Appearance of a unique cell type in the fusion sites of facial processes

The contact sites between the medial and lateral nasal processes during the period of facial formation of the mouse embryo were examined by light and electron microscopy. Characteristic superficial cells were observed at the transitional regions between the surface ectoderms and the nasal epithelia at the end of the isthmus, where the initial contact of the opposing nasal processes took place. At the later stage the contact sites extended to the bottom of the ravine formed by the two nasal processes, where the superficial cells always seemed to bridge the area between the nasal processes. These superficial cells had a large, clear nucleus and abundant cytoplasm as well as the common structural features characteristic of the embryonic cells. These cells were also observed on the surface near the contact site in the presumptive fusion area. These observations suggest that these superficial cells play a critical role in the epithelial adhesion of the medial and lateral nasal processes throughout the fusion.