11.5-Day Rat Embryos Cultured in vitro after Introduction of Immunoglobulin G into the Vitelline Circulation

The fate of rat immunoglobulin G (IgG) in the 11.5-day-rat conceptus cultured in vitro has been studied utilizing the intravitelline cannulation technique. When IgG bound to colloidal gold was introduced into the vitelline circulation, gold particles were detected on the luminal surface of embryonic endothelial cells, in both coated pits and vesicles and in various portions of the vacuolar system of the embryonic endothelial cell. By means of the ratiolabeled macromolecule, it has been demonstrated that the internalized IgG was not degraded. In comparison, digested products of radiolabeled bovine serum albumin (BSA) were detected in culture media after the macromolecule was introduced into the conceptus. It was therefore concluded that the 11.5-day rat embryo captures IgG probably by receptor-mediated endocytosis and does not degrade the macromolecule, indicating that IgG is not routed to the lysosomal compartment of the endothelial cell even though the embryo has the capacity to digest BSA. It appears therefore that the embryo is endowed with the capacity to handle the IgG macromolecule well before the macromolecule is introduced into it for passive immunity.