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The role of microtubules in adhesion and in the adhesion-stimulated release reaction of platelets from African green monkeys has been studied using conventional (100 kV) and high voltage (1000 kV) stereo electron microscopy. Upon exposure in vitro to either glass or a carbon-stabilized formvar surface, platelets were rapidly activated and extended numerous filopodia. The adhesion process evolved over a period of 10–20 min with extension of a delicate hyalomere across the surface between adjacent filopodia. Microtubules, which form a circumferential ring in the resting cell, were dissociated upon cell activation and then reassembled in patterns which conformed to the general cell shape. In dendritic cells with numerous filopodia the microtubules were oriented radially. With subsequent hyalomere development, the microtubules paralleled major cell axes. This morphological transition of platelet adhesion was accompanied by the release of β-thromboglobulin (B-TG) which was maximal at 20 min, a time that corresponded to maximal hyalomere extension. Treatment of platelets with colchicine or vinblastine sulfate prior to adhesion dissociated the microtubules, but affected neither the morphological transition nor β-TG release. Our observations separate microtubules from adhesion-related events and suggest that during adhesion the microtubules, by conforming to cell shape, may simply provide cytoskeletal support.

Adhesion of platelets in vitro resulted in rapid polymerization of the amorphous cytoplasmic ground substance into an organized cytoskeletal superstructure. This cytoskeleton, characterized through the use of whole-mount and stereo (3-D), high-voltage microscopy in conjunction with morphometrics and cytochemistry, comprised four major size classes of filaments organized in distinctive zones. The central matrix, or granulomere, at the center of the cell mass, was an ill-defined meshwork of 80-100-A filaments which enshrouded granules, dense bodies, and elements of the dense tubular system as identified through peroxidase cytochemistry. Demarcating this central matrix was a trabecular zone containing 30-50, 80-100, and 150-170 A filaments in an open and rigid-appearing lattice. Circumscribing the trabecular zone and extending to the margins of the hyalomere was the third region, the peripheral web, in which 70-A filaments were arranged in a tight honeycomb lattice. This organizational pattern was retained in cytoskeletons prepared by Triton x-100 extraction of the adherent cells, and was observed in basally located cells of aggregates which formed subsequent to adhesion. Our observations are consistent with biochemical studies of cytoskeletons prepared from suspended platelets and suggest a contractile protein composition for the superstructure during adhesion.