The behavior of platelets in capillary hemorrhage induced by snake venom.

The behavior of platelets in capillary hemorrhage has been investigated by using highly purified hemorrhagic principle from a snake venom. The first capillary hemorrhage was observed through a light microscope four minutes after topical application of the hemorrhagic principle to the microcirculatory system of the rat mesentery. With time hemorrhagic spots increased in num ber and size. The electron microscopic observations of the affected capillaries showed a partial discontinuity of the endothelial lining. The basement mem brane occasionally maintained its continuity even when the endothelial cells did not. When the endothelial gap was narrower than one micron, a single platelet which had not degranulated, was frequently seen to have a pseudopod within it. When the gap was 1-2 microns wide, platelets appeared to change their shape so as to fit the entire cell into the gap. Most of these platelets showed partial degranulation. When the gap was more than two microns wide, platelets collected and degranulated into it. In general, the wider the endo thelial gap was, the more platelets collected and degranulated in it. Some degranulated platelets were found in the area enclosed by endothelial cells, a basement membrane and a pericyte. The experimental results suggest that the endothelial injury itself may provoke the reaction of platelets before the platelets make contact with extra-capillary tissue constituents.