Summary
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 rnicrofilamentous structure in the form of a cylinder.
Vacuoles appeared, some of which were located outside the rnicrofilamentous structure.
Microtubules were present, but many appeared disorientated. The shape of the rnicrofilamentous
structure suggests that the cytoplasm is not organized into putative platelets at
the time of process formation.
Keywords
Megakaryocyte - Thrombin - Electron microscopy
