The effect of thrombocytopenia on the ultrastructure and reaction to injury of vascular endothelium.

Rats or guinea pigs in which the level of circulating platelets has been reduced to less than 20,000/cm3 by anti-platelet serum show no generalized change in the ultrastructure of small blood vessels in skeletal muscle even after several days of thrombocytopenia. A similar degree of reduction of platelet levels in rabbits induced by busulphan is also without effect on the ultrastructure of small vessels in muscle. As judged by leakage of colloidal antimony sulfide (Sb2S3), thrombocytopenia in rats does not alter the normal permeability of small blood vessels in skin or bowel mucosa or modify the magnitude of increased permeability in skin vessels induced by histamine or by mild thermal injury. Thrombocytopenia is also without effect on either exudate volume or rate of leakage of Sb2S3 in pleurisy induced by intrapleural injection of homologous serum or turpentine. In guinea pigs, thrombocytopenia does not alter leakage of Sb2S3 from uninjured small blood vessels, but escape of Sb2S3 from skin vessels injured by histamine or mild thermal burning is reduced significantly in thrombocytopenic animals. The decreased leakage appears to reflect a diminished reaction of endothelial cells to injury and not an increase in the adhesion between endothelial cells. None of the findings provides an adequate explanation of the endothelial supporting function of circulating platelets.