Platelet membrane interactions

The term 'viscous metamorphosis' was coined in early studies to describe the fusion and disintegration of platelets into an amorphous mass sealing sites of vascular injury. Improvements in morphological techniques revealed that disintegration was not involved in the platelet hemostatic reaction, and that part of the definition was eliminated. Fusion was retained, however, even though platelet fusion was never observed. The present study has examined the different forms of platelet membrane interaction to determine if fusion is an appropriate term for any aspect of platelet physiology or pathology. Fusion was not observed during platelet-platelet, platelet-surface or platelet-fibrin interaction at any stage in the development. Only fusion of secretory organelles with channels of the open canalicular system (OCS) during the release reaction represents involvement of the phenomenon. Pathological fusion occurs during long-term storage when alpha granules bind together to form giant organelles. The only examples of surface membrane fusion develops when platelets are incubated with the cholinergic agent, carbachol (carbamyl choline chloride). Ethylenediamine tetraacetic acid also causes intimate associations of OCS channels and interacting surface membranes, but not the fusion of lipid bilayers that carbachol induces. Thus, fusion does not appear to be an appropriate term to describe platelet-membrane interactions, except for the process of secretion.