Injury of brain microvessels with a helium-neon laser and Evans blue can elicit local platelet aggregation without endothelial denudation.

A helium-neon laser was used to produce endothelial lesions and local platelet aggregation in brain microvessels (pial arterioles and venules) sensitized by intravascular Evans blue. Electron microscopy showed that the aggregates formed in the absence of endothelial denudation or exposure of basal lamina. Initial lesions consisted only of endothelial lucencies, vacuoles, and distended rough or smooth endoplasmic reticulum. Aggregates consisted not only of degranulating platelets, rounded platelets, and platelets with pseudopods, but also of masses of discoid platelets. Red blood cells were uninjured and did not form the nidus of these collections. The possibility that aggregation occurred as a result of direct injury to the platelet, and independently of the endothelial damage, is discussed and considered unlikely. Rather, the data are consonant with the hypothesis that in the microcirculation, endothelial injury can produce aggregation prior to exposure of basal lamina.