Light-microscopical analysis of focal adhesions of retinal pigmented epithelial cells

Retinal pigmented epithelial (RPE) cells from eyes of chick embryos form colonies in vitro in which cells at the periphery of the colony are unpigmented, undifferentiated, and well spread, while those in center of the colony are cuboidal, polygonal, and pigmented, and resemble RPE cells in vivo. The differentiated RPE cells in the center of the colony display predominantly cell-cell adhesiveness, and their microfilaments are organized in compact, circumferential rings. Undifferentiated RPE cells from the edge of the colony, in contrast, display predominantly cell-substratum adhesiveness and have numerous stress fibers spanning their cytoplasm. The well-spread RPE cells adhere to the substratum with focal contacts and unusually large focal adhesions. The focal adhesions which are typical of the spread chick RPE cells in vitro consist of several closely apposed focal contacts, arranged in a parallel fashion, which are often coalesced with each other along their sides. They occur at the termini of prominent microfilament bundles which contain F-actin and tropomyosin along their entire length. Myosin, which is also present in these bundles, however, is less abundant than actin and tropomyosin in the terminal, focal adhesion-associated parts of these bundles. On the other hand, myosin is more abundant than actin and tropomyosin outside the microfilament bundles in the bulk of the cytoplasm. Both focal adhesions and termini of microfilament bundles coincide with the restricted regions where high concentrations of vinculin, an adhesion-specific protein, are found. In contrast, an actin binding protein, spectrin, is distributed fairly uniformly throughout the entire cortex of RPE cells, and, unlike vinculin, does not seem to participate in the binding of microfilament bundles to the plasma membrane. Although extracellular matrix components laminin, fibronectin, and heparin sulfate proteoglycan are produced and deposited by the more differentiated RPE cells in the center of the colony, heparan sulfate proteoglycan has not been detected along the surface of the flat, undifferentiated RPE cells near the edge of the colony, while both laminin and fibronectin are present in very low amounts, the former along their ventral, and the latter along their dorsal cell surfaces. These data are discussed from a point of view that the formation of highly adhesive membrane-cytoskeleton complexes of the focal type in the spread, undifferentiated RPE cells is brought about by exposure of the cells to the rigid, unyielding substrata, such as glass or plastic.(ABSTRACT TRUNCATED AT 400 WORDS)