Attachment of cytoskeletons to cell membranes in human blood platelets as revealed by the quick-freezing and deep-etching replica method.

This study evaluated ultrastructures of cell membrane-cytoskeletal interactions in resting and activated human blood platelets. Some blood platelets were fixed with paraformaldehyde to inhibit activation and the others were activated with thrombin treatment. The replicas of inside-out cell membranes, cell membrane surfaces, and Triton shells were made by the quick-freezing and deep-etching replica method. A geometrical pattern consisting of main hexagons and partial pentagons was revealed to be formed with microfilaments which were attached laterally to cytoplasmic sides of cell membranes in resting platelets. After the activation, cytoskeletal meshworks in the cytoplasm were densely associated with cell membranes, and parallel microfilamentous bundles in filopodia were connected with small polygonal meshworks lying under the cell membrane. The cell membrane-cytoskeletal interactions are essential to keeping a discoid shape at the resting stage, and to changing the shape quickly after the activation.