Osteoblasts and Bone Formation

This chapter focuses on osteoblast biology and bone formation. Bone formation may be classicaIly defined as the process by which osteoblasts synthesize and mineralize an extracellular bone matrix during bone development bone remodeling, or bone repair. The sequence of events and the genes and proteins involved in bone formation differ during the fetal and adult life, during repair of fractures and during bone remodeling. Bone formation during remodeling is characterized by the deposition of a collagenous matrix at the site of previously resorbed bone. During the formation phase, the osteoblast differentiation pathway is characterized by a sequence of events involving the proliferation of osteoprogenitor cells, the progressive differentiation of preosteoblasts, and the sequential expression of genes of the osteoblast phenotype and the synthesis, organization, deposition and mineralization of a bone matrix by post-mitotic mature osteoblasts. The recruitment of osteoblast precursors is in part controlled by mitogenic factors acting on osteoblast precursor cells. The differentiation of preosteoblasts into mature osteoblasts, and the synthesis of the bone matrix by osteoblasts are modulated by cell-cell contact and cell-matrix interactions, and are regulated by hormonal and local factors affecting cell differentiation. The normal deposition of new bone matrix in the remodeling unit during the formation phase requires the coordinated succession of these events and the appropriate induction by systemic and local factors during the sequence of osteoblastic cell differentiation.