Abstract
In most cases, the rectal epithelium was abruptly changed into the anal canal epithelium at the pectinate line. In some cases, however, just proximal to the pectinate line there existed the transitional zone between the rectal epithelium and the anal canal one The epithelium occupying the transitional zone consisted of cuboidal cells with bulging surfaces or flattened cells. It is considered that the epithelium represents various stages which the rectal columnar epithelium transforms into the stratified squamous one. The epithelium lining the mouse anal canal showed an irregular paving-stone-like appearance, which was a typical surface appearance of stratified squamous epithelium, throughout from the anal margin to the pectinate line. The width of the microridges in the distal part of the anal canal was about doubled compared to that in the proximal part. It is suggested that the epithelium in the distal part is keratinized, while that in the proximal part is nonkeratinized.
