Remodeling involves resorption of bone from one surface and deposition of new bone on another closely juxtaposed surface. It occurs on all bone surfaces, including the periosteal, haversian, cortical, and endosteal. This dynamic equilibrium is achieved through the coupling of packets of specialized, interacting bone cells called remodeling units. Systemic hormones and local paracrine factors, as well as gravity and weight bearing, are known to regulate the remodeling process. Other cells in contact with the inner, or endosteal, bone surface that modulate the activities of osteoblasts and osteoclasts include monocytes, macrophages, and fibroblasts.
Osteoblasts are the bone-forming cells.
They synthesize the organic matrix and are important in the process of mineralization. Although its primary structure is similar to the type I collagen found in skin, the type I collagen in bone has secondary posttranslational modifications that chemically distinguish it from skin collagen. In the process of generating the extracellular organic matrix, osteoblasts become trapped within it and become osteocytes. They are structurally polarized, characteristically large, and multi-nucleate. Their basolateral membrane faces the bone marrow and possesses sodium pumps as well as a bicarbonate-chloride exchanger.