Effects of Large Latex Particle Uptake on the Surface Connected Canalicular System of Blood Platelets: A Freeze-Fracture and Cytochemical Study

The complex structural organization of the platelet's surface-connected or open canalicular system (OCS) and close physical relationships to the dense tubular system have been described in previous cytochemical and freeze-fracture ultrastructural studies. Despite observations that suggest that channels of the OCS are seldom, if ever, single, tubelike invaginations of the surface, others have indicated that the OCS is a readily available source of membrane for evagination onto the exposed surface of the platelet after activation or during phagocytosis. In the present investigation we have utilized freeze-fracture for the first time to evaluate the uptake of large (0.312 μm, SD ± 0.0022) latex particles by platelets. Results of the study leave no doubt that channels of the OCS serve as the major route for latex ingestion in the unstirred system employed. Prolonged exposure to latex and uptake of many spherules cause marked changes in platelet surface contour and internal organization. The OCS is transformed from a spiderweb of intercommunicating channels into one or two large vacuoles filled with latex. Conversion of the OCS into large sacs is associated with disk-to-sphere transformation and a decrease in the number of openings of the OCS on the platelet surface. Thus, the OCS appears to be effectively interiorized by the process of phagocytosis, rather than evaginated as others have suggested.