Prox1 is essential for LEC progenitors to leave the CV. Transverse sections (A-B) and sagittal sections (C-D) from E10.5 Prox1^{GFPCre/GFPCre} embryos (Prox1 null) showing that GFP-positive cells outside the CV are all located in the ISVs (arrowheads) and not in the surrounding mesenchyme (n = 3 embryos). (E-F) EM analysis showing that X-gal⁺ cells (stars) are colocalized with the venous ECs in the ISV outside the CV in E10.5 Prox1^LacZ/LacZ embryos. Scale bars: 100 μm (A-D), 10 μm (E-F).