In vivo proliferation assay. Pregnant females were injected intraperitoneally with 100 μg/g body weight BrdU (Sigma-Aldrich). After 1 hour, mice were euthanized and embryos recovered for immunohistochemistry as described in Supplemental Methods. BrdU-positive cells were detected with the BrdU staining kit (Zymed) followed by staining in Mayer hematoxylin. Percent proliferative cells was determined by blinded quantitation of BrdU-positive cells in the lymphatic or vascular endothelium divided by total hematoxylin-stained nuclei. Data were analyzed using the 2-tailed Student’s t test assuming unequal variance. A P value < 0.05 was considered significant.