Changes in globin gene mRNA accumulation. (A) E7.5 yolk sac, (B) E8.5 yolk sac, and (C) E10.5 peripheral blood. Relative levels (mean ± SEM of at least 3 independent experiments) of the β-globin genes (ϵy, βH1, β1, and β2) and of the α-globin genes (ζ, α1/α2) quantified by qPCR are shown on the left. Total β- and total α-like globin transcript levels are each considered as 1. Expression of these same genes by in situ hybridization is shown in the right panels. Arrowheads indicate blood islands. For comparison, at each developmental time point, similar exposures of all of the globin genes are shown. In panel B, short exposures of the βH1- and ϵy-globins, marked by asterisks, highlight their differential transcript accumulation. ζ- and α1/α2-globin transcripts were not consistently detected at E7.5 (data not shown). Images were processed as described in “Materials and methods.”