Loss of Isl1 eliminates the dorsal pyloric outer longitudinal muscle Sox9 expression. (A) Double immunostaining for Isl1 and Sox9 in the dorsal pylorus at E14.5. In the absence of Isl1, the domain of mesodermal cells (asterisks) expressing Sox9 was smaller. (B) Double immunostaining for Isl1 and Sox9 in the dorsal pylorus at E18.5. Inducible Isl1 knockout effectively eliminated Isl1 expression, with concomitant loss of Sox9 expression in the dorsal OLM cells (asterisks). Sox9 expression was normal in the stomach epithelium at both stages (arrowheads). Yellow dotted lines mark the epithelial basement membrane and white dotted lines separate ICM and OLM. Red staining is Isl1, green staining is Sox9, and DAPI nuclear counterstaining (DNA) is blue. Scale bars: 50 μm. ICM, inner circular muscle; OLM, outer longitudinal muscle.