Electron microscopic studies on the uptake of colloidal thorium dioxide particles by isolated fetal guinea-pig chondrocytes and the distribution of labeled lysosomes in cartilage formed by transplanted chondrocytes.

Chondrocytes isolated from fetal, guinea-pig epiphyses were grown in monolayer culture, exposed to thorium dioxide particles, and studied ultrastructurally after varying intervals. The exogenous marker was ingested by endocytosis and subsequently accumulated in lysosomes. After intramuscular injection into young guinea pigs, the thorium dioxide-labeled chondrocytes formed a typical hyaline cartilage. This consisted mainly of rounded or polygonal cells with large, eccentrically located nuclei. The cytoplasm showed an extensive granular endoplasmic reticulum and a well-developed Golgi complex, suggesting active synthesis and secretion of matrix components. Among the other cytoplasmic organelles, lysosomes containing variable amounts of marker particles were observed. After 2-3 weeks the transplants whowed signs of cellular degeneration and disintergration. During these processes, lysosomes remained structurally intact and, furthermore, retained the incorporated marker. Thus, thorium dioxide-labeled bodies were found in former chondrocyte lucunae and in the intercellular substance proper. In the latter location labeled bodies could be observed in close proximity to early mineral deposits. These results are discussed with special special reference to the cellular origin and lysosomal nature of matrix vesicles in calcifying cartilages.