Endochondral ossification and de novo collagen synthesis during repair of the rat Achilles tendon.

Ectopic endochondral ossification is the inevitable consequence of midpoint tenotomy of the rat Achilles tendon. After tenotomy, the tendon stumps retract and the intervening space fills with granulation tissue. The initiation of chondrogenesis is indicated by pre-chondrocytic cells forming a "whorled" pattern, both at the tendon stumps and within the granulation tissue and later clearly differentiating into cartilage nodules. The chondrocytes rapidly "hypertrophy" exhibiting an orientation similar to that in epiphyseal growth plates. The nodules of cartilage are then replaced, by bone. During this total process, a temporal and spatial pattern of new collagen synthesis can be demonstrated, both biochemically and immunocytochemically. Both the cartilage and the subsequent bone closely resemble the tissue in developing long bones enabling this model to be used to study the initial switching on of normal chondrogenesis and osteogenesis in a system not normally programmed to do so.