Platelets are covercytes, not phagocytes: uptake of bacteria involves channels of the open canalicular system.

Recent studies have suggested that uptake of bacteria by human platelets is more similar to ingestion of the same organisms by neutrophils than previously considered. Platelet pseudopods surround a bacterium by a process of circumferential adherence until the organism is fully enclosed in a vacuole consisting of internalized extracellular space surrounded by plasma membrane. The surface connected, open canalicular system (OCS) is not involved in the process of platelet phagocytosis. The present investigation has reviewed the literature on platelet-particle and platelet-bacterial interaction, and added a new study of the process of bacterial engulfment employing tannic acid as an electron dense tracer. The osmium black reaction product formed by tannic and osmic acids during fixation stained the glycocalyx on the platelet exterior surface and lining channels of the OCS. It also stained the vacuoles containing bacteria in platelets as well as channels of the OCS connected to the engulfment vacuole and exterior surface of the cell. Alpha granules releasing their contents into the OCS and into engulfment vacuoles were also stained by osmium black. The only way that tannic acid could reach and stain the interior surface of engulfment vacuoles during fixation was through patent channels of the OCS. In contrast, neutrophils do not contain an OCS, and phagocytic vacuoles containing organisms are never stained by tannic acid. Thus the platelet is a covercyte, not a phagocyte, and uptake of the bacteria does involve channels of the OCS.