Prox1-expressing LEC progenitors are also found on the ISVs. (A-C) Sagittal sections of E10.5 Tie2Cre;COUP-TFII^+/f embryos (n = 3 embryos) showing the presence of Prox1-expressing LEC progenitors in the ISVs (arrowheads), which are labeled by COUP-TFII β-gal reporter staining. (A-C) Shown from anterior to posterior. Short arrows in panel A highlight the Prox1-expressing LEC progenitors that are budding off from the ISVs. Long arrows point to Prox1-expressing LEC progenitors budding off from various points around the CV that appear to alternate with those from the ISVs. (D) EM analysis showing that X-gal–positive Prox1-expressing cells (stars) are intermingled with X-gal–negative venous endothelial cells in the ISVs at E10.5. Higher magnification of the X-gal expressing cells is shown in supplemental Figure 1. (E-F) At the 20ss, Prox1-expressing LEC progenitors are already detected in the CV (white dashed line) and ISVs (arrowheads; n = 4 embryos). Panel F is high-power magnification of panel E. ISV, intersomitic vessels; Scale bars: 100 μm (A-C,E), 20 μm (D), and 50μm (F).