Coated pits and vesicles transfer plasma components to platelet granules.

Electron microscopy of mammalian blood platelets after contrast enhancement with tannic acid after an initial fixation in glutaraldehyde and osmium reveals numerous coated pits (c.p.) and vesicles (c.v.), indicating a process of receptor-mediated endocytosis. The c.p. may be located at any site of the plasma membrane or the canalicular, surface connected membrane system. C.v. fuse with platelet granules without losing their coat. Evidence for a continuous transfer of ambient fluid to granules via c.p. and c.v. was obtained by the use of fluid-phase markers. It is proposed that the endocytic process may play a role in blood platelet activation.