Changing patterns of plasma membrane-associated filaments during the initial phases of polymorphonuclear leukocyte adherence

Abstract

By utilizing a combination of several ultrastructural techniques, we have been able to demonstrate differences in filament organization on the adherent plasma membranes of spreading and mobile PMN as well as within the extending lamellipodia. To follow the subplasmalemmal filaments of this small amoeboid cell during these kinetic events, we sheared off the upper portions of cells onto glass and carbon surfaces for 30 s--5 min. The exposed adherent membranes were immediately fixed and processed for high-resolution SEM or TEM. Whole cells were also examined by phase contrast microscopy, SEM, and oriented thin sections. Observed by SEM, the inner surface of nonadherent PMN membranes is free of filaments, but within 30 s of attachment to the substrate a three-dimensional, interlocking network of globular projections and radiating microfilaments--i.e., a subplasmalemmal filament complex--is consistently demonstrable (with or without postfixation in OsO4). Seen by TEM, extending lamellipodia contain a felt of filamentous and finely granular material, distinct from the golbule/filament complex of the adjacent adherent membrane. In the spread cell, this golbule-filament complex covers the entire lower membrane and increases in filament-density over the next 2--3 min. By 3--5 min after plating, as the PMN rounds up before the initiation of amoeboid movements, another pattern emerges--circumferential bands of anastomosing filament bundles in which thick, short filaments resembling myosin are found. This work provides structural evidence on the organization of polymerized contractile elements associated with the plasma membrane during cellular adherence.