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Transferrin binding by the placenta increased progressively from Day 14 to Day 21 of gestation. Qualitatively similar changes in the rate of iron transfer to the fetus were also seen. These results suggest that the increase in iron transfer to the fetus occurs as a result of an increase in the number of transferrin receptors on the maternal surface of the placenta rather than being due to a change in the affinity or turnover rate of the existing receptors, although disproportionate change in the number of receptors and maternal placental surface area suggests that there may be a reduction of the density of transferrin receptors. The ratios of fetal and total (fetal plus placental) iron to transferrin showed that maturation of the mechanism of iron release from transferrin and the intra-placental iron transport system occurred during development. In contrast to that in the chorioallantoic placenta, iron and transferrin incorporation into the yolk sac decreased during gestation, indicating that the yolk sac does not have a physiological role in the transport of iron from the mother to the fetus in the last part of pregnancy. For at least 28 h after fetectomy, the placenta continued to accumulate iron at the same rate as the controls. The iron thus accumulated was stored mainly as insoluble haemosiderin, and was not refluxed into the maternal circulation. These results indicate that iron transport by placental cells is 'preprogrammed' and is not subject to feedback control by the fetus or the placenta itself.

Transferrin and iron movements in the rat conceptus during gestation