Regulatory aspects of placental iron transfer--a comparative study.

Receptor-mediated endocytosis is generally assumed to be the process by which the haemochorial placenta takes up iron from transferrin. The involvement of an additional nonendocytic process cannot, however, be excluded. It appears from a study of iron transport mechanisms and of the maturation of the transfer process that placental ferritin is involved in the transfer of iron from mother to fetus. The metabolic relationship between the ferritin pool and the placental transfer pool remains to be elucidated. There is no evidence for short-term regulation of placental transfer capacity in response to changes in the maternal iron supply or to changes in the trophoblastic iron content. This cannot yet be said of fetal feedback control of placental iron uptake because the experiments performed so far do not permit conclusions on this point. The capacity for iron uptake and transfer seems to increase in accordance with the ontogenetically determined placental growth pattern. This does not exclude long-range adaptive modifications of the transfer capacity in response to early maternal or fetal disturbances. The results obtained from studying placental maturation suggest that a possible long-term regulatory interaction between growing placenta and fetus may occur. Clinical evidence so far is inconclusive. The relatively moderate reductions in the fetal iron stores which are generally associated with severe iron-deficient pregnancies might be seen as an argument in favour of long-term placental control. The marked impact of pregnancy on maternal iron metabolism in rodents, as compared to other mammals, is possibly met by means of direct placental control of mucosal iron uptake. In primates, mucosal iron uptake during pregnancy seems to be governed by factors related to systemic iron deficiency only.