vhl mutants develop severe polycythemia. (A) vhl mutants develop a severe increase in circulating red blood cells (polycythemia), apparent by 40 hpf by blood accumulation under the heart on the yolk sack (). (B) o-Dianisidine blood staining at 48 hpf. (C) Circulating blood cells imaged just anterior to the cloaca. Whereas normally a single row of blood cells is transported through the dorsal aorta (DA) and posterior cardinal vein (PCV) at 8 dpf, several rows of circulating blood cells are observed in vhl mutants. Anterior is left. (D) Confocal analysis through the CHT (posterior to cloaca) of 5 dpf embryos expressing the CD41:egfp transgene reveals an enlarged population of CD41-GFP⁺ hematopoietic cells between the DA and PCV in vhl mutants. Anterior is left. Scale bar indicates 100 μm. (E) Whole-mount in situ hybridization for c-myb mRNA reveals an increase number of c-myb⁺ HSCs in the vhl^−/− CHT at 7 dpf. (F) BrdU incorporation assays demonstrate increased proliferation in all hematopoietic tissues in 7.5 dpf vhl^−/−, as shown in whole-mount and cross-sections (i-iii). Original magnifications, ×5 (A,B,E), ×20 (C,Fi-iii), and ×40 (D).