Prox1-expressing LEC progenitors bud from the CV. (A-D) EM analysis of E10.5 Prox1^+/LacZ embryos (n = 3 embryos) shows that Prox1-expressing cells (stars) exit the CV via an active budding mechanism. During this process, the budding cells are attached by cell junctions (arrowheads in panels B-D). LacZ negative cells are indicated by black arrows. Panels B and D are high-power magnifications of the black boxed area in panels A and C, respectively. (E-F) As the Prox1-expressing LEC progenitors bud off, they express high levels of the cell adhesion marker VE-cadherin (n = 5 embryos). (G) Diagramatic representation of the budding process of Prox1-expressing LEC progenitors from the CV (the border of LEC progenitors is painted in red). Adhesion junctions between the Prox1-expressing cells ensure the integrity of the cardinal vein during the budding process. CV indicates cardinal vein. Scale bars: 10 μm (A-C), 1 μm (B), 2 μm (D), and 20 μm (E-F).