The rat visceral yolk sac internalizes maternal transferrin and secretes hydrolyzed products towards the fetus

The uptake of transferrin by the rat visceral yolk sac membranes, and the f ate of this protein, were measured in a two-chambered system which allowed access to both surfaces of these membranes, i.e. that facing the maternal c ompartment and that facing the fetal compartment. I-125-labeled transferrin was internalized by the maternal surface of the visceral yolk sac but not by the fetal surface. Following internalization, this transferrin was degra ded and the amino acids were secreted exclusively towards the fetal compart ment. Transcytosis of intact transferrin was not detected in either directi on. These results suggest that transport across the rat visceral yolk sac b ound to maternally derived transferrin is not a major mechanism of iron tra nsport in vivo. These results support a role for the visceral yolk sac in f etal metabolism, of supplying the fetus with amino acids derived from degra dation of specific maternal plasma proteins, in this case, transferrin. (C) 2000 Elsevier Science Inc. All rights reserved.