Endokondral

Endochondral ossification is a process by which most bones of the skeleton are formed. In this process, a hyaline cartilage template is gradually replaced by bone tissue. It is one of the two major pathways of skeletal ossification; the other is intramembranous ossification, in which bone forms directly from mesenchymal tissue.

During fetal development, mesenchymal cells condense to form a cartilage model. Chondrocytes in the core become hypertrophic, the surrounding matrix calcifies, and the center of the model undergoes degeneration. A primary ossification center develops in the diaphysis as blood vessels invade the cartilage and bring osteoprogenitor cells. A bone collar forms around the diaphysis, and osteoblasts replace calcified cartilage with woven bone. As remodeling proceeds, a marrow cavity forms and lamellar bone replaces woven bone.

Secondary ossification centers appear in the epiphyses after birth, driven by independent vascular invasions. The epiphyseal growth plate (physis) remains between the diaphysis and epiphysis, containing zones for proliferation and hypertrophy, enabling longitudinal growth until the plate closes in late adolescence.

Clinical significance: defects in endochondral ossification can lead to skeletal dysplasias such as achondroplasia. It plays a major role in growth, development, and repair of most long bones.

Comparison: in contrast, intramembranous ossification forms flat bones (skull, clavicle) directly from mesenchyme without a cartilage intermediate; endochondral ossification accounts for most bones, especially long bones.