At the anal canal, there is a transition from the simple columnar epithelium of the intestinal mucosa to the keratinized stratified squamous ep-ithelium of the skin. Between these two distinctly different epithelia there is a narrow region (anal transitional zone) where the epithelium is first stratified columnar (or stratified cuboidal) and then nonkeratinized stratified squamous. At the level of the anal canal, the muscularis mucosae disappears. At the same level, the circular layer of the muscularis externa thickens to become the internal anal sphincter. The external anal sphincter is formed by the striated muscles of the pelvic floor.
Anal canal, human, H&E ×40.
A view of the anal canal is shown at low magnification. Mucosa characteristic of the large intestine (colorectal zone) is seen on the upper left of the micrograph. This region is the upper part of the anal canal, and the intestinal glands are the same as those present in the colon. The muscularis mucosae (MM ) is readily identified as the narrow band of tissue under the glands. Both the intestinal glands and the muscularis mucosae terminate within626 the left rectangular area of the field, and here, at the diamond, there is the first major change in the epithelium. This area called the anal transitional zone is examined at higher magnification in bottom left figure. The right rectangular area includes the stratified squamous epithelium (StS ) of the
Anal transitional zone, anal canal, human, H&E ×160; inset ×300.
The junction between the simple columnar (SC ) and the stratified (ST ) epithelium called the anal transitional zone is marked with the diamond. The simple columnar epithelium of the upper part of the anal canal contains numerous goblet cells, and as in the mucosa of the colon, this epithelium is continuous with skin in the squamous zone of the anal canal and is examined at higher magnification in bottom right figure.
Between the two diamonds in the rectangular areas shown is epithelium of the lower part of the anal canal. Under this epithelium, there is a lymphatic nodule that has a well-formed germinal center. Isolated lymphatic nodules under mucous membranes should not be construed to have fixed locations. Rather, they may or may not be present, according to local demands.
Also, at this low magnification, note the internal anal sphincter muscle (IAS ), i.e., the thickened, most distal portion of the circular layer of smooth muscle of the muscularis externa. Under the skin on the right is the subcutaneous part of the external anal sphincter muscle (EAS ). It is composed of striated muscle fibers, which are seen in cross section.
the epithelium of the intestinal glands (IG ). These glands continue to about the same point as the muscularis mucosae (MM ). Characteristically, the lamina propria contains large numbers of lymphocytes (Lym), particularly so in the region marked. A higher magnification of the stratified columnar epithelium (StCol ) and stratified cuboidal epithelium (StC ) found in the transition zone is shown in the inset.
Squamous zone, anal canal, human, H&E ×160.
The final change in epithelial type that occurs at the squamous zone of the anal canal is shown here. On the right is the stratified squamous epithelium of skin (StS(k)). The keratinized nature of the surface is apparent. On the other hand, the stratified squamous epithelium (StS ) below the level of the diamond is not keratinized, and nucleated cells can be seen all the way to the surface. Again, numerous lymphocytes (Lym) are in the underlying connective tissue, and many have migrated into the epithelium in the nonkeratinized area.