The initial phase of long bone development occurs through mesenchymal condensation to form a cartilage model. Following chondrocytes hypertrophy, the cartilaginous template is calcified by osteoblasts and then partially resorbed by osteoclasts, resulting in the formation of the primary and the secondary ossification centers. Through additional cycles of remodeling, the cartilage of the primary and secondary ossification centers is converted to the trabecular bone of the metaphysis and epiphysis, respectively. Longitudinal bone growth occurs at the growth plates and results from the elongation of the cartilaginous template. In adult bones, the growth plate is fully resorbed, so that one marrow cavity extends the full length of the bone.
