Intestines 05

This is a section of the recto-anal junction (human). The rectum (left) joins the anal canal (right) at a point (arrow) of very abrupt transition in the lumenal epithelium, from the simple columnar epithelium (with many goblet cells) that lines the rectum to the stratified squamous epithelium that lines the anal canal. The stratified squamous epithelium is NON-keratinized in the proximal portion of the anal canal but is keratinized within a few centimeters of the junction. Although it is somewhat difficult to see in this particular section, the muscularis mucosa ("M") ends fairly abruptly at the recto-anal junction; this fact allows one to distinguish this junction from the gastroesophageal junction, even if the glands of the mucosa (rectal vs. cardiac) cannot be identified.

Slide P-5, field O36 of finder slide B, 2.5X objective