endochondral

Endochondral refers to the formation of bone tissue from a cartilage template, particularly through the process of endochondral ossification. It is one of the main pathways of skeletal development in vertebrates and contrasts with intramembranous ossification, in which bone forms directly from connective tissue without a cartilage intermediate. The term derives from Greek endon meaning within and chondros meaning cartilage.

During development, mesenchymal cells condense and differentiate into chondrocytes, creating a cartilage model organized into a diaphysis (shaft) and epiphyses (ends). The primary ossification center appears in the diaphysis before birth. Chondrocytes enlarge and eventually die, the cartilage matrix calcifies, and blood vessels invade with osteogenic cells. This leads to breakdown of cartilage and replacement with woven bone, forming the medullary cavity.

Secondary ossification centers develop in the epiphyses around birth. Longitudinal growth occurs at the epiphyseal plates (physes), a layer of cartilage between the diaphysis and epiphysis. Proliferation of chondrocytes followed by their replacement with bone lengthens the bone during childhood and adolescence; growth generally ceases when the plates fuse after puberty. The articular cartilage at joints is also derived from the original cartilage template and remains as a thin, avascular surface.

Impairments in endochondral ossification underlie several skeletal disorders, including achondroplasia and metaphyseal dysplasia, and can disrupt normal growth and bone shape. Endochondral ossification is essential for forming most of the long bones, vertebrae, and parts of the pelvis and skull base.