The platelet pseudopodium and its involvement in aggregation and adhesion to vessel walls.

The ultrastructure of the phases of adhesion and aggregation of platelets in close association with damaged endothelium of the aorta and inferior vena cava of the rat was examined. Multivesicular membraneous sacs were found at or near the termination of platelet dendritic pseudopodia at the adhesive points between these structures and basement membrane, fibrin and pseudopodia from other platelets. Progression from a primary sac with closely packed secondary vesicles or subunits to separate subunits and bursting of the enclosing primary membrane was noted. Some of the multivesicular membranous sacs arose directly from the main cytoplasmic mass of the platelet though the more usual mode of origin was via platelet dendritic pseudopodia. Attachment of platelets to basement membrane permitted the release of the secondary vesicles from the multivesicular membraneous sacs. The multivesicular membranous sacs were on average 0·5-0·6 μm. in least and greatest diameters and contained up to 18 vesicles of about 0·08-0·11 μm. in diameter in section. These sacs occurred particularly at the stage of platelet aggregation in which there was complex intermingling of filiform platelet pseudopodia.