Epithelial-derived basal lamina regulation of mesenchymal cell differentiation

The mechanisms by which epithelial-mesenchymal interactions result in differentiation are not known. A number of recombinations between vertebrate tissues associated with epidermal organs (e.g. skin, feather, mammary gland, salivary gland, tooth organ) indicate that regional mesenchymal specificity is instructive for determination and differentiation of epithelial phenotypes. In epidermal organs within which mesenchyme becomes determined and differentiates into a unique phenotype, such as during tooth organogenesis and odontoblast differentiation. Does the epithelial-derived basal lamina regulate mesenchymal differentiation into odontoblasts and the expression of dentine extracellular matrix? Experiments were designed to test the hypothesis that murine or avian epithelial-derived basal lamina possess information which is instructive for determined dental mesenchyme to differentiate into odontoblasts. The strategy was to examine homologous and heterologous tissue recombinants between Theiler stage 25 C57BL/6 molar tooth organs and Hamburger-Hamilton equivalent stage 22-26 Japanese Pharoah quail mandibular processes. Trypsin-dissociated molar epithelium and mesenchyme, reconstituted, secreted a basal lamina within 8 hours and mesenchyme differentiated into odontoblasts and formed dentine matrix within 3 days. Isolated trypsin-dissociated mesenchyme did not differentiate in vitro, whereas heterologous recombinants between odontogenic mesenchyma and quail epithelia resulted in odontoblasts and dentine production. Mouse tooth or quail mandibular epithelia served to regulate odontogenic mesenchyme differentiation. EDTA-dissociated mouse molar mesenchyme, in the absence of epithelium but with adherent basal lamina, routinely differentiated into odontoblasts. Control tooth organs routinely formed both dentine and enamel extracellular matrices within 7-10 days in our serumless, chemically-defined organ culture system. Regulation of determined mesenchymal cells to differentiate into functional and highly specialized odontoblasts appears to be mediated by epithelial-derived basal lamina and is not species or organ-specific.