AGM definitive hematopoietic cell emergence shows a greater dependence on P2-Runx1 than P1-Runx1. Panels A, D, G, J, M, and P are representative bright-field images of dissected AGMs of P1 or P2 mutant, +23GFP transgenic compound embryos. Genotypes are as indicated. Panels B, E, H, K, N, and Q are corresponding fluorescent images of dissected AGMs. White arrowheads indicate position of the dorsal aorta. Levels of GFP expression in the aorta are dependent on P1 and P2 genotype. Settings for photomicrographs were identical within the P1 and within the P2 group. Panels C, F, I, L, O, and R are representative images of sections through the AGMs shown, displaying +23-mediated GFP expression in hematopoietic cell clusters (green, examples indicated by white arrowheads). Cell nuclei are shown in red (Topro-3-iodide stained). Total number of embryos analyzed: 2 P1N/N, 1 P1N/+, 2 P2neo/neo, 1 P2neo/+, and 3 wild-type littermates. Scale bar: 100 μm. *Compaction of the mesenchymal cell layers underlying the aorta.