Bone is a specialized connective tissue that comprises most of the skeleton, which supports the entire body. Unlike cartilage, bone tissue is extremely hard and inelastic. This allows the long bones to serve as levers on which muscles act to hold the body upright and move it through space. Bones also provide protective sites for housing the central nervous system (the brain and the spinal cord) as well as the hematopoietic tissue, which is responsible for forming blood cells. The extracellular matrix (ECM) of bone is hardened by the deposition of calcium salts. Bone is thus a reservoir of Ca2+, allowing it to play an important role in regulating the Ca2+ concentration in the blood, a vital activity. Bones begin forming early in embryogenesis and continue growing for about 20 years after birth. Although bone is hard, it does not become a static tissue after growth stops. Bone tissue is turned over by a mechanism that maintains the shape and strength of bone even while portions are being removed and replaced every day. Bone fractures are initially repaired by processes that recapitulate those used in early development, and the restored tissue then is returned to the normal adult form by the continuous activity of turnover. Features of the structure, development, and physiology of bone are intertwined, which complicate a simple understanding of this interesting tissue.
Bone is a connective tissue; therefore, the various resident bone cells and the ECM are its key elements. However, bone is surprisingly complicated and to understand it fully, one must also consider interrelated features of its gross and microscopic organization, development, and physiology.
Bone is produced, maintained, and turned over by the following four distinct resident cells:
Osteoprogenitor cells. Fibroblast-like stem cells that divide to become osteoblasts. In adult bone, osteoprogenitor cells are quiescent, except in the repair of bone fractures or during the normal, localized turnover of bone, when they produce osteoblasts.
Osteoblasts. Responsible for the synthesis and secretion of the organic components of bone matrix, called osteoid, and its initial mineralization. Osteoblasts become osteocytes when they encase themselves in this matrix (Figure 4-1A). Osteoblasts are nonmitotic and can live for a few months and then either die, become quiescent bone-lining cells, or remain as long-lived osteocytes when trapped in bone matrix.
Osteocytes. The main resident cells of bone. Osteocytes sit in lacunae in the matrix that they maintain. These cells project long, thin processes that link osteocytes together via gap junctions by coursing through small channels, or canaliculi (Figure 4-1B). Osteocytes can live 20 to 30 years.
Osteoclasts. Large, multinucleated cells responsible for the removal and turnover of bone matrix. Osteoclasts are derived from the same stem cells that produce macrophages; they are formed by the fusion of many cells ...