Defective erythropoiesis in Epo^−/− and EpoR^−/− embryos.

(A) Comparing the amount of red blood cells in the heart (H) and yolk sac (YS) regions of E9.5 mutant embryos (^−/−) with age-matched control embryos (^+/− ). Normal levels of EryP can be detected in both types of mutant embryos. Original magnification × 10. (B) Epo^−/− or EpoR^−/− embryos isolated at E10.5 or later showed profound decreases in levels of erythropoiesis in both the embryo proper and the yolk sac. Epo and EpoR null mutants had identical erythropoietic phenotypes at all stages studied. Original magnification × 10.