morpho3test

PALATE MORPHOGENESIS 3

Shelf Elevation Various mechanisms have been proposed for bringing about the elevation of the palatal shelves. Hydration of palatal glycosaminoglycans, principally hyaluronic acid has been implicated. Stout bundles of collagen I which run down the centre of the palatal shelf probably direct the force and the epithelial covering may exert differential traction on the shelf. It would also seem that palatal mesenchymal cells can contract under the control of neurotransmitters released from nerves and the palatal mesenchyme itself. All of this occurs when the head of the embryo is growing constantly in height but not in width so the position of least resistance for the shelves in the oral cavity comes to be above the dorsum of the tongue. (Ferguson, 1985) Very recent studies show that at the time of elevation, the jaw is drawn backwards and takes the tongue with it. The tongue becomes free from its place between the shelves, allowing them to elevate. (Kjær, 1999)
Fusion	d. 40mm human embryo
After elevating, the shelves approach each other and the medial edge epithelia of each shelf adhere to each other by means of cell surface glycoprotein and desmosomes. This forms a midline epithelial seam. (Ferguson, 1985) Seam Degeneration Formation of the definitive palate depends on removal of the medial edge epithelial cells that constitute the midline epithelial seam allowing merging and continuity of the core mesenchyme of the shelves. As seam degeneration progresses, there is thinning of midline epithelial cell layers, disruption of the basal lamina on which they rest and formation of epithelial islands. At nasal and oral surfaces of the palate, the seam expands into prominent trianguar areas. Conflicting views suggest that apoptosis or epithelial-mesenchymal transformation of the MEE cells is predominantly involved in seam degeneration. Using labelling of palate epithelia in culture, fate of MEE cells at fusion has been examined using histology and laser scanning microscopy. No evidence of cell death or transformation is found. Instead most, if not all MEE cells migrate nasally and orally out of the seam and are recruited into and constitute the epithelial triangles on both the oral and nasal aspects of the palate. Subsequently these cells become incorporated into the oral and nasal epithelia on the surface of the palate and the triangles disappear. (Carette and Ferguson, 1992)
Although both slides on the right are from 40mm embryos, they have been sectioned at different levels. As fusion proceeds from anterior to posterior, some areas are seen to have fused and some are still in the process of fusion. Click on either of the slide pictures to see the whole embryological sections with labels.	e. 40mm human embryo