Epo and EpoR gene expression are spatially and temporally segregated.

(A) Expression of EpoR and Epo at E8.0 (upper panel) and at E9.0 (lower panel), as detected by whole-mount in situ hybridization. EpoR expression starts in the blood islands of the yolk sac (top left). Histologic sections confirmed that EpoR staining is confined to the cells in the blood islands of the yolk sac (inset). At E9.0, EpoR expression can be observed in the vasculature of the yolk sac and in the embryo proper (lower left). No Epo expression can be detected in the yolk sac or the embryo proper at E8.0 or earlier (top right). Epo expression starts at approximately E9.0, when staining is present in the vitelline of the embryo proper but is not present in the yolk sac (lower right). Original magnifications, all main panels, × 10; original magnification, inset, × 40. (B) Histologic section showing EpoR and Epo expression present in the urogenital ridges in the AGM region of the embryo proper at E10.0 (upper panels). Positive staining of EpoR and Epo can also be observed in the vitelline vessels leading to the hepatic primordium (lower panels). Sense strand probes derived from either Epo or EpoR were used on littermate controls, and nonspecific binding was not observed (data not shown). Original magnifications, all panels, × 20. BI indicates yolk sac blood islands; YS, yolk sac; EP, embryo proper; UV, umbilical vein; YSV, vasculature of the yolk sac; V, vitelline; UR, urogenital ridge; VV, vitelline vessel.