Wnt-activated cells represent progenitor cells within the intestinal crypt. (A-C) Cryopreserved intestinal tissue sections from TOPGAL adult mice co-stained with antibodies to β-gal (red) and Ki67 (green) then counter-stained with Hoechst (blue). Arrow indicates a cell with β-gal staining and arrowhead designates a cell co-staining for both markers. (D-G) Co-localization of BrdU (green) and β-gal (red) expression in crypt and villus epithelial cells from adult TOPGAL mice injected with BrdU 2 days prior to sacrifice. Green arrow denotes β-gal-positive cells at the lagging edge of migrating BrdU-positive cells up the villus. Red bracket indicates β-gal-positive villus epithelium. White asterisk marks β-gal and BrdU double-positive crypt cells. (F-G) Higher magnification of crypt regions in D-E. Solid white line demarks the epithelial-mesenchymal boundary. Dashed white line outlines the apical epithelial border. Counter-stained with Hoechst dye (blue). Bar = 25 μm.