Figure 2.

Endochondral ossification of long bones. Condensation of mesenchymal cells forms the general shape of the long bone. Differentiation of these cells into chondrocytes begins the process of bone formation in the primary ossification center. In late differentiation, chondrocytes undergo apoptosis, leaving behind a mineralized scaffold onto which osteoblasts brought in by the invading vasculature lay down bone, lengthening the bone while forming the marrow cavity. As development continues, secondary ossification centers form in the epiphyses. Bone increases in width through deposition of bone on the periosteal side and, through endochondral ossification, continues to increase in length until the growth plates fuse.