Cartilage ultrastructure after high pressure freezing, freeze substitution, and low temperature embedding. I. Chondrocyte ultrastructure--implications for the theories of mineralization and vascular invasion.

Electron microscopic examination of epiphyseal cartilage tissue processed by high pressure freezing, freeze substitution, and low temperature embedding revealed a substantial improvement in the preservation quality of intracellular organelles by comparison with the results obtained under conventional chemical fixation conditions. Furthermore, all cells throughout the epiphyseal plate, including the terminal chondrocyte adjacent to the region of vascular invasion, were found to be structurally integral. A zone of degenerating cells consistently observed in cartilage tissue processed under conventional chemical fixation conditions was not apparent. Hence, it would appear that cell destruction in this region occurs during chemical processing and is not a feature of cartilage tissue in the native state. Since these cells are situated in a region where tissue calcification is taking place, the implication is that the onset and progression of cartilage calcification are, at least partially, controlled by the chondrocytes themselves. The observation that the terminal cell adjacent to the zone of vascular invasion is viable has important implications in relation to the theory of vascular invasion. This may now require reconceptualization to accommodate the possibility that active cell destruction may be a precondition for vascular invasion.