[The cytoskeletal organization of epithelial cells in culture]

Cytoskeleton organization of cultured normal epithelial cells (epithelium of newborn mouse kidney, mouse and rat hepatocytes) was studied using electron microscopy of platinum replicas. These cells in culture were firmly connected with each other and formed multicellular islands. Pseudopodial activity was observed only at the free edges of marginal cells of the islands. Cytoskeleton in the vicinity of such active edges included several structurally different zones. The most peripheral zone contained dense actin meshwork. More inner "sparse" zone contained loose actin filament network. Next zone in the same direction was the lamella proper. It contained individual microfilaments and their bundles or meshwork patches. Microtubules and intermediate filaments were also present in the lamella proper. The characteristic feature of the central (endoplasmic) region of the marginal cells of the islands was the presence of the submembranous microfilament sheath. Microfilaments in the sheath were densely packed. Individual fibers were visible along a significant distance. The inner cells in the epithelial islands had no zonal organization of the cytoskeleton. The endoplasmic microfilament sheath occupied the whole dorsal cell surface in these cells. Different epithelia studied here had some variations in the relative width of cytoskeletal zones. The organization of cytoskeleton in the epithelial cells has many features in common with that in fibroblasts. Possible mechanisms of establishment of the zonal cytoskeletal organization in both the cell types are discussed.