Reorganization of actin microfilaments and microtubules in regenerating retinal pigment epithelium.

Retinal pigment epithelium (RPE) regenerating after experimental damage in rabbits exhibits major changes in cell shape, polarity and junctions--features that depend on the cytoskeleton. This report correlates these changes with the redistribution of actin microfilaments and microtubules, using electron microscopy and confocal laser scanning microscopy. We compare immature cells with the more mature cells that form the new epithelial monolayer. Two populations of immature RPE cells are interspersed at the edge of the regenerating RPE sheet. One population of immature cells makes few junctions with their neighbors or the basement membrane. They form pseudopodia and exhibit a prominent network of actin microfilaments beneath the plasma membrane. These cells are probably motile and advance the epithelial sheet. Another population of immature cells contains numerous stress fibers that insert into large basement membrane attachments. The cells make focal adhesions with their neighbors, rather than the junctional complexes characteristic of mature RPE cells. These cells are probably not motile and mature into the cells forming the new monolayer--cuboidal cells with numerous basal folds and apical villi and a complete belt of intercellular junctions. Stress fibers are lost as the circumferential bundle associated with the zonula adherens re-forms. Microtubules, which form prominent longitudinal bundles running through the processes of immature cells, take on the meshwork organization characteristic of mature RPE as the immature cells differentiate.