The fate of the secondary cartilage present on the membrane bones of the embryonic chick has been studied after immobilization. Immobilization was achieved by the in vivo injection of paralysing drugs (tubocurare or decamethonium), by grafting membrane bones onto the chorioallantoic membrane, or by organ-culturing membrane bones in vitro. In all three situations the cartilage was transformed into a bone-like tissue, the matrix losing its acid muco-polysaccharide, accumulating collagen and undergoing calcification. The chondrocytes shrank in size, came to resemble osteoblasts (osteocytes) and acquired alkaline phosphatase activity.
In normal development this cartilage is not transformed into bone but is partly replaced by bone and partly converted into a fibrocartilage which forms the definitive articular cartilage. Immobilization prevented this normal sequence.
Past studies on the transformation of cartilage to bone are reviewed and are seen to be adaptations of a highly labile tissue to functional demands.