Proteoglycan core protein and type II collagen gene expressions are not correlated with cell shape changes during low density chondrocyte cultures.

Chondrocytes isolated from chicken embryo sterna were cultivated in low density monolayer cultures to induce their dedifferentiation. At different stages of the long-term cultures, changes in expression of a cartilage-specific sulfated proteoglycan and cartilage-characteristic type II collagen have been examined and related to the shape change of cells using in situ hybridization and immunocytochemistry. At the beginning of the culture, all cells exhibit a round shape and express the cartilage phenotype. Then, during the course of the culture, chondrocytes flatten and become fibroblast-like, but this morphological modification does not start for all the cells at the same time. Interestingly, the loss of cartilage proteoglycan or type II collagen expression did not occur for all polygonal or fibroblast-like cells. Moreover, we observed a variability in the steady state levels of RNA or protein accumulation among chondrocytes exhibiting a similar shape, as judged by the intensity of hybridization signal or immunofluorescence over the cells. These observations support the hypothesis that the shape change does not have a causative role in the chondrocyte phenotype expression, but is rather a secondary effect of the dedifferentiation process. Furthermore, the disappearance of hybridizable core protein or type II collagen mRNA during the dedifferentiation process was coincident with the disappearance of the proteins for which they code as detected by immunohistochemical staining. This suggest that core protein and type II collagen gene expressions are controlled primarily at the transcriptional level in long-term chondrocyte cultures.