Immunocytochemical localization of antibody during placental transmission of immunity in rats.

Autologous anti-horseradish peroxidase antibody has been localized in pregnant rats at various levels through placental membranes en passage to fetal circulation. The antibody was complexed in situ with the antigen and the resulting complex was made visible electron microscopically by immunohistochemical techniques. Visceral yolk-sac membranes from fetuses during the last half of gestation, viz. at 12, 17, and 22 days gestation, were so examined; differences were noted in the localization of cytochemical reaction product at increasing gestational ages. In yolk-sac endodermal epithelium at 12 days, the product was identified on receptor areas at the free surface of the epithelium, in a system of endocytosed clathrin-coated vesicles, or endosomes, within a closely related apical canalicular system, and stored in subapical vacuoles. Uptake and storage appeared markedly reduced in membranes taken at 17 days and was least apparent at 22 days (the day before term). Moreover, the product of antibody localization was not identified in the apical canaliculi at the two later stages. It is believed that most antibody had been hydrolyzed by these later stages. However, at the latest stage, reaction product also was identified in small vesicles in the basal cytoplasm of the endodermal epithelium and in the endothelium of vitelline (i.e., fetal) capillaries. These morphological results suggest that during the latter half of pregnancy antibody is absorbed; it is stored and hydrolyzed early on, and is transferred to fetal circulation only near term, a conclusion in close accord with what is known physiologically about prenatal transmission of immunity in this species.