Role of uteroferrin in transplacental iron transport in the pig.

The pig possesses a noninvasive, diffuse type of epitheliochorial placentation in which the blood supply of the mother is well separated from the absorptive surface of the chorion, a feature that must complicate the movement of nutrient molecules across the placenta. Evidence is presented that a protein synthesized and secreted by the glandular epithelial cells of the maternal uterus of the pig is involved in iron transport to the fetus. This protein, uteroferrin, is induced by progesterone; is purple, which results from an unusual iron center; and possesses acid phosphatase activity. Secreted uteroferrin is taken up by specialized chorionic epithelial cells located in domed structures, called areolae, overlying the mouth of each uterine gland. Uteroferrin then enters the placental venous drainage and its iron is efficiently incorporated into fetal hemoglobin. It is taken up by the fetal liver or cleared by the kidney. The liver is the main site of erythropoiesis in the fetus. From the kidney uteroferrin enters the allantoic sac where it exchanges its iron with fetal transferrin. The rate of uteroferrin biosynthesis in the uterus and its rate of metabolism in the fetus can theoretically provide sufficient iron for the needs of pregnancy, at least until around day 70 of the 115-day gestation. Uteroferrin and transferrin, the iron transport protein of plasma, appear to be unrelated proteins.