Zonal analysis of cytoplasmic components of articular cartilage chondrocytes.

The cytoplasmic components of chondrocytes in the various zones of articular cartilage of the medial femoral condyle of adult male New Zealand white rabbits were quantitated from electron micrographs. A progressive increase in the content of endoplasmic reticulum, Golgi apparatus, mitochondria, and electrondense bodies (presumed to be lysosomes) from the tangential zone through the top half of the calcified zone was observed. These organelles then showed a decrease in content in the bottom half of the calcified zone. This was associated with an even greater increase in content of holes in the cytoplasm at that level, providing clear evidence of cellular swelling. Based on these quantitative morphologic studies, it is concluded that the chondrocytes in all zones of articular cartilage are metabolically active, but those cells in the calcified zone show signs of degeneration. The ultimate fate of the chondrocytes in the bottom of the calcified zone is not clear from this study.