Cartilage resorption and endochondral bone formation during the development of long bones in chick embryos

Cartilage resorption during long bone development in the chick embryo varies in several crucial aspects from that in mammals. Cartilage does not calcify prior to resorption, the pattern of vascular tunnelling is not as regular as that in mammals and no distinct growth plate or secondary ossification centre is apparent. Primary cartilage resorption is preceded by diffusion of substances which produce a change in the cartilage matrix and necrosis of adjacent chondrocytes. Mononuclear phagocytes resorb the bulk of uncalcified cartilage, but resorption is slowed down by the formation of an osteoid band along the walls of the marrow tunnels. This prevents resorbing cells from gaining access to the cartilage matrix. If this osteoid band is in alignment with the trabecular structure, it may contribute to structural bone. In some areas chondrocytes transform into, or are replaced by, bone-producing cells and osteoid is observed within the chondrocyte lacunae. Some calcification of cartilage is observed after 16 days in ovo, but this is independent of cartilage resorption. Remnants of calcified cartilage frequently persist in the marrow cavity and multinucleated chondroclasts are required to resorb these remnants.