The surface-connected canalicular system of blood platelets--a fenestrated membrane system.

Recent reports have suggested that channels of the surface-connected or open canalicular system (OCS) in discoid blood platelets represent a reservoir of membrane that can be evaginated following activation and shape change and contribute to an increased ratio of surface area to volume. The present study has used electron cytochemistry and freeze-fracture to examine the organization of the OCS in unaltered platelets. Results of the investigation indicate that channels of the OCS are seldom if ever single tubular invaginations of the surface membrane. Each channel joins with other canaliculi of the OCS to form an anastomosing network of fenestrated conduits spreading throughout the cytoplasm from one side of the platelet to the other. The multiple connections of the interlocked channels to different sites on the platelet surface, their association in a continuous labyrinth, and participation with elements of the dense tubular system to form membrane complexes suggest that the OCS would have to be torn apart or undergo radical rearrangement before it could be evaginated and contribute to an increased surface area on activated cells.