Bone Formation and Development

Bone formation occurs through two main processes: intramembranous ossification, where mesenchymal cells directly differentiate into osteoblasts and deposit bone matrix, and endochondral ossification, where a cartilage model is replaced by bone. In endochondral ossification, chondrocytes in the cartilage model undergo hypertrophy, degenerate, and are replaced by osteoblasts that deposit bone, forming a primary ossification center in the diaphysis and secondary ossification centers in the epiphyses after birth. Bone growth in length occurs at the epiphyseal plate, where cartilage proliferation and ossification allow the bone to elongate until the plate fuses in early adulthood, while appositional growth increases the bone's diameter through the activity of osteoblasts and osteoclasts