Bone is a composite tissue consisting of mineral, matrix, cells, and water. The mineral is an analog of the naturally occurring crystalline calcium phosphate, hydroxyapatite. Physiologic mineral crystals, as distinct from geologic apatites, are very small and imperfect, containing fewer hydroxyl groups and many impurities such as carbonate, fluoride, acid phosphate, magnesium, and citrate. The small size of the crystals makes them ideally suited for their function in mineral ion homeostasis, because smaller crystals generally dissolve before larger crystals. Hydroxyapatite crystals also form in tissues that are not normally calcified, for example, in atherosclerotic plaque, soft tissues of some patients with abnormally high circulating calcium or phosphate, and articular cartilage of some patients with degenerative joint diseases (1). These abnormal tissues generally appear distinct from bone because of the larger size of the crystals and the nature of the matrices upon which the crystals are deposited.

In addition to serving as a source for calcium, magnesium, and phosphate ions, the mineral crystals in bone provide strength and rigidity to the matrix upon which they are deposited. The second major function of bone is mechanical. Bone provides protection for internal organs and facilitates mobility. The bone matrix is essentially type I collagen. The collagen fibrils are arranged in the extracellular matrix in patterns related to the function of the tissue in which they are found. The unique triple helical structure of collagen provides strength and flexibility to most of the connective tissues (2). The mineral crystals add extra rigidity to the collagen fibers. Collagen is stabilized by cross-links formed post-translationally in the extracellular matrix. The nature of these cross-links differs in the mineralized and non-mineralized connective tissues (3). Analyses of the cross-links that stabilize bone collagen, as opposed to skin and tendon collagen, provide a useful marker for diseases such as osteoporosis in which breakdown of the matrix is increased (4). In addition to collagen, about 5% of the extracellular matrix of bone is made up of noncollagenous proteins. These proteins play crucial roles in mineral homeostasis, bone metabolism, bone formation, and bone turnover (5,6).

The metabolism, formation, and turnover of bone is governed by cells. Bone has three cell types: osteoblasts, osteocytes, and osteoclasts (F ig. 1). Osteoblasts and osteocytes are derived from the mesenchymal cell lineage and appear to be closely related. Osteoblasts synthesize the bone matrix, while osteocytes, which are enmeshed in an existing bone matrix, appear to be more important in conveying nutrition and information throughout bone. Osteoclasts, multinucleated giant cells believed to be of macrophage origin, are responsible for removing bone. In normal tissues, the functions of osteoblasts and osteoclasts are coupled such that signals from one affect the other (7). The distribution of these bone cells and their relative activities vary with type of bone, age, and disease state. Similarly, distributions of mineral and matrix proteins differ in various bones, and these also change with disease and age.

Bone formation occurs through two distinct processes: endochondral ossification, in which bone replaces a cartilage model, and intramembranous ossification, in which bone forms directly. During embryonic development, the long bones are formed by the former process and the bones of the skull by the latter.

During endochondral ossification, mesenchymal cells differentiate into chondrocytes. These chondrocytes produce a cartilage anlage that is modified to facilitate mineralization, vascular invasion, and replacement by bone. As they mature, chondrocytes change shape switch from making colla en found in all cartilage ( e and IX) to type X collagen, a form unique to the "hypertrophic" chondrocytes at the interface of cartilage and bone. Although the precise function of type X collagen is not known, defects in its production have been found in several families with various forms of spondylepiphyseal dysplasias, a disease characterized by extreme curvature of the spine (kyphosis), implying that type X collagen has a role in stabilizing the endochondral (8). Transgernc mice expressing an abnormal type X collagen develop lymphocytopenia in addition to skeletal kyphosis (9), indicating that type X collagen may have some other, yet-to-be determined function in immunoregulation in addition to preparing the calcifying cartilage matrix for bone formation.

A second alteration that occurs in the cartilaginous matrix as it matures and prepares for calcification is a modification of proteoglycan structure. Several different types of proteoglycans are present in the developing epiphyseal plate. The large chondroitin sulfate/keratan sulfate molecules (aggrecan) associate with hyaluronic acid (hyaluronan) to form high-molecular weight, space-filling molecules (aggregates) that expand to 50 times their volume. The related nonaggregating molecules found in a variety of connective tissues (versican) and a unique but related nonaggregating molecule (epiphysican) may have similar functions. In addition, there is a smaller dermatan sulfate-containing molecule with only one glycosaminoglycan chain, called decorin, that trims the surface of collagen fibrils, and biglycan, a related molecule with two component glycosaminoglycan chains per core protein. Chondroitin sulfate analogs of these dermatan sulfate proteoglycans are found in bone and tendon. The large cartilage proteoglycans (aggrecan and its aggregates and versican) function to keep the matrix hydrated and prevent calcification. The smaller proteoglycans are believed to regulate collagen fibril formation and, because of their ability to bind growth factors, to play a key role in cartilage metabolism. Aggrecan and proteoglycan aggregates in solution are effective inhibitors of mineral crystal growth and formation, a finding that may explain the observation that in severe osteoarthritis, calcification occurs around chondrocytes where the proteoglycans have been degraded. As the hypertrophic cartilage is modified, mineralization commences. Initial calcification in cartilage occurs in association with both collagen and extracellular membrane-bound bodies known as matrix vesicles (Fig. 3C-3). These vesicles are enriched in en zymes that facilitate both the transport of calcium and phosphate ions needed for mineralization into the vesicle and the degradation of the matrix around the vesicle. These vesicles thus provide a protected environment in which ions can accumulate and in which initial mineral deposition can occur in the absence of inhibitors. Mineral crystals break through the vesicles and may fuse with collagen-based mineral as cartilage calcification proceeds (10).

Vascular invasion of the calcified cartilage mediated by growth factors and replacement of the underlying matrix by one containing type I collagen results in the replacement of the calcified cartilage by bone. Osteoblasts produce this bone, sequentially laying down an underlying fibrous network (consisting of fibronectin and vitronectin), type I collagen, and a variety of matrix proteins. Mineral deposition then occurs by processes described below. Once the bone is formed, it remains in a dynamic state, remodeling aim to provide maximum strength with minimum mass (Wolff's law), to allow growth , and to provide a source of mineral ion homeostasis. The remodeling processes (formation and resorption) are linked so that factors produced by bone-forming cells (osteoblasts) activate remodeling cells (osteoclasts) and vice versa (7). Table 3C-1 provides a partial list of the hormones and growth factors implicated in the regulation of the bone-forming and bone-resorbing cells. The receptors for many of these factors have been identified. Many of these receptors activate kinases, which in turn activate DNA-regulating proteins altering protein synthesis or enzymes that modulate matrix proteins.

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9. Jacenko O, LuValle PA, Olsen BR: Spondyloepiphyseal dysplasia in mice carrying a dominant negative mutation in a matrix protein specific for cartilage-to-bone transition. Nature 365:56-61,1993

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