Regulation of Erythropoiesis. XV. Neonatal Erythropoiesis and the Effect of Nephrectomy *

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Abstract
Normal newborn rats have a hemolytic anemia characterized by hypochromic macrocytosis. The hemolysis, although substantial, is largely compensated for by intense erythropoiesis so that the anemia itself is relatively mild. The character of the erythropoietic response, i.e., production of hypochromic macrocytic cells, and the high mitotic index suggest that red cell production is governed by a different mechanism than in adults. Further support for this thesis was provided by studies on nephrectomized newborns. Nephrectomy did not importantly affect erythropoiesis during the first 20 days of life. Transfer of maternal erythropoietin through the milk was excluded by suppressing maternal erythropoietin production by hypertransfusion. It is concluded that red cell production in the newborn rat operates independently of renal-produced erythropoietin. It is further suggested that red cell production in the newborn rat is analogous to fetal erythropoiesis in human beings.