Microtubules, microfilaments and adhesion patterns in differentiating chick retinal pigment epithelial (RPE) cells in vitro.

The distribution of microtubules (MT), microfilaments (MF), and patterns of cell-to-substratum adhesion were studied by tubulin antibody labeling, NBD-phallacidin staining and by reflection interference contrast (RIC) microscopy respectively in colonies of differentiating RPE cells obtained from explants after 10 days in culture. In each colony three zones could be identified: a central zone of packed well-differentiated cuboidal cells (zone 1), an intermediate zone of more flattened, pleomorphic cells (zone 2) and a peripheral zone of very spread cells at the edge of the colony (zone 3). As visualized with antibodies to tubulin, the MT distribution in cells of each zone was distinctly different and correlated well with differences in cell shape. Changes in the distribution of MF were more striking. In the cuboidal well-differentiated cells of zone 1, prominent cortical bands but no stress fibers were observed after staining with NBD-phallacidin and RIC microscopy showed that the cells lacked strong adhesion to the substratum. Stress fibers, in addition to cortical bands of MF, were seen in the more spread, less differentiated cells of zone 2 and focal contacts were observed when these cells were examined by RIC microscopy. The flattened least differentiated cells in zone 3 lacked cortical bands but had prominent stress fibers. These cells displayed a variety of adhesion forms ranging from a mosaic of far and close contacts to numerous focal contacts and broad focal adhesions. Our results show that as the RPE cells display less differentiated morphologies, i.e. are more flattened and less densely packed towards the edge of the colony, there is a gradual decrease in the cortical bands of MF and an increase in the number and prominence of stress fibers. This increase in numbers of stress fibers is correlated with an increase in the cell adhesiveness to the substratum, as estimated by RIC microscopy. These results strongly support the general observation that normal epithelial cells in colonies tend to adhere to the substratum more strongly by marginal cells than by the more differentiated centrally located cuboidal cells which have well developed intercellular contacts.