Differential expression of laminin-5 subunits during incisor and molar development in the mouse

Rodent incisors are continuously growing teeth and enamel deposition is restricted to the labial side. In the present study, the expression of laminin-5 subunits (alpha3, beta3 and gamma2) has been analyzed by in situ hybridization in developing mouse lower incisors and compared to that reported in the molar. At the bud stage (E12), mRNAs for all subunits were detected in the whole epithelial thickening. At E14, when histogenesis had started, transcripts for alpha3 and gamma2 subunits were restricted to the outer dental epithelium (ODE), whereas the beta3 subunit was intensely expressed in the inner dental epithelium (IDE). A transient expression for alpha3 subunit was seen in the enamel knot area and disappeared at E15. Subsequently, all laminin-5 subunit genes were re-expressed in differentiating ameloblasts on the labial side. Similar patterns of transcription were observed in incisor and molar, suggesting that the differential expression of laminin-5 subunits in the IDE might be involved in the histogenesis of the IDE and ameloblast differentiation. At E16.5, cells of the IDE at the anterior extremity of the incisor and in the anterior part of the lingual IDE expressed transcripts for alpha3 and beta3 but not for gamma2 subunit. Similar expression patterns were observed in the enamel-free areas of the E18 molar. This specific expression might thus be related to cells that do not differentiate as functional ameloblasts. Throughout incisor development, intense expression for all laminin-5 subunits was restricted to the labial side of the cervical loop. The asymmetrical expression of laminin-5 might be related to incisor morphogenesis and to the differences in histogenesis and cytodifferentiation of the IDE that exist in the labial versus lingual aspect of the cervical loop.