Role of cartilage canals in osteogenesis and growth of the vertebral centra.

Bone formation in the vertebral centra commences within the centrum and is in this respect analogous to the secondary ossification which occurs in the epiphysis of a long bone. Bone tissue first appears at about the 85 mm C.R. stage and not in the 55 mm C.R. length embryo; at the latter stage blood vessels and calcification alone were observed. The connective tissue cells within the cartilage canal appear to assist osteogenesis by providing osteogenic cells which lay down bone in the walls of the cartilage canal, and provide cells which remove calcified cartilage found at the periphery of the canal; they assist growth by producing an appreciable number of chondrocytes that permit lateral expansion of the centra. Osteogenesis appears to occur in multiple foci within the growth plate of the older embryos and could account for the rapid rate of growth of vertebrae. Bone formation occurs in both mineralised and unmineralised matrix (as seen on the walls of the cartilage canals). The blood vessels within the growing vertebra tend to follow the zone of cartilage hypertrophy.