The lung lymphatic vascular network is functional during late gestation. (A) Rhodamine-labeled dextran (dextran-TMR) injected into the lungs of live E18.5 Prox1^GFP transgenic embryos is selectively taken up by GFP⁺ lymphatic vessels. Bars, 250 µm. (B and C) Rhodamine-labeled dextran (dextran-TMR) injected into the lungs of live E17.5 Prox1^GFP transgenic embryos is detected in the lumen of GFP⁺ lymphatic vessels (C). Bars, 250 µm. Representative images in A–C are shown from 6 embryos from 4 litters. (D and E) H-E staining of Ccbe1^−/−, Vegfr3^kd/kd, and littermate control embryonic lungs harvested after uterine immersion in ice-cold saline to preserve their in utero state at E16.5, E17.5, and E18.5. Bars, 200 µm. Representative images are shown from 3–7 control and 3–7 lymphatic-deficient embryos per time point examined from 2–4 independent litters. (F) Measurement of alveolar septal thickness and alveolar area in Ccbe1^−/− and littermate control lungs at E17.5 and E18.5. Quantitative data represent mean and SEM from 3–7 control and 3–7 Ccbe1^−/− embryos per time point from 2–4 independent litters (P = 0.015 and P = 0.005 for septal thickness and alveolar area at E18.5, respectively). (G) Measurement of alveolar septal thickness and alveolar area in Vegfr3^kd/kd and littermate control lungs at E17.5 and E18.5. Quantitative data represent mean and SEM from 3 control and 3 Vegfr3^kd/kd embryos per time point examined from 3 independent litters (P = 0.02 and P = 0.05 for septal thickness and alveolar area at E18.5, respectively). (H) DNA content in E18.5 Ccbe1^−/− and littermate control lungs. Quantitative data are represented as mean and SEM from 13 control and 8 Ccbe1^−/− embryos examined from 3 independent litters (P = 0.58). (I) Dry weight of E18.5-19.5 Ccbe1^−/− and littermate control lungs. Quantitative data are represented as mean and SEM examined from 21 control and 7 Ccbe1^−/− embryos from 5 independent litters (P = 0.29). Asterisks indicate P < 0.05 compared with control.