In an attempt to elucidate some of the mechanisms utilized in the placental transfer of large molecules morphological changes in the visceral yolk sac have been studied in relation to the disposition of ferritin molecules at different stages of gestation.
From the tenth through the eighteenth days of pregnancy, ferritin molecules were readily absorbed by the visceral endodermal cells when this material was injected either into the maternal blood stream or directly into the uterine lumen. These large molecules entered the endodermal cells through a progressively developed apical canalicular system and became localized within relatively large, heterogenous vacuoles. These vacuoles were seen to retain the ferritin for long periods, except during the last few days of gestation. At the end of pregnancy, when normally antibodies are being transferred in the largest quantities to the fetus, relatively smaller vesicles which contain ferritin appeared to have migrated away from the larger apical storage vacuoles and to have approached the endodermal basal plasma membrane. Once the large protein molecules left these cells they appeared to pass either directly into vitelline capillaries, or through mesothelial cells which line the exocoelom.
Transport of large protein molecules through visceral endodermal cells involves an apical tubular system, large storage vacuoles, and smaller vesicles which appear to migrate basally near the end of pregnancy. Regulation of this process appears not to depend on the failure or success of entry of such a substance into these cells, but rather on its subsequent disposition once it is intracellular.