Runx1 expression in the mouse dorsal aorta during EHT. (A) Diagram of the ventral wall of the mouse E10.5 aorta showing Runx1-expressing cells (blue). Cells undergoing EHT are indicated as hemogenic endothelial (he) or hematopoietic cluster (h) cells. Aortic endothelial cells are indicated in gray, and Runx1-expressing hemogenic endothelial cells are in blue. Some mesenchymal cells (beige) underlying the aorta are Runx1 expressing. Runx1-expressing hematopoietic cell clusters are closely associated with the ventral luminal wall of the aorta. (B) Runx1 in situ hybridization analysis on a transverse section through the E10.5 mouse dorsal aorta. Runx1 expression (blue) is detected mainly at the ventral aspect of the aorta where it is seen in the subaortic mesenchymal cell layer, in some endothelial cells, and in hematopoietic cluster cells. In situ hybridization, microscopy, and image processing were performed as described in Bee et al.⁴⁶  Original magnification ×400. (C) Schematic of the Runx1 +23 enhancer-reporter transgene construct (23GFP) and resulting GFP expression (green) in a transverse section through the E10.5 aorta of a 23GFP transgenic embryo. Endothelial cells (VE-cadherin labeled) are in red and nuclear stain (TO-PRO-3) in blue. The 23GFP transgene marks hemogenic endothelium in the vessel wall and all hematopoietic stem and progenitor cells (HSPCs) in the aortic clusters.^55,135  Immunofluorescent labeling, confocal microscopy, and image processing were performed as in Swiers et al.⁵⁵  Original magnification ×300.