Fig. 5. Dorsalization of the abnormal foregut of Nkx2.1^-/- mutants. Histological sections were taken through the undivided foregut of Nkx2.1 mutant embryos just anterior to the abnormal lung sacs, and through the trachea and esophagus of wild-type embryos at about the same level. (A,B) Immunohistochemical analysis of wild-type sections showing high levels of Sox2 (A) and p63 (B) in basal cells of the esophagus, and lower levels in the trachea (higher magnification of trachea shown in the insets). By contrast, the foregut of Nkx2.1-null mutants shows high levels of Sox2 (D) and p63 (E) throughout the basal layer of the undivided foregut (high magnification in the insets). The ectopic expression of Sox2^hi and p63^hi was less pronounced in more rostral sections of the undivided foregut (data not shown). Immunohistochemistry for smooth muscle actin shows only a localized region of smooth muscle in the wild-type trachea (C) but a continuous layer around the mutant foregut (F). Scale bars: 100 µm. fg, undivided foregut; SMA, smooth muscle actin.