Scanning electron microscopic observations on human blood platelets and their alterations induced by thrombin.

The surface structure of human blood platelets and their alterations by thrombin were studied by scanning electron microscopy.The majority of the native platelets were discoid or lentiform as already reported by previous authors using the transmission electron microscope. The surface was relatively smooth, but some platelets had a few slight elevations and conic depressions which may correspond to the openings of the endoplasmic reticulum. A few platelets possessed at the margin one or two spiny short projections.The platelets collected by careful centrifugation partly showed a few long pseudopods which were issued from the margin of the platelet and supposed to be the elongation of the spiny projections described above. About 78% of the collected platelets were discoid in form, 12.5% spheroidal, 5.5% spherical and 4% irregular.The addition of thrombin to the citrated, platelet rich plasma caused the following changes. At first small protrusions appeared on the entire surface of the platelets except for their margin. The discoid platelets then transformed into spheres covered by the grown and rounded protrusions and aggregated, while the formation of pseudopods proceeded. Fibrin strands about 0.1μ in thickness appeared to form networks into which the aggregated platelets were arrested more and more firmly.