoblasts. This region of the perichondrium is now called the periosteum. (2) Osteoblasts elaborate matrix deep to the periosteum, and via intramembranous bone formation, form the subperiosteal bone collar. (3) Chondrocytes within the core of the cartilaginous model undergo hypertrophy and degenerate, and their lacunae become confluent, forming large cavities that eventually become marrow spaces. (4) Osteoclasts create perforations in the bone collar that permit the periosteal bud (blood vessels, osteoprogenitor cells, and mesenchymal cells) to enter the newly formed spaces in the cartilaginous model. The cartilage that constitutes the walls of these spaces then becomes calcified. (5) Newly developed osteoblasts elaborate bone matrix that becomes calcified on the surface of the calcified cartilage, forming a calcified cartilage-calcified bone complex. In histological sections, the calcified cartilage stains basophilic, whereas the calcified bone stains acidophilic. (6) The subperiosteal bone collar becomes thicker and elongates toward the epiphysis. (7) Osteoclasts begin to resorb the calcified cartilage-calcified bone complex, thus enlarging the primitive marrow cavity. (8) Repetition of this sequence of events results in bone formation spreading toward the epiphyses.

Gross observation of cross sections of bone reveals two types:

1. Gross observation 2. Microscopic observation

Organization of lamellae

1. Haversian systems (osteons) are long cylindrical structures that run approximately parallel to the long axis of the diaphysis. a. Haversian systems are composed of 4 to 20 lamellae surrounding a central haversian canal, which contains blood vessels, nerves, and loose connective tissue. They are lined by osteoprogenitor cells and osteoblasts. b. They are often surrounded by an amorphous cementing substance. c. They are interconnected by Volkmann canals, which also connect to the periosteum and endosteum and carry the neurovascular supply. 2. Interstitial lamellae are irregularly shaped lamellae between haversian systems. They are remnants of remodeled haversian systems. 3. Outer and inner circumferential lamellae are located at the external and internal surfaces of the diaphysis, respectively (Figure 7.6).

Osteomalacia (rickets of adults) results from calcium deficiency.

1. It is characterized by deficient calcification in newly formed bone and decalcification of already calcified bone. 2. This disease may be severe during pregnancy because the calcium requirements of the fetus may lead to calcium loss from the mother.

Role of hormones in bone formation

1. PTH activates osteoblasts to secrete osteoclast-stimulating factor, which then activates osteoclasts to resorb bone, thus elevating blood calcium levels. Excess PTH (hyperparathyroidism) renders bone more susceptible to fracture and subsequent deposition of calcium in arterial walls and certain organs, such as the kidney. 2. Calcitonin is produced by parafollicular cells ( C cells) of the thyroid gland. It eliminates the ruffled border of osteoclasts and inhibits bone matrix resorption, preventing the release of calcium. 3. Pituitary growth hormone (somatotropin) is produced in the pars distalis of the pituitary gland. It stimulates overall growth, especially that of epiphyseal plates, and influences bone development via insulin-like growth factors (somatomedins), especially stimulating growth of the epiphyseal plates. Children deficient in this hormone exhibit dwarfism, whereas adults with an excess of somatotropin in their growing years display pituitary gigantism and acromegaly.

Repair of a bone fracture. A bone fracture damages the matrix, bone cells, and blood vessels in the region and is accompanied by localized hemorrhaging and blood clot formation.

1. Proliferation of osteoprogenitor cells occurs in the periosteum and endosteum in the vicinity of the fracture. As a result of this proliferation, cellular tissue surrounds the fracture and penetrates between the ends of the damaged bone. 2. Formation of a bony callus occurs both internally and externally at a fracture site. a. Fibrous connective tissue and hyaline cartilage are formed in the fracture zone. b. Endochondral bone formation replaces the cartilage with primary bone. c. Intramembranous bone formation also produces primary bone in the area. d. The irregularly arranged trabeculae of primary bone join the ends of the fractured bone, forming a bony callus. e. The primary bone is resorbed and replaced with secondary bone as the fracture heals.

Role of vitamins in bone formation

1. Vitamin D is necessary for absorption of calcium from the small intestine. Vitamin D deficiency results in poorly calcified (soft) bone, a condition known as rickets in children and osteomalacia in adults. Vitamin D is also necessary for bone formation (ossification), whereas an excess of vitamin D causes bone resorption. 2. Vitamin A deficiency inhibits proper bone formation and growth, whereas an excess accelerates ossification of the epiphyseal plates. Deficiency or excess of vitamin A results in small stature. 3. Vitamin C is necessary for collagen formation. Deficiency results in scurvy, characterized by poor bone growth and inadequate fracture repair.

Bone Repair.

After severe injury where segments of bone have been lost or must be removed, the remaining bone is prevented from forming a bony union followed by a bony callus that over time would result in completed bone repair. When the bony union is not possible, a bone graft is required. For this purpose, bone fragments that a restored frozen to maintain osteogenic potential may then may be utilized in bone grafts.

Schematic drawing of cells associated with bone

All cells except osteoclasts originate from the mesenchymal stem cells, which diff erentiate into osteoprogenitor cells, osteoblasts, and fi nally osteocytes and bone-lining cells. Bone-lining cells on external bone surfaces are part of the periosteum, hence the term periosteal cells. Bone-lining cells on internal bone surfaces are frequently called endosteal cells. Note that osteoprogenitor cells and bone-lining cells have a similar microscopic appearance and are often diffi cult to distinguish from each other. Osteoclasts originate from hemopoietic progenitor cells, which diff erentiate into bone-resorbing cells. The specifi c details of osteoclast diff erentiation are illustrated in Figure 8.15.

Bone remodeling

Bone is constantly being remodeled as necessary for growth and to alter its structural makeup to adapt to changing stresses in the environment throughout life. 1. Early on, bone development outpaces bone resorption as new haversian systems are added and fewer are resorbed. 2. Later, when the epiphyseal plates are closed, ending bone growth, bone development and resorption are balanced. Several factors, including calcitonin and PTH, are responsible for this phenomenon regarding compact bone (see Section II J). Remodeling of cancellous bone is under the control of many factors within the bone marrow.

Morphology osteoclasts

Osteoclasts activated by an osteoclast-stimulating factor produced and released by osteoblasts display four regions in electron micrographs. (1 ) Basal zone is that part of the osteoclast housing most of the organelles and is the farthest from the subosteoclastic compartment. (2) The ruffled border is the site of active bone resorption. It is composed of irregular fingerlike cytoplasmic projections extending into the subosteoclastic compartment, a slight depression that deepens as the osteoclast resorbs bone, and then that depression is referred to as Howship lacuna. The ruffled border of an inactive osteoclast is collapsed as the cell is in a resting stage. (3) The clear zone surrounds the ruffled border. It contains actin filaments at the periphery that help osteoclasts maintain contact with the bony surface. This isolates and seals the region of osteolytic activity. Further, osteopontin, secreted by osteoblasts, is used to seal the zone between osteoclasts and the subosteoclastic compartment. (4) The vesicular zone contains exocytotic vesicles that transfer lysosomal enzymes to Howship lacunae and endocytotic vesicles that transfer degraded bone products from Howship lacunae to the interior of the cell.

Osteons

The Haversian systems consist of concentric lamellae (sing., lamella) of bone matrix surrounding a central canal, the osteonal (Haversian) canal, which contains the vascular and nerve supply of the osteon. Canaliculi containing the processes of osteocytes are generally arranged in a radial pattern with respect to the canal (Plate 11, page 244). The system of canaliculi that opens to the osteonal canal also serves for the passage of substances between the osteocytes and blood vessels. Between the osteons are remnants of previous concentric lamellae called interstitial lamellae (see Fig. 8.3). Because of this organization, mature bone is also called lamellar bone.

The newly formed osteoclast must be activated to become a bone-resorbing cell. During this process, it becomes highly polarized. When actively resorbing bone, osteoclasts exhibit three specialized regions:

The ruffled border The clear zone (sealing zone) The basolateral region

Heterographs

a re the least successful because the donor bone comes from another species. However, calf bone that has been frozen can serve as a viable bone g raft when necessary.

Intramembranous bone formation (Figure 7.7) is the process by which most of the flat bones (e.g., parietal bones of the skull) are formed. It involves the following events:

a. Mesenchymal cells, in the presence of a vascular zone, condense into primary ossification centers, differentiate into osteoblasts, and begin secreting osteoids in a rather haphazard form, known as woven bone. b. As appositional bone growth continues and calcification occurs, osteoblasts become trapped in their own matrix and become osteocytes. These centers of developing bone are called trabeculae (fused spicules). c. Fusion of the bony trabeculae produces spongy bone as blood vessels invade the area and other undifferentiated mesenchymal cells become hematopoietic cells forming blood cells of the bone marrow. d. The periosteum and endosteum develop from portions of the mesenchymal layer that do not undergo ossification. e. Mitotic activity of the mesenchymal cells gives rise to osteoprogenitor cells, which undergo cell division and form more osteoprogenitor cells or differentiate into osteoblasts within the inner layer of the developing periosteum. f. Finally, intramembranous bone may then be converted to lamellar bone.

Osteoblasts

a. Osteoblasts are derived from osteoprogenitor cells under the influence of members of the BMP family and also transforming growth factor-b. They possess receptors for PTH (see Chapter 13 V). Osteoblasts are responsible for the synthesis of organic protein components of bone matrix, including type I collagen, proteoglycans, and glycoproteins, which they secrete as osteoid (uncalcified bone matrix), which is mineralized under the influence and control by the osteoblasts. Additionally, they produce macrophage colony stimulating factor (M-CSF), a receptor for the activation of nuclear factor kappa B (RANKL), osteoprotegerin, osteocalcin (for bone mineralization), osteopontin (for formation of sealing zone between osteoclasts and the subosteoclastic compartment), osteonectin (related to bone mineralization), and bone sialoprotein (binding osteoblasts to extracellular matrix). b. On bony surfaces, they resemble a layer of cuboidal, basophilic cells as they secrete organic matrix (see Figure 6.1). c. They possess cytoplasmic processes with which they contact the processes of other osteoblasts and osteocytes and form gap junctions. d. When synthetically active, they have a well-developed RER and Golgi complex. e. These cells become entrapped in lacunae but maintain contact with other cells via their cytoplasmic processes. Entrapped osteoblasts are known as osteocytes.

Osteocytes

a. Osteocytes are mature bone cells housed in their own lacunae. b. They have narrow cytoplasmic processes that extend through canaliculi in the calcified matrix (see Figures 6.1 and 7.3). c. They maintain communication with each other via gap junctions between their processes. d. They are nourished and maintained by nutrients, metabolites, and signal molecules carried by the extracellular fluid that flows through the lacunae and canaliculi. In addition, calcium released from bone enters the extracellular fluid located within these spaces. e. They contain abundant heterochromatin, a paucity of RER, and a small Golgi complex.

Osteoclast cytoplasm is

acidophilic. Osteoclasts function in the resorption of bone (osteolysis). They form and reside in depressions known as Howship lacunae, which represent areas of bone resorption.

Calcification of bone is not clearly understood, however.

a. Osteonectin, proteoglycans, and bone sialoprotein are known to stimulate calcification. b. Bone matrix contains high concentrations of calcium Ca2+ along with several other organic compounds and enzymes. Osteocalcin and sialoproteins further concentrate the calcium, resulting in osteoblasts secreting alkaline phosphatase, thus concentrating P043- ions, which further concentrates the calcium ions. Small matrix vesicles are released into the bone matrix from the osteoblasts, resulting in crystallization of calcium phosphate within the matrix vesicles. c. Calcium pumps in the matrix vesicle membranes bring in more calcium, concentrating it and forming calcium hydroxyapatite crystals that grow and eventually puncture the matrix vesicle expelling its contents. d. Calcium hydroxyapatite crystals that become free in the matrix become nidi of crystallization. e. Released enzymes free phosphate ions that unite with the calcium forming calcium phosphate. f. Calcium phosphate then begins to calcify the matrix around the nidi of crystallization. g. Water is removed from the matrix, permitting hydroxyapatite crystals to be deposited into gaps within the collagen fibrils. h. Nidi of mineralization enlarge and fuse with neighboring nidi, eventually calcifying the entire matrix.

Microscopic observation of bone reveals two types:

a. Primary bone, also known as immature or woven bone (1) Primary bone contains many osteocytes and large, irregularly arranged type I collagen bundles. (2) It has a low mineral content. (3) It is the first compact bone produced during fetal development and bone repair. (4) It is remodeled and replaced by secondary bone except in a few places (e.g., tooth sockets, near suture lines in skull bones, and at insertion sites of tendons). b. Secondary bone, also known as mature or lamellar bone (1) Secondary bone is the compact bone of adults. (2) It has a calcified matrix arranged in regular layers, or lamellae. Each lamella is 3 to 7 um thick. (3) It contains osteocytes in lacunae between, and occasionally within, lamellae.

Gross observation (Figure 7.3) of cross sections of bone reveals two types:

a. Spongy (cancellous) bone, which is composed of interconnected trabeculae. Bony trabeculae surround cavities filled with bone marrow. The trabeculae contain osteocytes and are lined on both surfaces by a single layer of osteoblasts. Spongy bone is always surrounded by compact bone. b. Compact (dense) bone has no trabeculae or bone marrow cavities.

Bone matrix

a. The inorganic (calcified) portion of the bone matrix (about 65% of the dry weight) is composed of calcium, phosphate, bicarbonate, citrate, magnesium, potassium, and sodium. It consists primarily of hydroxyapatite crystals, which have the composition Ca10(P04)6(OH)2• b. The organic portion of the bone matrix (about 35% of the dry weight) consists primarily of type I collagen (95%) and a minor contribution of type V collagen. It has a ground substance that contains chondroitin sulfate, keratan sulfate, and hyaluronic acid. (1) Osteocalcin, stimulated by vitamin D, inhibits osteoblast function, and osteopontin, both glycoproteins, binds to hydroxyapatite and to integrins on osteoblasts and osteoclasts. Osteonectin, produced mostly by osteoblasts, is located in bone undergoing remodeling. Cytokines, growth hormones, and bone morphogenic proteins (BMPs), among others, contribute to the bone matrix. (2) Bone sialoprotein is a matrix protein that also binds to integrins of the osteoblasts and osteocytes and is thus related to the adherence of bone cells to the bone matrix.

Zones of the epiphyseal plates are histologically distinctive and arranged in the following order:

a. The zone of reserve cartilage is at the epiphyseal side of the plate. It possesses small, randomly arranged inactive chondrocytes. b. The zone of proliferation (of chondrocytes) is a region of rapid mitotic divisions giving rise to rows of isogenous cell groups. c. The zone of cell hypertrophy and maturation is the region where the chondrocytes are greatly enlarged. d. The zone of calcification is the region where hypertrophied chondrocytes die and the cartilage becomes calcified. e. The zone of ossification is the area where newly formed osteoblasts elaborate bone matrix on the calcified cartilage, forming a calcified cartilage-calcified bone complex, which is resorbed and replaced by bone.

Osteoprogenitor cells

a. These spindle-shaped cells are derived from embryonic mesenchyme and are located in the periosteum and the endosteum, and persist throughout life as stem cells that line bone. They can be activated later for the bone repair of fractures or other repair. b. They are capable of differentiating into osteoblasts. However, at low oxygen tensions, they may change into chondrogenic cells.

Overview. Osteoclasts

are large, motile, multinucleated cells (up to 50 nuclei) that resorb bone. They are derived from cells of the mononuclear phagocyte system, comprising blood-borne monocytes that enter the connective tissue spaces where they differentiate into various types of macrophages and osteoclasts (see Section (3)). (1 ) Osteoclasts possess cell surface receptors: colony-stimulating factor-1 receptor, calcitonin receptor, and RANK (nuclear factor kappa B). (2) Osteoblasts that have been stimulated by PTH promote osteoclast formation, whereas osteoblasts that have been stimulated by calcitonin inhibit osteoclast formation by stimulating osteoid synthesis and calcium deposition. (3) Via a series of three osteoblast signals, osteoclast precursors (macrophages) are stimulated by M-CSF to undergo mitosis. Another signaling molecule, RANKL, binds to the precursor, inducing it to differentiate into the multinucleated osteoclast, thus activating it to commence bone resorption. A third signal, osteoprotegerin (OPG), a member of the tumor necrosis factor receptor (TNFR) family produced by osteoblasts and other cells, can prohibit RANKL from binding to the macrophage, thus prohibiting osteoclast formation.

Autographs

are most successful since the bone donor is the recipient.

Homographs

are of bone donated from a different individual.

Intramembranous Ossification

bone formation is initiated by condensation of mesenchymal cells that differentiate into osteoblasts. The first evidence of intramembranous ossification is seen around the eighth week of human gestation within embryonic connective tissue, the mesenchyme. Some of the spindle-shaped, pale-staining mesenchymal cells migrate and aggregate in specific areas (e.g., the region of flat bone development in the head), forming ossification centers. This condensation of cells within the mesenchymal tissue initiates the process of intramembranous ossification (Fig. 8.18a). Mesenchymal cells in these ossification centers elongate and differentiate into osteoprogenitor cells. These cells express CBFA1 transcription factor, which is essential for osteoblast diff erentiation and the expression of genes necessary for both intramembranous and endochondral ossification. The osteoprogenitor cell cytoplasm changes from eosinophilic to basophilic, and a clear Golgi area becomes evident. These cytologic changes result in the differentiated osteoblast, which then secretes the collagens (mainly type I collagen molecules), bone sialoproteins, osteocalcin, and other components of the bone matrix (osteoid). The osteoblasts accumulate at the periphery of the ossification center and continue to secrete osteoid at the center of the nodule. As the process continues, the osteoid undergoes mineralization and the entrapped osteoblasts become osteocytes (Fig. 8.18b). Within the bony matrix, osteocytes increasingly separate from one another as more matrix is produced, but they remain at- tached by thin cytoplasmic processes. With time, the matrix becomes mineralized, and the interconnecting cytoplasmic processes of osteocytes are contained within canaliculi.

Endochondral bone formation (Figure 7.8) is the process

by which long bones are formed. It begins in a segment of hyaline cartilage that serves as a small model for the bone. The two stages of endochondral bone formation involve the development of primary and secondary centers of ossification.

The key factor that triggers differentiation of osteoprogenitor cells is a transcription factor called

core-binding factor alpha-1 (CBFA1) or runt-related transcription factor 2 (RUNX2). This protein prompts the expression of genes that are characteristic of the phenotype of the osteoblast. IGF-1 and IGF-2 stimulate osteoprogenitor cell proliferation and differentiation into osteoblasts. Recently, several studies from clinical practice have demonstrated that pulsed electromagnetic field stimulation has been beneficial in healing bone fractures due to an increase in bone tissue regeneration. These effects related to increased differentiation of osteoprogenitor cells after stimulation with an electromagnetic field. In the future, this approach may be explored as an effective tissue- engineering strategy to treat bone defects in the head, neck, and vertebral column.

Both OPG and RANKL are detected in free form in the blood, and their concentrations can be measured

for diagnostic purposes and to monitor the therapy of many bone diseases. Initially, cells committed to become osteoclasts (osteoclast precursors) express two important transcription factors, c-fos, and NF-kB; later, a receptor molecule called receptor activator of nuclear factor-kB (RANK) is expressed on their surface. The RANK receptor interacts with RANK ligand molecule (RANKL) produced and expressed on the stromal cell surface (Fig. 8.15). The RANK-RANKL signaling mechanism is essential for osteoclast differentiation and maturation. Alternatively, during inflammation, activated T lymphocytes can produce both membrane-bound and soluble RANKL molecules. Therefore, inflammatory processes can stimulate osteoclast-mediated bone resorption. This pathway can be blocked by osteoprotegerin (OPG), which serves as a "decoy" receptor for RANKL. Lack of available ligand affects the RANK-RANKL signaling pathway and acts as a potent inhibitor of osteoclast formation. OPG is produced mainly by osteoblasts and is regulated by many bone metabolic regulators, such as IL-1, TNF , TGF-?, and vitamin D.

basolateral region

functions in the exocytosis of digested material (see Fig. 8.17). Transport vesicles containing degraded bone material endocytosed at the ruffled border fuse here with the cell membrane to release their contents. TRAP has been found within these vesicles, suggesting its role in the fragmentation of endocytosed material.

Structure of a typical long bone.

have a shaft, called the diaphysis, and two expanded ends, each called an epiphysis (see Fig. 8.2). The articular surface of the epiphysis is covered with hyaline cartilage. The flared portion of the bone between the diaphysis and the epiphysis is called the metaphysis. It extends from the diaphysis to the epiphyseal line. A large cavity filled with bone marrow, called the marrow or medullary cavity, forms the inner portion of the bone. In the shaft, almost the entire thickness of the bone tissue is compact; at most, only a small amount of spongy bone faces the marrow cavity. At the ends of the bone, the reverse is true. Here, the spongy bone is extensive, and the compact bone consists of little more than a thin outer shell (see Fig. 8.1). Elsewhere, periosteum, a fibrous connective tissue capsule, covers the outer surface of the bone.

Osteoporosis

is a disease characterized by low bone mass (low bone mineral density) and structural deterioration of bone tissue, making the bone fragile and susceptible to fracture. Osteoporosis is associated with an abnormal ratio of mineral to matrix. 1. It results from increased bone resorption, decreased bone formation, or both. 2. Estrogen activates bone formation by osteoblasts, and in its absence an imbalance causes osteoclastic activity to render bones fragile and susceptible to fracture. a. Osteoporosis is most common in postmenopausal women because of diminished estrogen secretion and in immobile patients because of lack of physical stress on the bone. b. Estrogen therapy was employed for decades to minimize the onset of osteoporosis. Recently, it was determined that estrogen replacement therapy increases the risk of heart disease, stroke, breast cancer, and blood clots. Now, instead of estrogen, a recent new group of drugs, the bisphosphonates, has been developed that reduces the incidence of osteoporosis. c. Preventive measures include a balanced diet rich in calcium and vitamin D and weight bearing exercise.

periosteum

is a layer of noncalcified connective tissue covering bone on its external surfaces, except at synovial articulations and muscle attachments. a. It is composed of an outer dense fibrous collagenous layer and an inner cellular osteoprogenitor (osteogenic) layer. b. Sharpey fibers (type I collagen) attach the periosteum to the bone surface. c. The periosteum functions to distribute blood vessels to the bone.

Clear zone/Sealing zone

is a ring-like perimeter of cytoplasm adjacent to the ruffled border that demarcates the bone area being resorbed. Essentially, the clear zone is a compartment at the site of the ruffled border where resorption and degradation of the matrix occurs. It contains abundant actin filaments but essentially lacks other organelles. The actin filaments are arranged in a ring-like structure surrounded on both sides by actin-binding proteins such as vinculin and talin (Fig. 8.17). The plasma membrane at the site of the clear zone contains cell and extracellular matrix adhesion molecules that are responsible for providing a tight seal between the plasma membrane and mineralized matrix of the bone. Several classes of integrin extracellular receptors (i.e., avb3 vitronectin receptor, a2b1 type I collagen receptor, or avb1 vitronectin/fibrinogen receptor) help maintain the seal.

endosteum

is a thin specialized connective tissue that lines the marrow cavities and supplies osteoprogenitor cells and osteoblasts for bone growth and repair.

ruffled border

is the part of the cell in direct contact with bone. It contains numerous deep plasma membrane infoldings forming microvillous-type structures responsible for increasing surface area for the exocytosis of hydrolytic enzymes and secretion of protons by ATP-dependent proton pumps, as well as endocytosis of degradation products and bone debris. The ruffled border stains less intensely than the remainder of the cell and often appears as a light band adjacent to the bone at the resorption site (see Fig. 8.14). At the electron microscopic level, hydroxyapatite crystals from the bone substance are observed between the processes of the ruffled border (Fig. 8.16). Internal to the ruffled border and in close proximity are numerous mitochondria and lysosomes. The nuclei are typically located in the part of the cell more removed from the bone surface. In this same region are profiles of rER, multiple stacks of Golgi apparatus, and many vesicles.

Histogenesis of bone

occurs by two processes, intramembranous and endochondral bone formation. Both processes produce bone that appears histologically identical. Bone histogenesis is accompanied by bone resorption. The combination of bone formation and resorption, termed remodeling, occurs throughout life, although it is slower in secondary than in primary bone.

Rickets

occurs in children deficient in vitamin D, which results in calcium deficiency. It is characterized by deficient calcification in newly formed bone and is generally a c companied by deformation of the bone spicules in epiphyseal plates; as a result, bones grow more slowly than normal and are deformed by the stress of weight-bearing.

spicules of bone

tiny, needle-like bits of bone (connect to form trabeculae)