Biomineralization, Life-Time of Odontogenic Cells and Differential Expression of the Two Homeobox Genes MSX-1 and DLX-2 in Transgenic Mice

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Abstract
Msx and Dlx homeobox genes encode for transcription factors that control early morphogenesis. More specifically, Msx‐1, Msx‐2, and Dlx‐2 homeobox genes contribute to the initial patterning of the dentition. The present study is devoted to the potential role of those homeobox genes during the late formation of mineralized tissues, using the rodent incisor as an experimental system. The continuously erupting mandibular incisor allows (1) the coinvestigation of the whole sequences of amelogenesis and dentinogenesis, aligned along the main dental axis in a single sample in situ and (2) the differential characterization of transcripts generated by epithelial and ectomesenchymal odontogenic cells. Northern blot experiments on microdissected cells showed the continuing expression of Msx‐2 and Dlx‐2 in the later stages of dental biomineralization, differentially in epithelial and ectomesenchymal compartments. Transgenic mice produced with LacZ reporter constructs for Dlx‐2 and Msx‐1 were used to detect different components of the gene expression patterns with the sensitive β‐galactosidase histoenzymology. The results show a prominent epithelial involvement of Dlx‐2, with stage‐specific variations in the cells involved in enamel formation. Quantitative analyses identified specific modulations of Dlx‐2 expression in ameloblasts depending on the anatomical sites of the incisor, showing more specifically an inverse linear relationship between the Dlx‐2 promoter activity level and enamel thickness. This investigation extends the role of homeoproteins to postmitotic stages, which would control secretory cell activity, in a site‐specific manner as shown here for Dlx‐2.