ldb1 is required for primitive erythropoiesis. (A) E9.0 embryos were photographed, paraffin embedded, and sectioned for H&E staining. Left, E9.0 ldb1^+/+ and ldb1^−/− embryos from an ldb1^+/− × ldb1^+/− mating showing yolk sacs and blood vessels. Bars, 500 µm. Center, H&E staining of E9.0 embryos showing yolk sac blood islands. Arrows designate hematopoietic cells in yolk sac blood islands. Bars, 50 µm. Original magnification, 400×. Right, Giemsa staining of cytospins prepared from E9.0 yolk sac. Bars, 50 µm. Original magnification, 400×. Data are representative of 3 ldb1^−/− embryos and 11 littermate controls. Controls consisted of ldb1^+/+ and ldb1^+/− embryos, which were phenotypically indistinguishable (not depicted). (B) Single cell suspensions from yolk sacs were prepared for in vitro methylcellulose culture. Erythroid (E) and nonerythroid (non-E) CFCs in E9.0 ldb1^−/−, ldb1^+/−, and ldb1^+/+ yolk sacs. Colonies were counted on days 6 and 14 after the initiation of the in vitro culture. One representative of two experiments is shown. Values are expressed as means ± SD. (C) Giemsa staining of cytospins prepared from day 14 methylcellulose cultures derived from E9.0 ldb1^+/+ or ldb1^−/− YS. Bars, 50 µm. Original magnification 400x. Images are representative of two experiments.