The platelet contacts during aggregation.

APD-stimulated and aggregated platelets show dense structures (DS) on their free surfaces and filament bridges within 40-50 nm wide spaces of contacts between aggregating platelets (bridge contacts). Within the bridge contacts tight contacts are observed. Adjacent to tight contacts plasmalemmal openings into a canalicular system are seen. Pits and the central ends of these membranes are coated as seen in serial sections. Cationized ferritin (CF) added before stimulation binds to the whole negatively charged surface. Closer packed CF particles were observed on the DS and in the contacts on the central part of the bridges. CF did not introduce qualitative changes to the formation of typical platelet contacts, in particular of the tight contacts. Surface bound CF was found in the adjacent plasmalemmal invaginations and in the canalicular endomembranes. This result suggests an endocytosis mechanism which cleans the platelets of surface material during formation of tight contacts in aggregates.