Cell Differentiation/physiology; Ectoderm/metabolism; Gene Expression Regulation, Developmental/physiology; Mice/embryology; Stem Cells/metabolism; Transcription Factors/metabolism; Animals; Cell Differentiation/drug effects; Cell Line; Cell Lineage/drug effects; Cell Lineage/physiology; DNA, Complementary/analysis; DNA, Complementary/genetics; Ectoderm/cytology; Fetus; Gene Expression Regulation, Developmental/drug effects; Genes, Lethal/drug effects; Genes, Lethal/genetics; Genes, Regulator/drug effects; Genes, Regulator/physiology; Mice/metabolism; Mice, Transgenic/genetics; Mice, Transgenic/metabolism; Molecular Sequence Data; Muscle Proteins; Repressor Proteins/drug effects; Repressor Proteins/metabolism; Sequence Homology, Amino Acid; Sequence Homology, Nucleic Acid; Species Specificity; Stem Cells/cytology; Transcription Factors/genetics; Transcription Factors/isolation & purification; Tretinoin/pharmacology