Bone is efficient. While forming and remodeling, mass is kept to a minimum. Bone is divided into two structural types: cortical (bark-like) and trabecular. Cortical bone is dense, with its collagen aligned in the direction of applied forces; the cortex of bone is found at the periphery and is responsible for skeletal homeostasis.
Whereas osteoblasts and osteocytes are responsible for the production of bone, osteoclasts break it down. Osteoclasts are large cells, derived from stem cells in the bone marrow that give rise to the monocytemacrophage cell line. Macrophages and osteoclasts, both formed by the fusion of monocyte precursors, are multinucleated.
Osteocytes, the cells of established bone, are derived from osteoblasts. In fact, an osteocyte is simply an osteoblast that has surrounded itself with bone matrix. Of course, the morphology of the cell changes: because the osteocyte is less metabolically active, it contains fewer organelles, and the nucleus occupies a greater proportion of the entire cell body.
Osteoblasts are bone-forming cells. Although this is a useful definition for the purpose of discussion, osteoblasts do not actually form complete bone. Rather, they synthesize the organic (ie, nonmineral) component of bone, the primary element of which is osteoid (a protein matrix of type I collagen).
Three types of cells, osteoblasts, osteocytes, and osteoclasts, are essential to the development of bone and its dynamic remodeling (ie, the process of bone resorption and deposition that releases and deposits ions, adapts the bone to new loads, and repairs microscopic damage).
At the physis, chondrocytes proliferate, ma- ture, and secrete extracellular matrix, which eventually ossifies. The physis is polar: mov- ing away from the epiphysis and joint sur- face toward the metaphysis, there is increas- ing maturation of the cells. Just below the epiphysis is a zone of immature resting cells; just above the metaphysis, calcification takes place in the setting of programmed cell death (apoptosis) of the hypertrophic chondro- cytes. New bone formation thus occurs at the point farthest from the articular surface, at the junction between the physis and the metaphysis. This region is named the zone of provisional calcification.
Bone forms by one of two mechanisms: en- dochondral or intramembranous ossifica- tion. Endochondral ossification is the most common form, producing the long bones; “long” refers to the longitudinal orientation of the bone. (Even the distal phalanx of the fifth toe is a long bone.) The flat bones, such as the skull and clavicle, form via intramem- branous ossification. The key distinction be- tween the two processes is that endochon- dral ossification employs a cartilage template whereas intramembranous ossification does not.
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