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Effect of intrauterine growth on erythropoiesis.

Erythropoietic activity, as reflected in the reticulocyte count and the incorporation of 59Fe into red cells, was studied in relation to intrauterine growth of rabbit fetuses during the last third of the period of gestation. Radioiron (10 microci) was administered by i.v. injection into the mothers and placental transfer to the fetuses. The fetuses were extracted after 48 hours. The amount of radioiron received by each individual fetus was determined by whole body counting. Radioactivity in red cells was determined and calculated as a percentage of the dose received. From day 21 to day 31 of gestation, the hemoglobin concentration increased by a factor of 1.5, from 8 to 12.2 g/100 ml, while the body mass increased by a factor of 7, from 6.6 to 47.5 g. During the same period the rate of growth declined from 45 to 20% in 24 hours. This was paralleled by a drop in 59Fe incorporation from 35 to 12% and in the reticulocyte count from 85 to 14.5%. It is concluded that the rate of erythropoiesis in fetal life is largely controlled by the rate of growth and the corresponding increase in hemoglobin mass.