(A–G’) Whole-mount and sagittal section images of Isl1Cre/+;Rosa26eYFP/+ embryos, in which platelet endothelial cell adhesion molecule (PECAM), eYFP, and Prox1 were labeled at embryonic day (E) 11.5, E12.0, or E14.5, are shown. (A, B) eYFP+/Prox1+ cells were observed around the cores of the first and second pharyngeal arches at E11.5 (white arrowheads). (C–D’’) eYFP+/Prox1+ LECs migrated from the first and second pharyngeal arches to the lymph sac-forming domain (white dotted region) adjacent to the anterior cardinal vein at E12.0 (white arrows). (E–G’) Some of the eYFP+/Prox1+ LECs expressed PECAM and formed lymphatic capillaries in the lower jaw and tongue at E14.5 (white arrows). (H–N’) Whole-mount and sagittal section images of Isl1CreERT2/+;Rosa26eYFP/+ embryos, in which PECAM, eYFP, and Prox1 were labeled at E12.0, are shown. Tamoxifen was administered at E8.5 (H–L’) or E9.5 (M–N’). (I, I’) eYFP+/Prox1+ LECs (white arrows) migrated from the first and second pharyngeal arches to the lymph sac-forming domain (white dotted region) at E12.0. (J) There were no eYFP+/Prox1+ cells in or around the cardinal vein (n=4). (K–N) eYFP+/Prox1+/PECAM+ LECs were seen in the first and second pharyngeal arches of the embryos at E12.0, when tamoxifen was administered at E8.5 or E9.5 (white arrows), although the number of these cells was decreased in the E9.5 group (M–N’). (O) The results of a quantitative analysis of the percentage of eYFP+/Prox1+ cells among Prox1+ cells in the first and second pharyngeal arches at E12.0 after tamoxifen treatment at E8.5 (the number of eYFP+/Prox1+ cells [10.83 (mean)±1.249 (SEM)]: Prox1+ cells [30.83±4.549]) or E9.5 (the number of eYFP+/Prox1+ cells [2.833±1.108]: Prox1+ cells [35.50±5.847]) are shown. **p=0.0022. All of the data are presented as the mean ± SEM, and statistical analyses were performed using the non-parametric Mann-Whitney U-test. V, ventricle; PA1, first pharyngeal arch; PA2, second pharyngeal arch; CV, cardinal vein; LS, lymph sac-forming domain; L, liver; T, tongue; RA, right atrium; OFT, cardiac outflow tract. Scale bars, 100 μm (A, B, D, F, G, I, J, L, N), 500 μm (C, E, H, K, M); **p<0.01.