Serum erythropoietic activity in the rat around birth.

Using a fetal rat liver cell bioassay, serum erythropoietic activity was tested in non-suckled newborn rats comparatively to 21-day-old rat fetuses at time 0 or after 2 h of surviving. The number of erythroid colonies (CFUe) is significantly greater in cultures with neonatal and fetal rat serum after 2 h of surviving than in cultures with fetal rat serum sampled at time 0. These results suggest a release of erythropoietin or erythropoietin-like substances in response to a short hypoxic phase following the parting of the fetus with the mother and preceding the autonomous oxygenation of the newborn by breathing.