vhl mutants display an altered blood composition and develop anemia. (A) May-Grünwald-Giemsa–stained blood smears of 8 dpf vhl mutants and siblings. Relative to the number of (more) mature erythrocytes, vhl mutants display a marked increase in circulating orthochromatophilic erythroblasts (40%, n = 9) compared with siblings (5%, n = 9). Similar to human embryonic blood, zebrafish erythrocytes are nucleated. The cytoplasm of the morphologically more mature vhl^−/− erythrocytes appears paler (hypochromic). (B) To investigate whether vhl mutants are iron deficient, blood cells were stained with o-dianisidine. Whereas in siblings uniformly stained dark brown erythrocytes are observed (Bi), in vhl mutants no () or variable heme levels are detected at 6.5 dpf (Biii). Injection of 100 mM iron-dextran at 2 dpf rescued the observed anemic phenotype (Biv). (C) Rescue is even observed in live vhl mutants by darker red circulating blood cells. Original magnifications, ×40 (A,B) and ×5 (C). Scale bar indicates 25 μm.