Author: Dr. Thomas Caceci
At this site there's an immediate disappearance of the crypts and goblet cells of the rectum, as well as the equally sudden appearance of the stratified squamous portion. The epithelium makes an abrupt transition from simple columnar to stratified squamous. Remember that there is a similar site at the cardiac junction, where the esophageal epithelium (stratified squamous) abruptly transits to the epithelium of the stomach (simple columnar). How do you tell these two junctions apart? Look at the columnar epithelium. If it has goblet cells in the sheet, it's the ano-rectal junction. Another giveaway is the deep straight crypts of the rectum, and the absence of glands in the lamina propria.
A second "transition" occurs in the anus: it's the mucocutaneous junction at the nether end of the digestive tract (M-CJ). At this point there's no change of epithelial type (it's stratified squamous on both sides) but the post-junctional epithelium is keratinized. The lining of the anus makes a transition to the general integument. This transition is better seen at higher magnification below.
To the left you can see typical adnexal structures associated with the integument: a couple of hair follicles (HF) complete with sebaceous glands are visible. These sebaceous glands (and some other glands in the region) contribute to the individual scent signature. When one dog meets another for the first time, canine protocol demands that he offer his butt to be sniffed, and that he sniff the other's butt in turn. The breakdown products of these glands, the fecal material, and the secretions of the glands around the anal sacs (not seen here) all provide components of the scent signature.
The anal epithelium is
derived from surface
ectoderm. The forming embryo has a surface depression (the
proctodeum) which is the rudiment that invaginates to
form the anal canal. Eventually the membrane closing the bottom of
this depression perforates, opening the lumen of the rectum to the
outside. Occasionally this fails to happen and an individual is born
with an imperforate anus. Obviously this can lead
to very serious problems, but it's surgically correctable.