FIGURE 17.32 • Photomicrographs of the anal canal. a. This photomicrograph shows a longitudinal section through the wall of the anal canal. Note the three zones in the anal canal: the squamous zone (SQZ) containing stratified squamous epithelium; the anal transitional zone (ATZ) containing stratified squamous, stratified cuboidal, or columnar epithelium and simple columnar epithelium of the rectal mucosa; and the colorectal zone (CRZ) containing only simple columnar epithelium like the rest of the colon. Note the anal valve that demarcates the transition between the ATZ and SQZ. The internal anal sphincter is derived from the thickening of the circular layer of the muscularis externa. A small portion of the external anal sphincter is seen subcutaneously. 10. b. This high magnification of the area indicated by the rectangle in a shows the area of the anal transitional zone. Note the abrupt transition between stratified cuboidal and simple columnar epithelium. The simple columnar epithelium of anal glands extends into the submucosa. These straight, mucus-secreting tubular glands are surrounded by diffuse lymphatic tissue. 200.
secretion of these glands acts as a sex attractant. Hair follicles sion). There are no teniae coli at the level of the rectum; the and sebaceous glands are also found at this site. longitudinal layer of the muscularis externa forms a uniform
The submucosa of the anal columns contains the terminal sheet. The muscularis mucosae disappears at about the level ramifications of the superior rectal artery and the rectal ve-of the ATZ, where the circular layer of the muscularis ex-nous plexus. Enlargements of these submucosal veins consti-terna thickens to form the internal anal sphincter. The extute internal hemorrhoids, which are related to elevated ternal anal sphincter is formed by striated muscle of the venous pressure in the portal circulation (portal hyperten-pelvic floor.
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The esophagus is a muscular tube that conveys food and other substances from the pharynx to the stomach. The mucosa that lines the length of the esophagus has a nonkeratinized stratified squamous epithelium. The underlying lamina propria is similar to the lamina propria throughout the alimentary tract; diffuse lymphatic tissue is scattered throughout, and lymphatic nodules are present. The deep layer of the mucosa, the muscularis mucosae, is composed of longitudinally organized bundles of smooth muscle fibers. The submucosa consists of dense irregular connective tissue that contains the larger blood and lymphatic vessels, nerve fibers, and ganglion cells. The nerve fibers and ganglion cells make up the submucosal plexus (Meissner’s plexus). The muscularis externa consists of two muscle layers, an inner circular layer and an outer longitudinal layer. The upper one third of muscularis externa consists of striated muscle, a continuation of the muscle of the pharynx. Striated muscle and smooth muscle bundles are mixed and interwoven in the muscularis externa of the middle third of the esophagus. The muscularis externa of the distal one third consists only of smooth muscle, as in the rest of the digestive tract.
Esophagus, monkey, H&E ×60; inset ×400.