Fig. 1. Temporal and spatial differences in Cre recombinase activity in the PdxCre^early and PdxCre^late transgenic lines. Staining for lacZ expression marks cells that have undergone Cre-mediated recombination in PdxCre^late and PdxCre^early R26R mice. (A) Control E10.5 embryo. (B) No ß-galactosidase staining is detectable within the PdxCre^late R26R embryo at E10.5. (C) Strong ß-galacotosidase staining is present within the pancreas (arrow) of a PdxCre^early R26R E10.5 embryo. (D-F) Histological examination of pancreas sections immunostained for Pdx1 (brown) and enzymatically stained for ß-galactosidase activity (blue). (F) The majority of Pdx1⁺ cells in the PdxCre^early R26R animal, are also positive for lacZ at E10.5. (G-I) ß-Galactosidase (blue) and Pdx1 (brown) staining at E12.5. ß-Galactosidase staining is detectable within a subset of the Pdx1⁺ cells in the PdxCre^late R26R animals (H). (I) Equivalent pancreas sections of a PdxCre^early R26R animal show robust ß-galactosidase staining in the majority of Pdx1⁺ cells. (J-L) Adult animals stained with Eosin (red) and for ß-galactosidase (blue). Islets are outlined in black. (K) In the adult PdxCre^late R26R animal, only a subset of islet cells exhibits ß-galactosidase staining. Cre expression within the exocrine tissue is also mosaic in these animals. (L) A greater percentage of islet cells and exocrine cells exhibit strong ß-galactosidase staining in the adult PdxCre^early R26R pancreas. (M-O) Adult pancreatic ducts. (O)The majority of pancreatic ducts (stained with an antibody against mucin 1, red) are also ß-galactosidase ⁺ (green) in the PdxCre^early R26R pancreas (non-specific staining sometimes encountered within the center of ducts indicated with an asterisk). (N) ß-Galactosidase⁺ cells are seldom encountered within the PdxCre^late R26R pancreas. No ß-galactosidase staining is observed in control animals (A,D,G,J,M) at any of the time points characterized. Scale bars: 25 µm.