Fig. 4. VEGF activity determines the hematopoietic output during primitive erythropoiesis. (A) Frequency of Ery^P per individual E8.5 (5-14 somite pairs) yolk sacs from Vegf^+/+ (+/+), Vegf^lo/+ (lo/+) and Vegf^lo/lo (lo/lo) embryos. Yolk sacs were isolated, dissociated into single-cell suspension and assayed for their content in hematopoietic precursors. Plots illustrate pooled data from four individual litters and n is the number of embryos of a corresponding genotype. Horizontal bars represent median values. P<0.05 compared to either heterozygous () or wild-type embryos (*). (B) Gene-expression analysis of E8.5 Vegf^lo hypomorph yolk sacs. Globally amplified cDNA from single yolk sacs were probed for gene expression. The stage of development of the embryos, expressed in somite pairs (sp), is indicated at the top of each lane. No RT served as a control of genomic DNA contamination. (C) Plots illustrate the level of gene expression within individual yolk sacs as ratio of the indicated genes over L32 taken as an internal control. Horizontal bars represent median values.