Loss of chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII) does not affect the formation of proepicardium. A, Immunofluorescent staining for COUP-TFII (green) in wild-type and COUP-TFII−/− littermates at E9.5. Nuclei were counterstained with 4′,6-diamidine-2′-phenylindole dihydrochloride. B and C, Hematoxylin and eosin staining of sagittal sections obtained from wild-type and COUP-TFII−/− mutants at E9.5. C, Higher magnification of the regions shown in B. In both wild-type and COUP-TFII−/− mutants, a substantial number of epicardial progenitors cell cysts are formed and bud out from the proepicardium (PE). D, Immunohistochemistry for GATA4 in sagittal sections of E9.5 wild-type and COUP-TFII−/− embryos. All tissues are labeled with GATA4. E, Immunohistochemistry reveals Wilms tumor 1 expression in the PE and epicardial cells in wild-type and COUP-TFII−/− littermates at E9.5. A indicates atrium; OFT, outflow tract; V, ventricle.