Mesenchymal influences on epithelial differentiation in developing systems

Mesenchyme tissue: cells, matrix and soluble factors, influence the morphogenesis, proliferation and differentiation of a variety of embryonic epithelia, e.g. in the tooth, skin, mammary and salivary glands. Mesenchyme derivatives also 'maintain' adult epithelia, e.g. the local proliferation rate and cytokeratin composition of oral mucosa. Abnormalities in such epithelial-mesenchymal interactions lead to a variety of pathologies such as premalignant lesions, e.g. leukoplakia, tumours and psoriasis, whilst therapeutic manipulation of such interactions can prevent the exfoliation of dental implants. In all of these systems it is critical to understand, at the cellular and molecular levels, how the mesenchyme signals to the epithelium and how the latter processes and responds to such signals. We have investigated such questions using the developing embryonic palate both as a model system and as an important organ: failure of mesenchymal signalling leads to the common and distressing birth defect of cleft palate. Bilateral palatal shelves arise from the maxillary processes of embryonic day 11 (E11) mice, grow initially vertically down the sides of the tongue, elevate on E13.8 to a horizontal position above the dorsum of the tongue and fuse with each other in the midline on E14. The medial edge epithelia of each shelf fuse with each other to form a midline epithelial seam, suprabasal cells die, and the basal (stem) cells synthesize extracellular matrix molecules and turn into mesenchymal cells. Simultaneously the oral epithelia differentiate into stratified squamous cells and the nasal epithelia into pseudostratified ciliated columnar cells. Oral, medial and nasal epithelial differentiation is specified by the underlying mesenchyme in vivo and in vitro. Signalling involves a bifurcating action of a combination of soluble growth factors e.g. TGF-alpha, TGF-beta, PDGF and FGF on palatal epithelia and mesenchyme. These factors stimulate the synthesis of specific extracellular matrix molecules by palate mesenchyme cells, and the appearance of receptors for such molecules on epithelial cells. In this way, a combination of mesenchymal soluble factors and extracellular matrix molecules direct palatal epithelial differentiation. These signals act on epithelial basal (stem) cells, causing them to synthesize unique proteins, which may direct subsequent differentiation of daughter cells. In the most extreme example, namely the medial edge epithelia, these signals result in the basal epithelial cells transforming into mesenchymal cells, thus demonstrating that they are indeed multipotential stem cells.