Platelet adhesion and aggregation without endothelial denudation or exposure of basal lamina and/or collagen.

In vivo microvascular studies and postmortem studies of large and small blood vessels in a variety of species and vascular beds show that platelet adhesion and aggregation can occur over endothelium that is not denuded; the basal lamina and collagen need not be exposed. Moreover, evidence suggests that, at least in arterioles, locally adherent degranulating platelets can actually produce disruption and denudation of endothelial cells. Therefore, one should not assume, at least in small vessels, that observed sites of denudation were the cause rather than the result of adhesion/aggregation. All of this evidence should encourage a greatly increased emphasis on causes of adhesion/aggregation that do not depend upon collagen and/or collagen-bound von Willebrand factor (vWF). This emphasis does not deny the importance of collagen or collagen-bound vWF as the trigger for adhesion/aggregation when such exposure occurs. However, the emphasis on a structurally intact endothelial surface does lead to the corollary caution: even when endothelium is interrupted and potential triggers of adhesion/aggregation are exposed, this does not mean that these substances were, in fact, the cause of the locally observed adhesion/aggregation. Local exposure of key endothelial cell adhesion molecules such as PECAM may contribute to the adhesion/aggregation of platelets over structurally intact but injured endothelium. Adhesion/aggregation over injured but intact endothelium can also be modified by maneuvers that alter the local production of antiplatelet paracrine substances like endothelium-derived relaxing factor/nitric oxide. This supports the hypothesis that local decrements in the release of antiplatelet paracrine substances from perturbed but structurally intact endothelium leads to local adhesion/aggregation especially of platelets activated by a preexisting pathology. Coronary artery disease, ischemic stroke and diabetes are examples of diseases associated with both hyperaggregable platelets and with impaired endothelial synthesis/release of antiplatelet paracrine mediators. In addition, repetitive stereotypic symptoms in transient ischemic attacks may be related to repetitive and increasing damage to endothelium produced by successive episodes of platelet adhesion/aggregation/degranulation at the same sites.