Fusion of nasal swellings in the mouse embryo: Regression of the nasal fin

The nasal fin is an epithelial seam that develops by fusion between the epithelial linings of the medial and lateral nasal swellings. Shortly after its formation the nasal fin regresses and is replaced by mesenchyme, with exception of its most posterior portion which remains as the bucconasal membrane. Since the regression of the nasal fin plays an important role in the formation of the upper lip and primary palate, the breakdown of the fin was examined with the electron microscope in 12‐day‐old mouse embryos. Shortly before the epithelial linings of the opposing nasal swellings make contact, cell degeneration characterized by condensation and fragmentation occurs in the epithelial linings of the prospective fusion areas. Apparently direct contact between the nasal swellings is not necessary to cause cell death. After fusion has established the nasal fin, epithelial cells continue to degenerate by condensation and fragmentation. Cell degeneration, however, can not account for complete regression of the fin, since many morphologically healthy epithelial cells are always present. Since no indication was found that the surviving epithelial cells transform into mesenchymal cells, it is suggested that they are incorporated into the adjacent epithelial linings of the expanding primitive nasal and oral cavities. The extracellular dense bodies that result from degenerating cells are engulfed by neighboring epithelial cells, in which they form phagocytic vacuoles. Some of these epithelial cells containing numerous cytoplasmic vacuoles appear to transform into large round macrophages. When the acid phosphatase procedure was applied, many heterophagic vacuoles contained a black precipitate, indicating that, after being phagocytosed, the dense fragments are digested by lysosomal enzymes. A similar black precipitate appeared over autophagic vacuoles.