The response of megakaryocytes with processes to thrombin

The response of megakaryocytes to thrombin (1-10 U/ml) has been examined by time-lapse cinemicrography and electron microscopy. The study was confined to mature megakaryocytes which had developed processes following incubation in vitro. The initial response of all cells was to undergo retraction of processes, behaviour thought to be linked with the depolymerization of microtubules which extend longitudinally through the processes. The majority of cells completely withdrew their processes, but about 30% responded differently and underwent only limited retraction, followed by secretion. Analysis of time-lapse film showed that processes from the latter group of cells had formed attachments with the coverslip prior to exposure to thrombin. Within the partially retracted processes of these cells, secretory granules were found to be clustered centripetally and enveloped by a microfilamentous structure in the form of a cylinder. Vacuoles appeared, some of which were located outside the microfilamentous structure. Microtubules were present, but many appeared disorientated. The shape of the microfilamentous structure suggests that the cytoplasm is not organized into putative platelets at the time of process formation.