Rcor1 null embryos exhibit defective embryonic erythropoiesis. (A) E13.5 control (Ctrl) and mutant (Mut) littermates. Note pale liver in mutant (arrow). Scale bar: 1 mm. (B) Comparison of E13.5 control and mutant fetal livers. The mutant fetal liver is smaller and paler than the control fetal liver. Scale bar: 1 mm. (C) May-Grunwald Giemsa staining of E13.5 fetal liver cytospin preparations. The control liver contains early erythroid progenitors (i: BFU-E or CFU-E like cells; ii: proerythroblast) and late erythroid precursors (iii: early basophilic erythroblast; iv: late basophilic erythroblast; v: orthochromatophilic erythroblast). The mutant fetal liver contains primarily early-stage erythroid progenitors. Scale bar: 10 μm. (D) May-Grunwald Giemsa staining of E15.5 peripheral blood smears showing enucleated definitive erythrocytes (arrow) in the control that are lacking in the mutant. In the mutant, nearly all the circulating blood cells are primitive wave erythroid cells of normal appearance. Scale bar: 20 μm. (E) Mean of enucleated RBC frequency in control (n = 5) and mutant (n = 3) E15.5 peripheral blood. Error bars show standard deviation (SD). ***P < .001. Images for (A-B) were taken with a Zeiss SteREO Lumar.V12 microscope, a Neo-Lumar S ×0.8 FWD 80-mm objective and an AxioCam HRc camera. Images for (C-D) were acquired with a Zeiss Axiovert S-100 (Carl Zeiss), an AxioCam HRc camera, and either (C) a Zeiss plan-neofluar ×100/1.30 oil lens or (D) a Zeiss plan-neofluar ×40/1.30 oil lens.