Light and electron microscopy study of fusion of facial prominences. A distinctive type of superficial cells at the contact sites.

The contact site between the medial nasal prominence (MNP) and the lateral nasal prominence (LNP) during the period of primary palate formation in the mouse embryo was examined by light and electron microscopy. Throughout this period, a distinctive type of superficial cell was observed at the contact site. These superficial cells had a large nucleus and abundant cytoplasm as well as structural features characteristic of embryonic cells. At earlier stages, these cells were seen at the transitional region between the surface ectoderm and the epithelia of the nasal pit at the end of the isthmus, where initial contact of opposing MNP and LNP took place. At later stages, these superficial cells appeared to bridge the gap between MNP and LNP at the contact sites, which extended to the bottom of the valley formed by MNP and LNP. These cells were also observed on the surface near the contact sites, that is, the presumptive fusion area. These superficial cells displayed well-developed junctional complexes (intermediate and gap junctions, and desmosomes). Many filaments were observed subjacent to the plasma membranes of these superficial cells, some of which were associated with junctional complexes. These observations suggest that this kind of distinctive superficial cell may play critical roles in the contact of MNP and LNP throughout the fusion process.