The ultrastructure of initial stages of platelet aggregation and adhesion to damaged vessel walls in vivo.

The ultrastructure of the early aggregation of platelets was studied in homografts of lung and heart in 5 dogs. Because of the vascularity of the lung this tissue provided an opportunity to observe various stages in the aggregation of platelets following endothelial damage in vivo. The different steps involved in platelet aggregation from the formation of spiky processes to the production of the complete platelet aggregate associated with fibrin were examined. Damaged endothelial cells in this situation did not induce the adhesion of platelets, even when their immediately underlying basement membrane was covered by a platelet layer 3 cells thick. In some areas fibrin strands occurred between the platelets of this layer, but these fibrin strands did not extend beyond the platelet laminae.