Early determination and permissive expression of amelogenin transcription during mouse mandibular first molar...

The expression of tissue-specific enamel matrix genes is believed to require both instructive and permissive interactions of enamel organ epithelium with dental papilla mesenchyme and/or extracellular matrix during a restricted period of development. Biosynthesis of amelogenin gene products has been found to be associated with the terminal differentiation of inner enamel organ epithelium. The developing mouse first mandibular molar was used for a detailed examination of the temporal initiation and developmental pattern of amelogenin transcription. These studies define temporally instructive versus permissive influences on amelogenin transcription. During in vivo development, amelogenin transcripts were detected in late cap (15 days in utero; E15) through bell stage (E16 through E19) mouse molar tooth formation utilizing reverse transcription coupled to polymerase chain reaction amplification. Alternatively spliced amelogenin transcripts were detected in late bell stage (E18) molars. Amelogenin transcripts were also detected in isolated late cap stage (E15) enamel organ epithelium dissected free of dental papilla mesenchyme and cultured within a substitute basement membrane gel, but not in identical cap stage enamel organ epithelium cultured on plastic or a laminin-coated filter. Amelogenin transcripts were also found in early cap stage (E14) isolated enamel organ epithelium cultured within a basement membrane gel, but were not detected in enamel organ epithelium isolated from earlier stages of odontogenesis and cultured within a basement membrane gel. The results of these experiments indicate that a basement membrane gel is a useful extracellular substrate which provides permissive interactions required for the expression of amelogenin transcripts by enamel organ epithelium and that instructive interactions which determine enamel organ epithelium to become committed to amelogenin transcription occur prior to the early cap stage (E14) of odontogenesis. The results also suggest that continued interactions of enamel organ epithelium with dental papilla mesenchyme serve to regulate amelogenin transcription and post-transcriptional amelogenin RNA splicing in a complex manner during odontogenesis.