A mouthful of epithelial-mesenchymal interactions.

The oral cavity is a complex environment. It serves as a portal between the outside and the inside of an organism. It is the primary organ regulating what is or is not allowed to enter the gut as food. It performs the ¢rst line of food selection (by physical feel and chemical taste) and processing (by mechanical and enzymatic breakdown). It also performs the additional functions of speech and expression. Specialized tissue types are required to accomplish these complex functions. During embryonic development of the face, an invagination of the ectoderm forms the stomodeum and connects to the archenteron, the presumptive gut. The primitive oral cavity is lined with both ectoderm and endoderm (Moore and Schmitt, 1998). The ectoderm gives rise to the anterior two thirds of the tongue and all of the hard palate. The endoderm forms the posterior third of the tongue, the £oor of the mouth, the palato-glossal folds, the soft palate, and others. The oral epithelium is composed of strati¢ed squamous epithelium. This oral squamous epithelium keratinizes to various degrees in di¡erent regions (Presland and Dale, 2000). Additional complexity is added when parts of the oral epithelia are morphologically transformed into di¡erent epithelial appendages in speci¢c locations: teeth are induced, taste buds appear on the tongue, and salivary glands invaginate in the buccal region. Along the muco-cutaneous junction, lips form. Thus, morphogenesis of the oral epithelia in the oral cavity sets up the basis for the diverse functions of the mouth. How does this tissue variety form during development? The epithelia come from the ectoderm and share the same developmental origin as the skin. Generally the di¡erent developmental fates are speci¢ed by interactions with the mesenchyme. However, aberrant situations can arise. Hairs can be induced from the oral epithelium in LEF 1 over-expressing transgenic mice (Zhou et al, 1995). Ectopic teeth can form on the skin of the chin.While these are pathological conditions, they give us a clue: the fates of skin or oral epithelium can be switched. Furthermore, di¡erent ectodermal dysplasia syndromes frequently have multiple defects in hairs, teeth, glands, etc. due to defects in one gene, as demonstrated in the recently characterized Eda pathway (Wisniewski et al, 2002). These ¢ndings support the notion that the epithelial varieties result from epithelial^mesenchymal interactions and are variations superimposed on a common theme (Chuong, 1998). Yet, the molecular basis of the epithelial^mesenchymal interactions underlying oral epithelia phenotype determination remains mostly unknown. Driven by the desire to tissue engineer the oral epithelium, Costea et al (2003) in this issue developed in vitro organotypic cultures consisting of primary human oral keratinocytes grown on top of a reconstituted collagen matrix with or without oral ¢broblasts. It has long been suggested that suboral mesenchyme is essential for epithelial proliferation, but the molecules involved were unknown (Hill and Mackenzie, 1989). Costea et al are able to produce reconstituted oral epithelia in a de¢ned medium, therefore providing an experimental model for determining the growth factors involved. The oral epithelium formed on a collagen matrix was thin and had a dominant basal layer.When KGF, or FGF-7, was added, there was a concentration dependent increase of the epithelial thickness.When ¢broblasts were incorporated in the matrix, the reconstituted epithelium was strati¢ed and there was a clear expansion of the spinous cell layer.When both FGF7 and ¢broblasts were present, the reconstructed oral epithelium reached optimal growth and di¡erentiation. The thickness of the reconstituted epithelium was not signi¢cantly di¡erent from that of native oral epithelium. Using a set of carefully designed experiments, they measured cell proliferation, apoptosis, di¡erentiation, and the thickness of each strati¢ed epithelial layer. They concluded that FGF-7 can drive keratinocyte proliferation, but not di¡erentiation. Fibroblasts provided other unknown factors required for epithelial di¡erentiation and could modulate the thickness of the reconstituted oral epithelium by balancing cell division, apoptosis and terminal di¡erentiation. In histology, we start by teaching that the basic con¢gurations of epithelia are cuboidal, columnar, or squamous, and that the epithelia can be either simple or strati¢ed. Yet, we know very little about these fundamental processes at the cellular and molecular level. The researchers ought to be commended for their success in forming reconstituted strati¢ed squamous oral epithelia using dissociated oral keratinocytes obtained from the super£uous oral tissue after wisdom tooth extraction of a normal person. The di¡erentiation of the re-constructed epithelia could have been assessed more rigorously with more molecular markers to demonstrate intercellular integrity and appropriate cyto-di¡erentiation. The histological appearance is reasonably good, but there are spaces in the supra-basal layer and fewer interpapillary rete pegs at the epithelial^mesenchymal interface, indicating reduced cell interactions. Another signi¢cant aspect of this work is the demonstration that underlying ¢broblasts are important for this basic histogenetic process, and that the FGF pathway is involved in the initial strati¢cation step. Many questions remain unanswered.What are the other factors produced by ¢broblasts? What are the e¡ects of other types of ¢broblasts? What molecular pathways are involved? The establishment of this in vitro organ culture model with de¢ned medium opens doors for testing many candidate molecules. A complementary approach is to use an in vivo model and genetics. One line of exciting work showed that p63, a molecular homolog of p53, is essential for epithelial strati¢cation. Mice lacking p63 showed persistent simple epithelia and missing epithelial appendages that require epithelial ^ mesenchymal interactions including hairs, teeth, mammary glands, etc. (Mills et al, 1999). Obviously, oral epithelial appendages involve more complicated epithelial ^ mesenchymal interactions than oral epithelia. For example, some regions are induced to form teeth while others are not. For example, mice have no canine teeth. Taken to the RJ Gorlin, personal communication.