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Figure 3. Remodeling of bone in a bone multi-cellular unit starts with osteoblastic activation of osteoclast differentiation, fusion, and activation (A,B). When resorption lacunae are formed, the osteoclast leaves the area, and mononucleated cells of uncertain origin appear and "clean up" the organic matrix remnants left by the osteoclast, also possibly forming the cementum line (dotted line) at the bottom of the lacunae (C). During the resorption process, coupling factors, including IGF-I and TGF- ß, are released from the bone extracellular matrix, and these growth factors contribute to the recruitment and activation of osteoblasts to the resorption lacunae (D). The osteoblasts will then fill the lacunae with new bone, and when the same amount of bone is formed as that being resorbed, the remodeling process is finished, and the mineralized extracellular matrix will be covered by osteoid and a one-cell layer of osteoblasts (E).
