Large Intestine Colon

I. ANAL CANAL

A. The upper anal canal is separated from the lower anal canal by the pectinate line (Figure 15-1).

1. The upper anal canal is lined by a typical simple columnar (colonic) epithelium arranged as intestinal glands (Figure 15-IB).

2. The colonic epithelium undergoes a transition at the pectinate line to a nonkera-tinized stratified squamous epithelium (Figure 15-1C).

3. The upper anal canal is derived embryologically from the hindgut (Table 15-1).

B. The lower anal canal is lined by a keratinized stratified squamous epithelium (Figure 15-1D). It is derived embryologically from the proctodeum (see Table 15-1).

II. MUCOSA of the large intestine is lined by:

A. Surface absorptive cells

B. Goblet cells

C. M cells

III. INTESTINAL GLANDS contain:

A. Stem cells

B. Enteroendocrine cells

IV. GUT-ASSOCIATED LYMPHATIC TISSUE (GALT) is prominent.

V. REPAIR (REGENERATION). Stem cells have a high rate of mitosis and replace surface absorptive cells and goblet cells every 5-6 days.

VI. CLINICAL CONSIDERATIONS

A. Hirschsprung disease (colonic aganglionosis) is a congenital defect that results from the failure of neural crest cells to form the myenteric plexus within the sigmoid colon and rectum. This results in a loss of peristalsis in the colon segment distal to the normal innervated colon. Patients present with fecal retention and abdominal distention.

B. Familial adenomatous polyposis coli (FAPC) is the archetype of adenomatous polyposis syndromes in which patients develop 500-2000 polyps (Figure 15-2) that carpet the mucosal surface of the colon and invariably become malignant.

Figure 15-1. (A) Division of the upper and lower anal canal by the pectinate line. (B) Typical simple columnar (colonic) epithelium arranged as intestinal glands lining the upper anal canal. (C) Transition of the colonic epithelium at the pectinate line to a nonkeratinized stratified squamous epithelium. (D) Keratinized stratified squamous epithelium lining the lower anal canal.

Figure 15-1. (A) Division of the upper and lower anal canal by the pectinate line. (B) Typical simple columnar (colonic) epithelium arranged as intestinal glands lining the upper anal canal. (C) Transition of the colonic epithelium at the pectinate line to a nonkeratinized stratified squamous epithelium. (D) Keratinized stratified squamous epithelium lining the lower anal canal.

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