Lymph sacs form between E11.5 and E12.5. (A) Sagittal section of the posterior region of an E10.5 embryo revealed that at this early stage fewer Prox1-expressing LECs budded off from CV and ISVs (arrowheads) in the posterior region of the embryo (compare with anterior region in Figure 4B). These Prox1-expressing LECs are negative for podoplanin (compare with podoplanin expression in the anterior region shown in Figure 3A). (B) At approximately E11.0, migrating LECs form a capillary-like structure (arrowheads) also in the posterior area. (C) Prox1-expressing LECs organize into a lymph sac-like structure (white dashed line) at approximately E11.5. (D) The lymph sac (white dashed line) was obvious at approximately E12.5. All are sagittal sections (100-μm thick). LS, lymph sac (n = 4 embryos of each stage). Scale bars: 100 μm (A-D).