Rearrangement of the open-canalicular system of the human blood platelet after incorporation of surface-bound ligands. A high-voltage electron-microscopic study.

The three-dimensional configuration of the membrane system in human blood platelets following administration of cationized ferritin (CF) was reconstructed by means of thick serial sectioning in combination with high-voltage electron microscopy (HVEM). Surface-bound CF was rapidly internalized at 37 degrees C by the platelets, and a conglomerate of vacuolar and tubular structures containing CF was formed in their interior. Three-dimensional reconstruction of these incorporated membranous structures revealed that most of the internalized structures were interconnected with each other and that the number of sites at which they opened onto the cell surface was significantly reduced compared with the number found in intact platelets. In some cases, the openings completely disappeared. This may indicate that most of the internalized membranous structures originate from the open canalicular system (OCS), and that, following the incorporation of the ligand, the OCS gradually loses its connections with the surface membrane forming a conglomerate of the ligand-containing membranous structures in the interior of the platelet.