The intravascular fibrinoid which is deposited in glomerular capillaries of the rabbit during the generalized Shwartzman reaction has been studied with the aid of the electron microscope. In one group of animals the reaction was produced by two intravenous injections of Escherichia coli lipopolysaccharide endotoxin, spaced 24 hours apart. In another, a single dose of endotoxin was followed, 1 hour later, by intravenous liquoid (sodium polyanethol sulfonate).

The appearance of fibrinoid was the same in the two groups. Initially, fibrinoid deposition occurred on the irregular, swollen surfaces of the endothelial cells within the capillary lumen. Subsequently, the fibrinoid mass increased to such proportions that the capillary lumen was completely occluded.

Fibrinoid was found to be composed of unbranched fibrils, having a diameter of 200–300 angstroms and an axial repeating structure of 120 A.

The basement membrane (lamina densa) underwent no change in appearance during the time when fibrinoid was being laid down.

Balloon-like vesicles were consistently encountered in endothelial cells of glomerular capillaries after two doses of endotoxin, and also in animals given one injection of endotoxin followed by liquoid.

The possible significance of the observations are discussed. It is suggested that they are compatible with the hypothesis, proposed earlier, that intravascular fibrinoid, in the generalized Shwartzman reaction, is derived from fibrinogen.

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