Dense cortical bone and the more reticulated trabecular bone are the two major bone types
Bone consists largely of an extracellular matrix composed of proteins and hydroxyapatite crystals, in addition to a small population of cells. The matrix provides strength and stability. The cellular elements continually remodel bone to accommodate growth and allow bone to reshape itself in response to varying loading stresses. Basically, bone has three types of bone cells. Osteoblasts promote bone formation. Osteoblasts and preosteoblasts are the principal target cells for PTH's action to stimulate bone growth. Osteoclasts promote bone resorption and are found on the growth surfaces of bone. Their activity is increased by cytokines, with RANK ligand being particularly important. Osteocytes are found within the bony matrix and are derived from osteoblasts that have encased themselves within bone. In response to mechanical loading, osteocytes produced both stimulatory and inhibitory cues. These cells stimulate the bone-forming activities of osteoblasts and lining cells by secreting growth factors such as osteocalcin and Wnt ligands. Osteocytes inhibit osteoblast activity by secreting antagonists of Wnt signaling, including sclerostin and dickkopf1. Osteocytes also appear to play a role in the transfer of mineral from the interior of bone to the growth surfaces. Bone remodeling consists of a carefully coordinated interplay of osteoblastic, osteocytic, and osteoclastic activities.
As shown in Figure 52-3, bone consists of two types of bone tissue. Cortical (also called compact or lamellar) bone represents ~80% of the total bone mass. Cortical bone is the outer layer (the cortex) of all bones and forms the bulk of the long bones of the body. It is a dense tissue composed mostly of bone mineral and extracellular matrix elements, interrupted only by penetrating blood vessels and a sparse population of osteocytes nested within the bone. These osteocytes are interconnected with one another and with the osteoblasts on the surface of the bone by canaliculi, through which the osteocytes extend cellular processes. These connections permit the transfer of Ca2+ from the interior of the bone to the surface, a process called osteocytic osteolysis. Dense cortical bone provides much of the strength for weight bearing by the long bones.
FIGURE 52-3 Cortical and trabecular bone. Under the periosteum is a layer of compact cortical bone that surrounds the more reticulated trabecular bone. The fundamental unit of cortical bone is the osteon, a tube-like structure that consists of a haversian canal surrounded by ring-like lamellae. The inset shows a cross section through an osteon. The superficial lining cells surround the osteoblasts, which secrete osteoid, a matrix of proteins that are the organic part of bone. The lining cells are formed from osteoblasts that become quiescent. Osteocytes are osteoblasts that have become surrounded by matrix. Canaliculi allow the cellular processes of osteocytes to communicate, via gap junctions, with each other and with osteoblasts on the surface. Trabecular bone has both osteoblasts and osteoclasts on its surface; this is where most bone remodeling takes place.
Trabecular (or cancellous or medullary) bone constitutes ~20% of the total bone mass. It is found in the interior of bones and is especially prominent within the vertebral bodies. It is composed of thin spicules of bone that extend from the cortex into the medullary cavity (see Fig. 52-3, inset). The lacework of bone spicules is lined in many areas by osteoblasts and osteoclasts, the cells involved in bone remodeling. Trabecular bone is constantly being synthesized and resorbed by these cellular elements. Bone turnover also occurs in cortical bone, but the fractional rate of turnover is much lower. When the rate of bone resorption exceeds that of synthesis over time, the loss of bone mineral produces the disease osteoporosis.
The extracellular matrix forms the nidus for the nucleation of hydroxyapatite crystals
Collagen and the other extracellular matrix proteins that form the protein matrix of bone are called osteoid. Osteoid provides sites for the nucleation of hydroxyapatite crystals, the mineral component of bone. Osteoid is not a single compound, but a highly organized matrix of proteins synthesized principally by osteoblasts. Type I collagen accounts for ~90% of the protein mass of osteoid. It comprises a triple helix of two α1 monomers and one α2 collagen monomer. While still within the osteoblast, monomers self-associate into helical structures. After secretion from the osteoblast, helices associate into collagen fibers; cross-linking of collagen occurs both within a fiber and between fibers. Collagen fibers are arranged in the osteoid in a highly ordered manner. The organization of collagen fibers is important for the tensile strength (i.e., the ability to resist stretch or bending) of bone. In addition to providing tensile strength, collagen also acts as a nidus for nucleation of bone mineralization. Within the collagen fibers, the crystals of hydroxyapatite are arranged with their long axis aligned with the long axis of the collagen fibers.
Several other osteoblast-derived proteins are important to the mineralization process, including osteocalcin and osteonectin. Osteocalcin is a 6-kDa protein synthesized by osteoblasts at sites of new bone formation. 1,25-Dihydroxyvitamin D induces the synthesis of osteocalcin. Osteocalcin has an unusual structure: it possesses three γ-carboxylated glutamic acid residues. These residues are formed by post-translational modification of glutamic acid by vitamin K–dependent enzymes. Like other proteins with γ-carboxylated glutamic acid, osteocalcin binds Ca2+ avidly. It binds hydroxyapatite, the crystalline mineral of bone, with even greater avidity. This observation has led to the suggestion that osteocalcin participates in the nucleation of bone mineralization at the crystal surface. Osteonectin, a 35-kDa protein, is another osteoblast product that binds to hydroxyapatite. It also binds to collagen fibers and facilitates the mineralization of collagen fibers in vitro. Additional proteins have been identified that appear to participate in the mineralization process. For instance, extracellular glycoproteins present in bone may inhibit mineralization and their removal may be necessary for mineralization to occur.
Bone remodeling depends on the closely coupled activities of osteoblasts and osteoclasts
In addition to providing the proteins in osteoid, osteoblasts promote mineralization by exporting Ca2+ and from intracellular vesicles that have accumulated these minerals. Exocytosis of Ca2+ and raises the local extracellular concentration of these ions around the osteoblast to levels that are higher than in the bulk ECF, which promotes crystal nucleation and growth (Fig. 52-4). Bone formation along spicules of trabecular bone occurs predominantly at sites of previous resorption by osteoclasts. The processes of bone resorption and synthesis are thus spatially coupled within an active basic multicellular unit (BMU). In adults, 1 to 2 million BMUs actively remodel bone.
FIGURE 52-4 Bone formation and resorption. PTH and vitamin D stimulate osteoblastic cells to secrete agents that induce stem cells to differentiate into osteoclast precursors, mononuclear osteoclasts, and finally mature, multinucleated osteoclasts. Thus, PTH indirectly promotes bone resorption. Osteoblasts also secrete Ca2+ and inorganic phosphate (Pi), which nucleate on the surface of bone. IL-6R, interleukin-6 receptor.
Vitamin D and PTH stimulate osteoblastic cells to secrete factors—such as macrophage colony-stimulating factor (M-CSF; see p. 431)—that cause osteoclast precursors to proliferate (see Fig. 52-4). These precursors differentiate into mononuclear osteoclasts and then, with further stimulation by RANK ligand (also released by PTH-stimulated osteoblasts), fuse to become multinucleated osteoclasts. Osteoclasts resorb bone in discrete areas in contact with the “ruffled border” of the cell (Fig. 52-5). The osteoclast closely attaches to the bone matrix when integrins on its membrane attach to vitronectin in the bone matrix. The osteoclast—in reality a one-cell epithelium—then secretes acid and proteases across its ruffled border membrane into a confined resorption space (the lacuna). The acid secretion is mediated by a V-type H pump (see pp. 118–119) and the ClC-7 Cl− channel at the ruffled border membrane. Abundant intracellular carbonic anhydrase provides the H+. Cl-HCO3 exchangers, located in the membrane that faces the blood, remove the formed as a byproduct by the carbonic anhydrase. The acidic environment beneath the osteoclast dissolves bone mineral and allows acid proteases to hydrolyze the exposed matrix proteins. Having reabsorbed some of the bone in a very localized area, the osteoclast moves away from the pit or trough in the bone that it has created. Osteoblastic cells replace the osteoclast and now build new bone matrix and promote its mineralization.
FIGURE 52-5 Bone resorption by the osteoclast. The osteoclast moves along the surface of bone and settles down, sealing itself to the bone via integrins that bind to vitronectins on the bone surface. The osteoclast reabsorbs bone by secreting H+ and acid proteases into the lacuna. Thus, the osteoclast behaves as a one-cell epithelium. The acid secretion is mediated by a V-type H pump and ClC7 Cl− channel at the ruffled border membrane facing the lacuna. Carbonic anhydrase (CA) in the cytosol supplies the H+ to the H pump and also produces as a byproduct. Cl-HCO3 exchangers—located on the membrane opposite the ruffled border—remove this . AC, adenylyl cyclase; IL-6R, interleukin-6 receptor; PKA, protein kinase A; TRAP, tartrate-resistant acid phosphatase.
RANK ligand (RANKL), previously called osteoprotegerin ligand, appears to be a major stimulator of both the differentiation of preosteoclasts to osteoclasts (see Fig. 52-4) and the activity of mature osteoclasts (see Fig. 52-5). RANKL is a member of the tumor necrosis factor (TNF) cytokine family and exists both as a membrane-bound form (mRANKL) on the surface of stromal cells and osteoblasts, and as a soluble protein (sRANKL) secreted by these same cells. RANKL binds to and stimulates a membrane-bound receptor of the osteoclast called RANK (receptor for activation of nuclear factor κB), a member of the TNF receptor family. The interaction is essential for the formation of mature osteoclasts.
The activity of RANKL is under the control of a soluble member of the TNF receptor family called osteoprotegerin (OPG; from the Latin osteo [bone] + protegere [to protect]). Like RANKL, OPG is produced by osteoblastic and stromal cells (see Fig. 52-4). By scavenging RANKL, OPG limits osteoclastogenesis, thereby protecting bone from osteoclastic activity. The precise role of RANKL, RANK, and OPG in the development of various forms of osteoporosis (diminished bone density) and osteopetrosis (increased bone density) is only beginning to be understood. However, the balance between OPG and RANKL production by the osteoblast/stromal cell appears to be a very important factor in the development of osteoporosis from either estrogen deficiency or glucocorticoid excess. In both cases, RANKL production rises and OPG production falls. In 2010, the U.S. Food and Drug Administration approved denosumab, a humanized monoclonal antibody against RANKL, for the treatment of postmenopausal osteoporosis.
In rare human monogenic syndromes, defects of the Wnt signaling system are associated with marked increases or decreases in bone mass. Moreover, targeted disruption of specific Wnt antagonists in mice reveals the major role of Wnt in osteoblastogenesis and in modulation of the activity of both osteoblasts and osteocytes. Wnt increases the differentiation of mesenchymal stem cells and preosteoblasts, thereby increasing bone-forming capacity. In addition, Wnt increases the production of OPG, which competes with RANKL and thereby decreases osteoclastogenesis. New therapies to treat or prevent bone loss based on manipulation of Wnt signaling offer substantial promise.