Fig. 3. Molecular genetic analysis of serosal mesothelial development. (A-D) Whole-mount analysis of lacZ expression marks the advance of the serosal mesothelium. (A) At E11.5, lacZ staining is prominent at the root of mesentery and intestine (small arrows), but is absent from the herniated gut (large arrows). Note the staining in nephric tubules (black arrowheads) and the urogenital ridge (white arrowhead). (B) At E12.5, lacZ-positive cells are seen covering more distal portions of the gut (arrows). (C) By E14.5, the intestine and mesentery (m) are positive for lacZ staining. (D) At E18.5, lacZ staining is strong over the entire gut tube and in the mesentery. (E-E'') Immunohistochemistry for ß-Gal (E, red) and Wt1 (E', green) reveal that, at E12.5, most of the serosal mesothelial cells express ß-Gal; ß-Gal staining colocalizes with Wt1 staining (arrows; E''). e, endoderm; in, intestine; st, septum transversum. Scale bars: 500 µm in A-C; 250 µm in D; 50 µm in E-E''.