Bulging medial edge epithelial cells and palatal fusion.
The surface of the medial edge epithelium of embryonic day 12, 13 and 14 mouse palatal shelves was observed utilising Environmental Scanning Electron Microscopy (ESEM). This technique offers the advantage of visualisation of biological samples after short fixation times in their natural hydrated state. Bulging epithelial cells were observed consistently on the medial edge epithelium prior to palatal shelf fusion. Additionally, we have used ESEM to compare the morphology and surface features of palatal shelves from embryonic day 13 to 16 mouse embryos that are homozygous null (TGF-beta3 -/-), heterozygous (TGF-beta3 +/-) or homozygous normal (TGF-beta3 +/+) for transforming growth factor beta-3 (TGF-beta3). At embryonic day 15 and 16 most TGF-beta3 +/- and +/+ embryos showed total palatal fusion, whilst all TGF-beta3 null mutants had cleft palate: the middle third of the palatal shelves had adhered, leaving an anterior and posterior cleft. From embryonic day 14 to 16 abundant cells were observed bulging on the medial edge epithelial surface of palates from the TGF-beta3 +/- and +/+ embryos. However, they were never seen in the TGF-beta3 null embryos, suggesting that these surface bulges might be important in palatal fusion and that their normal differentiation is induced by TGF-beta3. The expression pattern of E-Cadherin, beta-catenin, chondroitin sulphate proteoglycan, beta-Actin and vinculin as assayed by immunocytochemistry in these cells shows specific variations that suggest their importance in palatal shelf adhesion.