ULTRASTRUCTURAL ASPECTS OF PLATELET-ADHESION ON SUBENDOTHELIAL STRUCTURES

Platelet adhesion to ligands within the subendothelium induces platele t activation. Ultrastructural and immunocytochemical investigations we re carried out to obtain information on the microtopography of the adh esive interactions. Rabbit platelets which react in vivo with endothel ial lesions (V. jugularis), and human platelets which react in vitro w ith collagen were observed. It was shown that the reaction of adhering platelets depends on the micro-topography of the adhesive surface 1. On plane surfaces the platelets spread 2. Collagen fibers induce the f ormation of focal contacts. They are initiated by binding of ligands t o certain transmembrane receptors. The contact mediates the formation of the contractile gel, which acts as a constricting sphere internaliz ing surface bound ligands, i.e., retraction of collagen networks. Most of the experiments hitherto dealt with platelets that spread. The des cribed findings show that the reaction mentioned above plays a more im portant role than previously imagined and may be the physiological res ponse of platelets interacting with subendothelial components in vesse l lesions.