Aldehyde Oxidoreductases/deficiency; Aldehyde Oxidoreductases/metabolism; Pancreas/embryology; Tretinoin/metabolism; Aldehyde Oxidoreductases/genetics; Animals; Cell Differentiation/drug effects; Endoderm/cytology; Endoderm/metabolism; Epithelial Cells/cytology; Epithelial Cells/drug effects; Epithelial Cells/metabolism; Gene Deletion; Gene Expression Regulation, Developmental; Hedgehog Proteins; Heterozygote; Homeodomain Proteins/metabolism; Immunohistochemistry; In Situ Hybridization; Mesoderm/cytology; Mesoderm/drug effects; Mesoderm/metabolism; Metalloproteins/metabolism; Mice; Mice, Knockout; Mice, Mutant Strains; Pancreas/cytology; RNA, Messenger/metabolism; Trans-Activators/deficiency; Trans-Activators/metabolism; Transcription Factors/metabolism; Transgenes; Tretinoin/administration & dosage; Tretinoin/pharmacology; beta-Galactosidase/metabolism