Ultrastructural studies of the transitional zone in the nasopharyngeal epithelium, with special reference to the keratinizing process in the mouse.

In the nasopharynx of the SMA mouse, the 'intermediate epithelium' occupies the transitional zone between the ciliated columnar and the stratified squamous epithelia. The intermediate epithelium showed gradations ranging from ciliated stratified low-columnar through stratified cuboidal to stratified squamous type. It is suggested that the intermediate epithelium shows the various stages of the epithelium transforming from the ciliated columnar to the stratified squamous epithelium, and that the basal cells of the ciliated columnar epithelium serve as the germinal layer for the transformation. The intermediate epithelium containing a few keratohyalin granules and many membrane-coating granules represented earlier stages of keratinization. The width of the microprojections in the stratified squamous epithelium was about doubled compared to that in the intermediate epithelium. It is suggested that the difference in width is caused by cell membrane distortion associated with keratinization and is regarded as an important marker of the start of keratinization.