The release of vesicles from platelets following adhesion to vessel walls in vitro.

The ultrastructure of the adhesion of platelets to the subendothelial layers of arteries was examined in man (coronary artery), rabbit (aorta) and rat (aorta) In each case dendritic platelet pseudopodia proceeded from the platelets. These dendritic pseudopodia were frequently associated with multivesicular membranous sacs. These sacs appeared in various forms and every gradation from profiles containing closely packed vesicles to rupture of the primary sac and release of the contained vesicles was observed. Following initial contact with the subendothelial layer by dendritic pseudopodia (and on many occasions by associated multivesicular membranous sac) progressive stages from a free-floating platelet to one closely applied to the basement membrane were noted. Granules were not extruded and were present in the main cytoplasmic masses of the platelets in contact with the basement membrane. Vesicles were released from the membranous sacs directly from the main cell mass of the platelet on contact of platelets with the vessel wall and at or near the terminal bulb of platelet dendritic pseudopodia. Human platelets in contact with the basement membrane of human coronary artery tended to form a thin usually monocellular layer more rapidly than platelets in the other 2 species. It is postulated that the release of vesicles from the multivesicular membranous sacs is the morphological basis of the platelet release reaction.