1), but at a precise developmental stage they rapidly elevate to a horizontal position above the dorsum of the tongue ( Fig. In mammals (both eutherian and protherian), these bilateral palatal shelves at first grow vertically down the sides of the tongue ( Fig. Bilateral palatal shelves arise from these maxillary processes at embryonic day 12 in mice, day 6 in chickens, day 17 in alligators and day 45 in man. Mesenchymal cells from the neural crest migrate to the primitive oral cavity where in association with craniopharyngeal ectoderm they form the bilateral maxillary processes. Moreover, the developing palate is often used as a model system for investigating fundamental mechanisms common to the embryogenesis of many structures as it appears relatively late in embryogenesis, can be easily excised and cultured under chemically defined, serum-free conditions ( Ferguson, Honig & Slavkin, 1984) and exhibits morphogenetic movements ( Ferguson, 1978), extracellular matrix synthesis ( Brinkley & Morris Wiman, 1984, 1987 Pratt & King, 1971 Silver, Foidart & Pratt, 1981), neurotransmitter synthesis ( Zimmerman & Wee, 1984), cell adhesion ( Greene & Pratt, 1977), epithelial-mesenchymal in-teractions and regional patterning ( Ferguson & Honig, 1984). Thus palate embryogenesis has been the target of much research with the long-term aim of rational prevention or improved treatment for cleft palate. Using such an approach, it is possible to go from a morphological description of palate development to a cellular analysis of the mechanisms involved and then to identification of candidate genes that may be important for screening and diagnosis of cleft palate.ĭevelopment of the mammalian secondary palate is a complex and critical event, which, in man, is frequently disturbed resulting in the common and distressing birth defect of cleft palate. Molecular markers for nasal, medial and oral epithelial cell differentiation include the types of cytokeratin intermediate filaments and specific cell surface molecules recognized by monoclonal antibodies: the genes for such molecules are probably expressed in response to mesenchymal signals. These signals cause differential gene expression via second messenger systems e.g. The extracellular matrix molecules (and bound growth factors) synthesized by the mesenchymal cells may then directly affect the epithelium. These soluble growth factors have a biphasic effect: directly on the epithelia and on the mesenchyme where they stimulate or inhibit cell division and synthesis of specific extracellular matrix molecules. minor collagen types, tenascin, EGF, TGFα, TGFβ, PDGF, FGF. Signalling of this interaction is complex but involves both extracellular matrix and soluble factors e.g. In all vertebrates, this regional, temporal and species-specific epithelial differentiation is specified by the underlying mesen-chyme. In birds, the MEE keratinize resulting in cleft palate whereas, in alligators, the MEE migrate onto the nasal aspect of the palate. In mammals, the MEE of opposing shelves adhere to each other to form an epithelial seam which then disrupts by cell death and cell migration into the mesenchyme accompanied by an epitheliomesenchymal transformation. In all vertebrates the nasal epithelium differentiates into pseudostratified ciliated columnar cells and the oral epithelia differentiates into stratified squamous cells, but the medial edge epithelial (MEE) phenotype differs in different groups. Palatal shelf-elevation is the result of an intrinsic shelf elevating force, chiefly generated by the progressive accumulation and hydration of hyaluronic acid. Mammalian palatal shelves initially grow vertically down the side of the tongue, but elevate at a precise time to a horizontal position above the dorsum of the tongue and fuse with each other to form an intact palate. In crocodilians, shelf fusion occurs resulting in an intact secondary palate. In birds and most reptiles, these palatal shelves grow initially horizontally, but do not fuse with each other resulting in physiological cleft palate. In all vertebrates, the secondary palate arises as bilateral outgrowths from the maxillary processes.
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