[Active region of the lamellar cytoplasm and focal contacts--the most cytochalasin-sensitive structures of fibroblasts]. / Aktivnaia oblast' lamelliarnoi tsitoplazmy i fokal'nye kontakty--naibolee chuvstvitel'nye k tsitokhalazinam struktury fibroblastov.

Effects of low doses of cytochalasin B (CB, 2 micrograms/ml) and cytochalasin D (CD, 0.2 microgram/ml) on the spreading of normal mouse fibroblasts in culture were investigated. CB desorganized the cortical layer of actin microfilaments to cause partial or complete disappearance of microfilament bundles; focal contacts with the substrate seen by interference-reflection microscopy also disappeared. Low doses of CB did not inhibit the control, on the upper surface of these lamellas distal zones with convex outer edges ruffles were lacking. The disappearance of these ruffling active edges was accompanied with the loss of ability to clear the surface of lamellas from the concanavalin A receptors cross-linked by a corresponding ligand. Thus, ruffling active edges and focal contacts can be regarded as specialized parts of lamellas with increased sensitivity to cytochalasins; the presence of ruffling active edges is essential for the initiation of centripetal movement of the patches of cross-linked surface receptors.

Active cell edge and movements of concanavalin A receptors of the surface of epithelial and fibroblastic cells.

Movements of the receptors of concanavalin A on various parts of the surfaces of substrate-attached cells were compared. Cultured mouse embryo cells of several types were used: epithelial kidney cells and normal and transformed fibroblasts. Initial distribution of receptors was random on the cells of all types. Binding of concanavalin A induced patching of its receptors on all the cell parts. In contrast, directional centripetal movement of receptors was observed only on the surface of certain cell parts, namely, only the surface of peripheral lamellar cytoplasm was cleared of the receptors. Clearing was always initiated in the zone of lamellar cytoplasm located near active cell edges. In epithelial sheets, clearing was not observed on the surface of central cells that had no lamellar cytoplasm. Concanavalin A receptors on the cleared areas of cell surface were gradually restored after the end of incubation. It is suggested that anchoring of the patches of membrane receptors by cortical microfilaments is possible only on the surface of pseudopods and of lamellar cytoplasm but not on the surface of other cell parts. Besides receptor movements, this hypothesis may be explain differences in the adhesive properties of various parts of the cell surface.