Bone development

The main component of both bone and cartilage is the extracellular matrix secreted by the cellular component of the connective tissue. Three types of molecules characterize the matrix -- structural proteins, adhesive proteins, and proteoglycans. Chondrocytes, which are the cellular component of cartilage, also secrete collagen. Collagen is comprised of three polypeptide chains that wrap around each other to form a strong, yet flexible triple helix. It is not innervated and is avascular.

As a fetus, humans have a cartilaginous endoskeleton, which is gradually replaced with bone until adulthood. In the diaphysis of long bones, chondrocytes enlarge and then break down, leaving a honeycombed cartilage. The ossification of the bone occurs from the center portion of the bone out towards both ends, carried out by osteoblasts, which have differentiated from osteoprogenitor cells in the periphery of the bone called the perichondrium. Once the osteoblasts have laid down concentric layers of bone, vascular connections and nerves move into the center of these layers, forming an osteon.