Mammalian palatogenesis is a highly regulated morphogenetic process during which the embryonic primary and secondary palatal shelves develop as outgrowths from the medial nasal and maxillary prominences, respectively, remodel and fuse to form the intact roof of the oral cavity. The complexity of control of palatogenesis is reflected by the common occurrence of cleft palate in humans. Although the embryology of the palate has long been studied, the past decade has brought substantial new knowledge of the genetic control of secondary palate development. Here, we review major advances in the understanding of the morphogenetic and molecular mechanisms controlling palatal shelf growth, elevation, adhesion and fusion, and palatal bone formation.

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Palatogenesis: morphogenetic and molecular mechanisms of secondary palate development

Clefts of the palate are common birth defects requiring extensive treatment. They appear to be caused by multiple genetic and environmental factors during palatogenesis. This may result in local changes in growth factors, extracellular matrix (ECM), and cell adhesion molecules. Several clefting factors have been implicated by studies in mouse models, while some of these have also been confirmed by genetic screening in humans. Here, we discuss several knockout mouse models to examine the role of specific genes in cleft formation. The cleft is ultimately caused by interference with shelf elevation, attachment, or fusion. Shelf elevation is brought about by mesenchymal proliferation and changes in the ECM induced by growth factors such as TGF-betas. Crucial ECM molecules are collagens, proteoglycans, and glycosaminoglycans. Shelf attachment depends on specific differentiation of the epithelium involving TGF-beta3, sonic hedgehog, and WNT signaling, and correct expression of epithelial adhesion molecules such as E-cadherin. The final fusion requires epithelial apoptosis and epithelium-to-mesenchyme transformation regulated by TGF-beta and WNT proteins. Other factors may interact with these signaling pathways and contribute to clefting. Normalization of the biological mechanisms regulating palatogenesis in susceptible fetuses is expected to contribute to cleft prevention.

Biological Mechanisms in Palatogenesis and Cleft Palate

Cleft lip and cleft palate are frequent human congenital malformations with a complex multifactorial etiology. These orofacial clefts can occur as part of a syndrome involving multiple organs or as isolated clefts without other detectable defects. Both forms of clefting constitute a heavy burden to the affected individuals and their next of kin. Human and mouse facial traits are utterly dissimilar. However, embryonic development of the lip and palate are strikingly similar in both species, making the mouse a model of choice to study their normal and abnormal development. Human epidemiological and genetic studies are clearly important for understanding the etiology of lip and palate clefting. However, our current knowledge about the etiopathogenesis of these malformations has mainly been gathered throughout the years from mouse models, including those with mutagen-, teratogen- and targeted mutation-induced clefts as well as from mice with spontaneous clefts. This review provides a comprehensive description of the numerous mouse models for cleft lip and/or cleft palate. Despite a few weak points, these models have revealed a high order of molecular complexity as well as the stringent spatiotemporal regulations and interactions between key factors which govern the development of these orofacial structures.

The etiopathogenesis of cleft lip and cleft palate: usefulness and caveats of mouse models.

Forces Generated by Cell Intercalation Tow Epidermal Sheets in Mammalian Tissue Morphogenesis

Palatogenesis involves the initiation, growth, morphogenesis, and fusion of the primary and secondary palatal shelves from initially separate facial prominences during embryogenesis to form the intact palate separating the oral cavity from the nostrils. The palatal shelves consist mainly of cranial neural crest-derived mesenchymal cells covered by a simple embryonic epithelium. The growth and patterning of the palatal shelves are controlled by reciprocal epithelial-mesenchymal interactions regulated by multiple signaling pathways and transcription factors. During palatal shelf outgrowth, the embryonic epithelium develops a "teflon" coat consisting of a single, continuous layer of periderm cells that prevents the facial prominences and palatal shelves from forming aberrant interepithelial adhesions. Palatal fusion involves not only spatiotemporally regulated disruption of the periderm but also dynamic cellular and molecular processes that result in adhesion and intercalation of the palatal medial edge epithelia to form an intershelf epithelial seam, and subsequent dissolution of the epithelial seam to form the intact roof of the oral cavity. The complexity of regulation of these morphogenetic processes is reflected by the common occurrence of cleft palate in humans. This review will summarize major recent advances and discuss major remaining gaps in the understanding of cellular and molecular mechanisms controlling palatogenesis.

/pdf/cellular-and-molecular-mechanisms-of-palatogenesis-1g97rvdwa3.pdf

Cellular and Molecular Mechanisms of Palatogenesis

Malformations in secondary palate fusion will lead to cleft palate, a
common human birth defect. Palate fusion involves the formation and subsequent
degeneration of the medial edge epithelial seam. The cellular mechanisms
underlying seam degeneration have been a major focus in the study of
palatogenesis. Three mechanisms have been proposed for seam degeneration:
lateral migration of medial edge epithelial cells; epithelial-mesenchymal
trans-differentiation; and apoptosis of medial edge epithelial cells. However,
there is still a great deal of controversy over these proposed mechanisms. In
this study, we established a [ Rosa26 ↔C57BL/6] chimeric culture
system, in which a Rosa26 -originated `blue9 palatal shelf was paired
with a C57BL/6-derived `white9 palatal shelf. Using this organ culture system,
we observed the migration of medial edge epithelial cells to the nasal side,
but not to the oral side. We also observed an anteroposterior migration of
medial edge epithelial cells, which may play an important role in posterior
palate fusion. To examine epithelial-mesenchymal transdifferentiation during
palate fusion, we bred a cytokeratin 14- Cre transgenic line into the
 R26R background. In situ hybridization showed that the Cre 
transgene is expressed exclusively in the epithelium. However,β
-galactosidase staining gave extensive signals in the palatal
mesenchymal region during and after palate fusion, demonstrating the
occurrence of an epithelial-mesenchymal transdifferentiation mechanism during
palate fusion. Finally, we showed that Apaf1 mutant mouse embryos are
able to complete palate fusion without DNA fragmentation-mediated programmed
cell death, indicating that this is not essential for palate fusion in
vivo.

Jixiang Ding

Papers

Analysis of cell migration, transdifferentiation and apoptosis during mouse secondary palate fusion.