Morphological and histological changes in developing stomach of Isl1MCM/Delmutants. (A) Gross and microscopic evidence for stomach defects in Isl1MCM/Del mice. Whole mount views at E18.5 in Isl1F/+and Isl1MCM/Del mouse stomachs. Isl1MCM/Del mutant stomachs lacked a functional pyloric sphincter (arrowhead), thereby allowing reflux of fluid as observed in mutant embryos. Yellow fluid is denoted by asterisk. (B) Hematoxylin and eosin staining of Isl1F/+and Isl1MCM/Del mouse pylorus at E18.5. The dorsal pyloric smooth muscle (black boxed region) was prominent in Isl1F/+embryos, but was much thinner in Isl1MCM/Del embryos. The remainder of the pylorus was histologically normal. Green dotted lines mark the epithelial basement membrane. Enlarged images in boxed regions are shown below original photos. Scale bars of original photos: 200 μm; scale bars of enlarged images: 50 μm. H&E, hematoxylin and eosin.