Endocytosis by platelets during cationized ferritin-induced aggregation.

Localization of cationized ferritin (CF) particles in the process of CF-induced aggregation of rabbit platelets was investigated by electron microscopy. CF particles attached to the surface membrane of discoidal platelets immediately after the addition of CF. Some platelets were connected to each other through the CF particles located on their surfaces. At 30 s after the addition of CF, aggregation of platelets in round form was observed. During the time course of aggregation, CF particles moved to the interplatelet spaces. Also CF particles were found in the open canalicular system, the membrane component of which was stained with ruthenium red. On the other hand, CF particles were also found in ruthenium-red-negative vesicles in platelets. At 180 s after, CF particles containing vacuoles, which showed acid phosphatase activity, were observed in the aggregates. These results suggest that part of CF particles may be incorporated into the cytoplasma by endocytosis.