Alterations in blood platelet morphology during aggregate formation in the Asian elephant (Elephas maximus)

The ultrastructure of Asian elephant (Elephas maximus) platelets before and after activation with the agonist platelet activating factor (PAF) was studied. The unactivated platelet has a distinct ultrastructural appearance: the cytoplasm contains large randomly distributed granules but lacks the internal cristae that typify the open canalicular system in many types of mammalian platelets. Following PAF stimulation, large platelet aggregates form, but many platelets remain discrete, with little evidence of pseudopod formation or fusion of membranes. Two types of platelets are visible within the aggregates: those that are morphologically intact and those with gaplike features on the outer membrane and that have become degranulated, appearing as empty swollen sacs. The lack of platelet membrane fusion within the aggregates may permit the reversal of aggregation that is a characteristic response of elephant platelets to PAF.