The organization of actin filaments in human polymorphonuclear leukocytes

Actin constitutes a major component of the cytoskeleton of human polymorphonuclear leukocytes (PMNs). In this study, we present a comprehensive view of the organization of actin in various PMN regions and functional states. Transmission electron microscopic observations were made on whole mount, migrating, and phagocytizing PMNs. Positive identification of actin filaments was made through S-1 myosin subfragment labeling. In all PMNs studied, actin filaments were primarily organized as a three-dimensional meshwork. The density of this meshwork was greatest within the cell cortex. At peripheral regions of nonpolarized (viz., no distinct head or tail region) and polarized PMNs, actin filaments organized into parallel bundles or overlapping arcs. These bundles or arcs were oriented either perpendicular or parallel to the cell periphery. At the base of the PMN, actin filaments converged upon dense, plaquelike condensations. This latter pattern of actin organization was also observed in some pseudopods at the cell front and in phagocytic processes engulfing bacteria. In areas of internalized bacteria, the surrounding actin appeared as a loose meshwork. Treatment of PMNs with the antiactin drug, cytochalasin B, revealed shearing of the peripheral actin meshwork, condensation of the meshwork around the nuclear region, and dissolution of the basal plaquelike condensations.