EpoR-null embryos were rescued from severe anemia and embryonic death by hematopoietic lineage-specific expression of the EpoRtransgene.

A single litter obtained by crossingEpoR^+/−::Tg withEpoR^+/−mice was used in this study. Wild-type (A), Tg-B (B), EpoR^−/−(C, E), and Rescued-B (D, F) embryos are shown. Panels A to D show whole E10.5 embryos, whereas panels E and F show sections of the yolk sac of E11.5 embryos. Note that the yolk sac blood vessels of the rescued embryo were filled by nucleated erythrocytes (arrowheads in panel F), whereasEpoR^−/− embryo contained only a small number of erythrocytes (E). Scale bar, 50 μm. Whole E12.5 embryos are shown in panels G to J. In contrast to theEpoR^−/−embryo (I), the size and redness of the liver (arrowhead) in the Rescued-B embryo (J) were similar to those of the Tg-B (H) and wild-type (G) embryos. The E12.5 embryos shown in the fluorescence images K to N were from a single litter obtained by crossing EpoR^+/−::Tg-Awith EpoR^+/−mice. Because HG1-EpoRand HG1-GFP transgenes were coinjected, transgenic EpoR expression could be monitored by the intensity of green fluorescence. Green fluorescence was detected only in the livers (arrowheads) of Tg-A (L) and Rescued-A (N) embryos.