(C23-25) Slide 59: Recto-anal junction - Groat’s stain

This slide has several different epithelia in a relatively short span, because there must be a transition between two dramatically different epithelia. Let’s start on the outside and work our way back in. Scan the epithelial edge of the section to find the epithelium that is stained bright red, and is flaking off (C23). This is the epidermis of the anal skin; it is keratinized stratified squamous epithelium. Here, the basal layers of epithelial cells are alive and well, but the surface cells are dead and sloughing. When you are trying to decide whether an epithelium is keratinized or mucosal, look for nuclei in the surface cells. If you see nuclei, the cells are alive and it is of the mucosal type; if there are no nuclei, the cells are dead, and it is keratinized.

Now, move the slide to the opposite end of the section (C24, top). You have now entered the rectal zone. The rectum is lined by simple columnar epithelium with goblet cells, and more Goblet cells, and more goblet cells (C25). As you can imagine, the mucus is very useful to keep the epithelial surface lubricated. The epithelium is invaginated to form simple tubular intestinal glands without ducts.

So far, we have seen two extremely different epithelia in terms of both the number of layers, and the shape of the cells. To bridge them, there must be intervening epithelia with intermediate characteristics. As you trace the epithelium back toward the anus, you will see stratified columnar, stratified cuboidal, and mucosal type stratified squamous epithelia before reaching the keratinized stratified squamous epithelium of the anal skin.