An electron microscopic visualization of transport across rat visceral yolk sac


  • Roger O. Lambson

    1. Department of Anatomy, Tulane University, School of Medicine, New Orleans, Louisiana
    Current affiliation:
    1. Department of Anatomy, University of Kentucky Medical Center, Lexington, Kentucky 40506
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  • This work is part of a dissertation submitted to the Department of Anatomy, Tulane University, School of Medicine as a requirement for the Doctor of Philosophy degree. The author gratefully acknowledges his sincere appreciat on to Dr. William P. Jollie for his patient understanding and technical demonstration throughout this investigation and finally for his critical assistance in the preparation of the manuscript. The investigation was supported by N.I.H. Research grant GM-11484 and in part by N.I.H. Training grant 5T1-GM-793.


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.