Tooth morphogenesis is regulated by epithelial-mesenchymal interactions mediated by the basement membrane (BM). Laminins are major glycoprotein components of the BMs, which are involved in several cellular activities. The expression and localization of the ␣3, 3, and ␥2 laminin-5 subunits have been analyzed by in situ hybridization and immunohistochemistry during mouse molar development. Initially (E12), mRNAs of all subunits were detected in the entire dental epithelium and the corresponding proteins were located in the BM. During cap formation (E13-14), transcripts for the ␣3 and ␥2 subunits were localized in the outer dental epithelium (ODE), whereas the 3 subunit mRNA was present in the inner dental epithelium (IDE).
During the early bell stage (E16), immunoreactivity for all subunits disappeared from the BM along the IDE, although intense signals for 3 mRNA were detectable in cells of the IDE. Subsequently, when the dentinal matrix was secreted by odontoblasts (E18-19.5), mRNAs of all three subunits were re-expressed by ameloblasts, and the corresponding proteins were detected in ameloblasts and in the enamel matrix. Tissue recombination experiments demonstrated that when E16 IDE or ODE was associated with E18 dental papilla mesenchyme, immunostaining for all laminin-5 subunits disappeared from the BM, whereas when cultured with non-dental limb bud mesenchyme, they remained positive after 48 hr of culture. These results suggest that the temporospatial expression of laminin-5 subunits in tooth development, which appears to be differentially controlled by the dental mesenchyme, might be related to the enamel organ histo-morphogenesis and the ameloblast differentiation.