Cell Lineage; DNA-Binding Proteins/metabolism; Intestinal Mucosa/metabolism; Transcription Factors/metabolism; Animals; Basic Helix-Loop-Helix Transcription Factors/genetics; Basic Helix-Loop-Helix Transcription Factors/metabolism; Cell Differentiation; DNA-Binding Proteins/genetics; Enteroendocrine Cells/cytology; Enteroendocrine Cells/metabolism; Enteroendocrine Cells/ultrastructure; Flow Cytometry; Gene Expression; Green Fluorescent Proteins/genetics; Green Fluorescent Proteins/metabolism; Intestinal Mucosa/cytology; Intestinal Mucosa/ultrastructure; Mice; Mice, Knockout; Mice, Transgenic; Microscopy, Electron, Transmission; Microscopy, Fluorescence; Nerve Tissue Proteins/genetics; Nerve Tissue Proteins/metabolism; Paneth Cells/cytology; Paneth Cells/metabolism; Paneth Cells/ultrastructure; Reverse Transcriptase Polymerase Chain Reaction; Transcription Factors/genetics; Zinc Fingers