Hematopoietic clusters within small vessels around vitelline artery at E10.5. Whole-mount immunostaining of E10.5 (35 sp) Runx1-GFP transgenic mouse for GFP and CD31 expression. (A) Three-dimensional imaging of CD31⁺ vasculature in E10.5 embryo. To clearly visualize the vascular plexus of mesentery, the z-stacks containing extra-embryonic part of vitelline artery and umbilical vein were removed from 3D projection. Scale bar: 200 μm. (B-G) Higher magnification views of boxed region in panel A. Confocal sections (B-D) and 3D reconstruction (E-G). Arrows indicate large hematopoietic cluster. Scale bar: 100 μm. Supplemental Videos 1 and 2 show a 3D image and sequential images along the z-axis, respectively. DA indicates dorsal aorta; VA, vitelline artery; and UA, umbilical artery. Blue arrowheads demarcate the area where small vessels emerge from VA.