Platelet Adhesion Plaques: Anchors for Contraction of the Hemostatic Plug.

Abstract 5128

Exposure of GPIIb-IIIa and other receptors on the surface of activated platelets, binding of fibrinogen, molding of shape-changed cells into tight aggregates, internal assembly of actin molecules into filaments and movement of talin, an actin-binding protein, to the inner membrane surface provides the framework for clot retraction. However, the direction of contractile force towards the center of large aggregates or clots would lift the hemostatic plug away from the edges of vascular injury. Another mechanism must be present to facilitate the direction of contractile force toward the damaged vessel wall. This may be accomplished by development of adhesion plaques as platelets spread out on the injured vessel. The present study has used scanning (SEM) and transmission electron microscopy (TEM), confocal and immunofluorescence microscopy to detect adhesion plaques developing at sites of contact as platelets spread on surfaces. Rhodamine-phalloidin was used to detect actin filaments, and an anti-talin antibody identified by protein-A gold or Alexa Fluor 488 labeled rabbit anti mouse IgG to demonstrate talin. Normal human platelets were spread on clean glass slides or plastic chambers for intervals of up to 90 min, extracted with Triton X100 or fixed intact then labeled for talin and actin, and prepared for study by the several microscopic techniques. Triton-extracted spread platelets revealed attachment plaques well stained for talin and actin when examined by SEM or TEM. Inmunofluorescence studies of spread platelets stained with rhodamine-phalloidin and antibodies also revealed co-participation of actin filaments and talin in formation of the adhesion plaques. The association of actin and talin remained intact at all intervals for up to 90 min. Clearly adhesion plaques serve as the anchors for contraction and sealing of hemostatic plug to damaged vascular surfaces.