Endogenous erythroid colony-forming cells in fetal and newborn sheep.

In plasma clot cultures fetal, newborn, and adult sheep bone marrow cells respond to Ep, giving rise to large numbers of colonies of erythroid cells. In fetal and newborn, but not adult, sheep the marrow produced EEC in the absence of added Ep. The EEC-forming cells represent a transitory population of erythroid progenitors, persisting for approximately 2 month post-partum. The activity of these cells in the normal newborn sheep displayed a cyclic pattern, which was abolished in the presence of high circulating Ep levels. In both fetal and newborn sheep, EEC formation was inhibited by prior neutralization of Ep present in culture media with anti-Ep, suggesting that as in patients with PV the proliferation and/or differentiation of EEC-forming cells of sheep is subject to Ep control.