Phenotypic modulation of nasal septal chondrocytes by cytoskeleton modification.

The reversion to the initial round shape of chondrocytes in monolayer cultures is one of the initial events required for the expression of cartilage-specific macromolecules. Thus, considerable research efforts have focused on developing reliable procedures to maintain a round morphology of cultured chondrocytes. Our study focuses on evaluating the response of dedifferentiated fetal rat chondrocytes to cytochalasin D, an actin-disrupting agent, with special emphasis on the morphological events. Immediately after exposure to the drug, cells round up but flatten again after removing the agent. However, immunocytochemical procedures revealed a disorganization of microfilaments and intermediate filaments. Phase-contrast and scanning electron microscopic observations revealed that on day 6 of culture, cells located at the top of the cell layer adopted a spherical morphology. Prominent differences were noted in control cultures where cells had to aggregate prior to overt chondrogenesis. These morphological changes occurred parallel to the expression of type II collagen, marker of the chondrocytes phenotype, strongly expressed in experimental cultures, but relatively weak in control cultures, and only restricted on areas of polygonal cellular aggregates. Furthermore, [35S]-sulphate incorporation into sulphated glycosaminoglycans increased rapidly with the period of culture to a maximum after 7 days and was then two-fold in treated cultures. Taken together, these findings indicated that cytochalasin-D stimulates chondrogenesis in response to modification of cytoskeleton architecture and the subsequent rounding up of the cells.