Figure 1637

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 (ATI) 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. xlO. b. This high magnification of the area indicated by the rectangle in a shows the area of the anal transitional zone (ATZ). 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. X200.

simple columnar epithelium stratified cuboidal epithelium anal columns are called anal sinuses. The anal canal is divided into three zones according to the character of the epithelial lining:

• Colorectal zone, which is found in the upper third of the anal canal and contains simple columnar epithelium with characteristics identical to that in the rectum

• Anal transitional zone (ATZ), which occupies the middle third of the anal canal. It represents a transition between the simple columnar epithelium of the rectal mucosa and the stratified squamous epithelium of the perianal skin. The ATZ possesses a stratified columnar epithelium interposed between the simple columnar epithelium and the stratified squamous epithelium, which extends to the cutaneous zone of the anal canal (Fig. 16.37).

• Squamous zone, which is found in the lower third of the anal canal. This zone is lined with stratified squamous epithelium that is continuous with that of the perineal skin.

In the anal canal, anal glands extend into the submucosa and even into the muscularis externa. These branched, straight tubular glands secrete mucus onto the anal surface through ducts lined with stratified columnar epithelium. Sometimes the anal glands are surrounded by diffuse lymphatic tissue. They often lead to the formation of pathologic fistulas (a false opening between the anal canal and the perianal skin).

Large apocrine glands, the circumanal glands, are found in the skin surrounding the anal orifice. In some animals, the secretion of these glands acts as a sex attractant.

Functional Considerations: Immune Functions of the Alimentary Canal

Immunologists have shown that the GALT not only responds to antigenic stimuli but also functions in a monitoring capacity. This function has been partially clarified for the lymphatic nodules of the intestinal tract. The M cells that cover Peyer's patches and lymphatic nodules have a distinctive surface that might be misinterpreted in sections as thick microvilli. The cells are readily Identified with the scanning electron microscope because microfolds contrast sharply with the microvilli that constitute the striated border of the adjacent enterocytes.

It has been shown with horseradish peroxidase (an enzyme used as an experimental marker) that the M cells pinocytose protein from the intestinal lumen, transport the pinocytotic vesicles through the cell, and discharge the protein by exocytosis into deep recesses of the adjacent extracellular space (Fig. 16.38). Lymphocytes within the deeply recessed extracellular space sample the luminal protein, including antigens, and thus have the opportunity to stimulate development of specific antibodies against the antigens. The destination of these exposed lymphocytes has not yet been fully determined. Some remain within the local lymphatic tissue, but others may be destined for other sites in the body, such as the salivary and mammary glands. Recall that in the salivary glands, cells of the immune system (plasma cells) secrete IgA, which the glandular epithelium then converts into slgA. Some experimental observations suggest that antigen contact necessary for the production of IgA by plasma cells occurs in the lymphatic nodules of the intestines.

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