Epidermal growth factor (EGF) and receptor (-R) signaling pathway is required for epithelial cell growth and differentiation such as the degeneration of the medial edge epithelial cells during the fusion process of secondary palate formation. As epithelial fusion takes place during primary palate formation, we investigated the involvement of the EGF-R in fusion of the medial (MNP) and lateral (LNP) nasal prominences of the mouse embryo was examined. Immunoreactivity of EGF-R was investigated in embryonic day 10 embryos (32-37 somite stages). The EGF-R immunoreactivity was observed in the nasal epithelia of the presumptive fusion area before fusion. It became undetectable just prior to the fusion and faintly reappeared at the time of the fusion. In contrast, the non-fusing epithelial cells of the nasal groove maintained the immunoreactivity throughout these stages. In order to elucidate whether the EGF/EGF-R signaling pathway was involved in nasal epithelial fusion, EGF solution was injected into the exocoelum of explanted mouse embryos, and the embryos were cultured for 18-24 h by whole embryo culture (WEC). This exogenous EGF inhibited fusion of nasal prominences in 66.7-81.5% of the embryos. Treatment with EGF for 4-14 h showed that exogenous EGF disturbed the EGF-R disappearance and normal alteration of epithelial cell morphology in the fusion area. These results suggest that temporal disappearance of the EGF/EGF-R signaling from presumptive fusion of the nasal prominences is required for morphological change of the epithelial cells leading to the fusion of MNP and LNP.
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