Animals lacking lymphatic vessels exhibit cyanosis and respiratory failure immediately after birth. (A) Cyanotic state of Ccbe1^−/− neonates after natural birth. Representative littermates are shown. Bar, 5 mm. (B) Cyanotic state of Ccbe1^−/− neonates after C-section at E19.5. Bar, 5 mm. These animals are also shown in Video 1. Representative neonates are shown from 47 littermate control and 13 Ccbe1^−/− animals examined from 8 independent litters. (C) H-E staining of the diaphragms of Ccbe1^−/− and control littermate embryos. Frontal sections of the intact chest of the indicated E18.5 embryos were stained with H-E. Bars, 500 µm. D indicates diaphragm. The images are representative of 3 control and 3 Ccbe1^−/− embryos examined at E18.5 from 3 independent litters. (D) Cyanotic state of Vegfr3^kd/kd neonates after natural birth compared with control littermates. Bar, 5 mm. (E) Cyanotic state of Vegfr3^kd/kd neonates after C-section at E19.5 compared with control littermates. Bar, 5 mm. These animals are also shown in Video 2. Representative images are shown from 35 littermate control and 13 Vegfr3^kd/kd neonates examined from 6 independent litters. Images in A, B, D, and E were obtained 30 min after birth or C-section. (F and G) H-E staining of lung sections to show alveolar septae and alveolar air spaces in Ccbe1^−/− and cyanotic Vegfr3^kd/kd neonates compared with control littermates 60 min after birth. Bars, 50 µm. Representative images are shown from 5–6 control and 4–5 lymphatic deficient neonates examined from 3–4 independent litters in each group. (H and I) Measurement of alveolar septal thickness and area in Ccbe1^−/−, cyanotic Vegfr3^kd/kd neonates, and littermate controls. Quantitative data are represented as mean and SEM for septal thickness and alveolar area. n = 5–6 control and 4–5 lymphatic-deficient neonates per genotype from 3–4 litters in each group (Ccbe1^−/−: P < 0.001 and P < 0.01 for septal thickness and alveolar area, respectively; Vegfr3^kd/kd: P < 0.0001 and P < 0.001 for septal thickness and alveolar area, respectively). (J) Trichrome staining of alveolar septae and alveolar area in Ccbe1^−/− and cyanotic Vegfr3^kd/kd neonatal lungs compared with control littermates 60 min after birth. Bars, 50 µm. Representative images are shown from 3 control and 3 lymphatic deficient neonates from 3 independent litters examined in each group. (K) Ccbe1^−/− and control neonate lung wet/dry (W/D) ratios of lung weight. Quantitative data are represented as mean and SEM from 10 control and 3 Ccbe1^−/− neonates from 2 independent litters (P = 0.02). Asterisks indicate P < 0.05 compared with control.