Fusion of nasal swellings in the mouse embryo: surface coat and initial contact.

The nasal region of 12-day-old mouse embryos was examined with the electron microscope to determine whether a surface coat and membrane specializations are involved in epithelial fusion between the medial and lateral nasal swellings. Ruthenium red was used to examine the distribution of the surface coat. Prior to contact, a surface coat is always present over the epithelial linings of the nasal swellings in the region of presumptive fusion, and it is often heavier in the fusing than in the non-fusing regions. At the point of initial contact, the coat is present as a thin film between touching superficial cells, suggesting that it may mediate epithelial contact. The initial contact between the cells of the medial and lateral nasal swellings is made by short projections from one superficial cell to the surface of an opposing superficial cells meet. The contacting membranes, which are separated by a distance of approximately 6-10 nm and show an increased electron-density, probably provide a firm adhesion between the nasal swellings.