Ultrastructure of Ristocetin Aggregated Normal Human Platelets: Tortuous Boundaries and Swollen Granules

The effects of ristocetin on human platelets were investigated by aggregometry and electron microscopy. We confirmed the finding that ristocetin induces platelet aggregation. Biphasic platelet aggregation, such as is observed with ADP, was also seen with ristocetin. The gross appearance of ristocetin aggregated specimens differed markedly from those induced by ADP or collagen. On electron microscopic examination the boundaries of the aggregated platelets were noted to be extremely tortuous with electron dense lines between membranes, and there were many swollen α‐granules. At the periphery of the aggregates, the platelets were markedly degranulated. The plasma membranes of degranulated platelets were occasionally lysed. Morphological alterations of platelets exposed to ristocetin appeared to continue even after maximum aggregation, as recorded in the aggregometer, had been achieved.