OSTEOGENIC DIFFERENTIATION OF HYPERTROPHIC CHONDROCYTES INVOLVES ASYMMETRIC CELL DIVISIONS AND APOPTOSIS

We have investigated the early cellular events that take place during the change in lineage commitment from hypertrophic chondrocytes to ost eoblast-like cells. We have induced this osteogenic differentiation by cutting through the hypertrophic cartilage of embryonic chick femurs and culturing the explants. Immunocytochemical characterization, [H-3] thymidine pulse-chase labeling, in situ nick translation or end labeli ng of DNA breaks were combined with ultrastructural studies to charact erize the changing pattern of differentiation. The first responses to the cutting, seen after 2 d, were upregulation of alkaline phosphatase activity, synthesis of type I collagen and single-stranded DNA breaks , probably indicating a metastable state. Associated with the change f rom chondrogenic to osteogenic commitment was an asymmetric cell divis ion with diverging fates of the two daughter cells, where one daughter cell remained viable and the other one died. The available evidence s uggests that the viable daughter cell then divided and generated osteo genic cells, while the other daughter cell died by apoptosis. The resu lts suggest a new concept of how changes in lineage commitment of diff erentiated cells may occur. The concepts also reconcile previously opp osing views of the fate of the hypertrophic chondrocyte.