Regions and Internal Surface Relief
Anatomically and histologically, the rectum comprises three consecutive segments:
- Supraampullary part.
- Ampulla.
- Anal canal.
The internal surface of the upper segments (supraampullary and ampulla) forms two types of folds. Permanent transverse folds have a characteristic spiral course. Three layers of the wall participate in their formation: the mucosa, a thick submucosa, and the circular layer of the muscular coat. Inconstant longitudinal folds are numerous, formed exclusively by the mucosa, and flatten out easily when the bowel is distended by filling.
Structure of the Ampullary and Supraampullary Wall
The wall of these regions largely resembles other parts of the large intestine but features several key differences.
- Mucosa contains typical intestinal crypts and is covered by simple columnar epithelium.
- Submucosa is very well developed. It houses baroreceptors (lamellar corpuscles) that respond to pressure changes. Due to the high mobility of the mucosa relative to the muscular layer, there is a clinical risk of mucosal prolapse through the anus.
- Muscular coat is formed by smooth muscle tissue. A crucial feature: the outer longitudinal layer here is continuous and does not split into distinct taeniae.
- Outer coat in the initial segment is serous (has a mesothelial covering), transitioning inferiorly into an adventitia where the mesothelium is absent.
Histophysiology of the Anal Canal
The anal canal is divided into three zones that differ in relief and epithelial type. A common feature of all three zones is the complete absence of intestinal crypts.
1. Columnar zone The relief presents 8–10 longitudinal folds termed anal columns. Between them lie anal sinuses (crypts), which are longitudinal clefts. The epithelium here is stratified cuboidal, rich in enterochromaffin-like cells (ECL cells). The lamina propria contains a network of thin-walled venous lacunae. The submucosa houses 6–8 rudimentary anal glands (blind tubules) and baroreceptors. A thin muscularis mucosae is present.
2. Intermediate (hemorrhoidal) zone Separated from the columnar zone by the zig-zag anorectal (dentate) line. The surface is smooth. The epithelium transitions to stratified squamous non-keratinizing. The muscularis mucosae disappears. The lamina propria is rich in lymphocytes, mast cells, and elastic fibers. The hemorrhoidal venous plexus forms within the submucosa.
3. Cutaneous zone Covered by typical skin. The epithelium becomes stratified squamous keratinizing. The lamina propria and submucosa blend continuously into the dermis. Hairs, sebaceous glands, and apocrine sweat glands appear.
Sphincters and Vascular Structures
The muscular apparatus of the anal canal features two sphincters:
- Internal sphincter: formed by a thickening of the circular layer of smooth muscle tissue.
- External sphincter: located subcutaneously and composed of striated (skeletal) muscle tissue, providing voluntary control.
Specific vascular structures are found within the anal canal wall: thin-walled venous lacunae in the columnar zone and the hemorrhoidal plexus in the intermediate zone. Blood stasis in these areas leads to varicose vein enlargement, mucosal protrusion, and the formation of hemorrhoids. The outer coat of the anal canal is represented exclusively by connective tissue (adventitia).