Stepwise development of the mammalian lymph sacs. (A) Diagramatic representation of a transverse view of the CV of a mouse embryo showing that at ∼ E9.0, COUP-TFII, and Sox18 are expressed in VECs in the CV and ISVs. Sox18 and COUP-TFII expression induce Prox1 expression at ∼ E9.75 in a subset of competent VECs both in the CV and ISVs. These Prox1-expressing cells become LEC progenitors. Around E10.5 and as these LEC progenitors bud off from the CV and ISVs under the influence of mesenchymal Vegfc signals, they start to express podoplanin. Once LEC progenitors exit the CV and start expressing podoplanin, the LEC fate becomes specified. (B) Temporal representation of this process on sagittal sections. At E10.5 the number of LEC progenitors that budded off from the CV and ISVs is more abundant in the anterior region of the embryo and Prox1-expressing LECs in the migrating front start to express podoplanin around this stage. Around E11.0, all Prox1-expressing LEC progenitors outside of the CV and ISVs express podoplanin. As LECs bud off from the CV dorsally and from the ISVs longitudinally, they merge together in the mesenchymal tissue to form a capillary-like structure at E11.0 that is more obvious in the anterior region of the embryo. At around E11.5, these specified podoplanin-expressing LECs organize into a lymph sac-like structure.