Electron microscopical tracers in the uterine epithelium of the pregnant guinea-pig

Horseradish peroxidase and lanthanum nitrate were used in pregnant guinea-pigs as electron dense tracers to determine whether the 'permeability' characteristics of the uterine epithelium support the hypothesis that immunoglobulin G gains access to the uterine lumen by transepithelial diffusion. Horseradish peroxidase was injected intravenously in eight animals in experiments ranging from 1-43 min and directly into the uterine lumen in five animals in experiments of 1-8 min duration. Lanthanum nitrate was injected only into the uterine lumen of eight animals for exposures of 1-8 min. Horseradish peroxidase did not traverse the junctional complexes regardless of injection site; lanthanum nitrate did not penetrate the complexes either except in one animal. We conclude that the uterine epithelium is a barrier that prevents the diffusional transfer of IgG from mother to fetus. Further studies are required to locate the site where maternal IgG is transferred to the uterine lumen.