Up-regulation of Fas expression correlates with anemia in Raf-1-deficient erythroblasts and embryos. (A) Anemia in E10.5 Raf-1-deficient embryos is rescued by the introduction of one lpr allele. (B) Faster differentiation correlates with increased Fas surface expression in WT and Raf-1^-/-fetal liver erythroblasts. Erythroblasts were cultured in erythroid medium. The progress of differentiation (shown as hemoglobin content per cell and cell volume; B) and Fas expression (C; determined by FACS) were determined at the indicated times during differentiation. For panels B and C, one representative experiment of 6 is shown. F2 embryos and embryo-derived cells originate from crosses of F1 Raf-1^+/-;lpr/+ 129/SvHsd:Bl6 mice. Embryo images were acquired using an MZ-Apo stereomicroscope equipped with a PLAN-APO 1.0 ×/12.5 NA bottom objective (Leica, Vienna, Austria). A DKC-5000 CCD camera (Sony, Cologne, Germany) was used to capture images, and Image Access v4.5 software (Imagic, Glattbrugg, Switzerland) was used to acquire them.