Actin cytoskeleton of spread fibroblasts appears to assemble at the cell edges

The action of metabolic inhibitors on actin cytoskeleton of cultured quail embryo fibroblasts has been studied using electron microscopy of platinum replicas and immunofluorescence microscopy. Sodium azide as well as other inhibitors (oligomycin and dinitrophenol) caused the disassembly of all types of actin structures: actin meshwork at the cell active edges, microfilament sheath underlying the cell surface, and microfilament bundles. Studying the time- and dose-dependence of the destruction process we have found that the active edge meshwork and microfilament sheath are much more labile than microfilament bundles. After the removal of metabolic inhibitors actin cytoskeleton restoration begins at the cell edges. The first sign of this process is the formation of actin meshwork along the whole cell perimeter (1-10 min of recovery). Sometimes fragments of this meshwork bend upwards forming ruffles. Later (10-20 min of recovery) the microfilament sheath appears at the cell periphery as a narrow band. The sheath seems to be formed from the edge meshwork, since ruffles in the process of transformation to sheath could be seen. During the following restoration the microfilament sheath gradually expands towards the cell centre. The last step of actin cytoskeleton restoration (60-120 min of recovery) is the formation of bundles. We suggest that the actin in spread fibroblasts polymerizes predominantly at the cell edges and then moves centripetally, forming the microfilament sheath and bundles.