Chondrocyte apoptosis in endochondral ossification of chick sterna

In the process of endochondral ossification, chondrocytes progress through a series of maturational changes, including division and hypertrophy, that culminate in chondrocyte loss and cartilage resorption. From an investigation of morphology, DNA fragmentation and collagen synthesis in the developing chick sterna we have characterized chondrocytes death in this process. Light microscopy of resorbing sterna demonstrated chondrocyte condensation at the interface with the invading vasculature and electron microscopy demonstrated a range of chondrocyte morphologies, including retraction from the pericellular matrix, cytoplasmic and nuclear condensation, and vesiculation suggestive of sequential changes characteristic of apoptosis. Isolation and end-labeling of DNA from chick primary ossification centers demonstrated fragmentation to nucleosome sized units, only in primary ossification centers exhibiting active resorption, and in situ detection of DNA fragmentation showed a restriction to chondrocytes at the interface with invading blood. We conclude that terminal differentiation of chondrocytes results in death by an apoptotic process prior to resorption of the tissue and invasion by blood vessels. The extent of DNA fragmentation correlated closely with the proportion of cells displaying a condensed phenotype in contralateral primary ossification centers and peaked at an early stage of resorption, suggesting that chondrocyte apoptosis may be an initiating event in tissue resorption and vascular invasion. Comparison of DNA fragmentation with expression of the hypertrophic chondrocyte phenotype, as indicated by type X collagen synthesis, suggested that DNA fragmentation was a late event in the process of chondrocyte hypertrophy and probably corresponded with chondrocyte condensation