INITIATION OF THE BONY EPIPHYSIS IN LONG BONES - CHRONOLOGY OF INTERACTIONS BETWEEN THE VASCULAR SYSTEM AND THE CHONDROCYTES

Many events occur concurrently during the initiation of the secondary ossification center in the cartilaginous epiphyses of long bones. We h ave investigated the chronology of interactions between the vascular s ystem and epiphyseal chondrocytes by culturing explanted heads of femu rs and humeri from pre-and neonatal rabbits on the chorioallantoic mem brane (CAM) of growing chick embryos. We confirmed that, on the whole, the epiphyseal cartilage was resistant to vascular invasion, whereas the physeal growth plate was resorbed, However, new CAM-derived cartil age canals occasionally penetrated through the articular surface. This caused death of those chondrocytes in the immediate vicinity of the c anal but no further reaction. If explants already contained a bony epi physis and were halved prior to culture, CAM-derived vessels were attr acted to the spongiosa, From there they pushed into the uncalcified ca rtilage, indicating that calcification was not a prerequisite for vasc ular invasion. Where at least two vessels were in apposition, a new ps eudo-ossification center was initiated: chondrocytes became hypertroph ic and the matrix calcified. This suggests that cumulative release of diffusible factors from more than one vessel was the trigger for chond rocyte hypertrophy, which, in turn, led to the initiation of the bony epiphysis, CAM cultures thus provide an experimental model for both th e quiescent angiogenesis of cartilage canal formation and the reaction ary angiogenesis associated with chondrocyte hypertrophy, By exploitin g the different anatomy of CAM-derived vascularity, events that occur concurrently in vivo can be spacially separated in CAM culture.