Parts and relief of the internal surface
Anatomically and histologically, three successive sections are distinguished in the rectum:
- Supraampullary part.
- Ampulla.
- Anal part (anal canal).
The internal surface of the upper sections (the supra-ampullary part and the ampulla) forms two types of folds. Permanent transverse folds have a characteristic spiral course. Three layers of the wall are involved in their formation at once: the mucous membrane, the robust submucosa, and the circular muscle layer. Inconstant longitudinal folds are numerous, formed exclusively by the mucous membrane, and easily straighten out when the bowel stretches as it fills.
Structure of the ampulla and supraampullary part wall
The wall of these areas largely resembles the other segments of the large intestine, but has a number of fundamental differences.
- Mucosa contains typical intestinal crypts and is lined with 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 probability of mucosal prolapse through the anus.
- Muscular coat is formed by smooth muscle tissue. A crucial feature: the outer longitudinal layer here is continuous, it does not separate into distinct taeniae.
- Outer coat in the initial segment is serous (has a mesothelial covering), and lower down it changes to adventitial, where mesothelium is absent.
Histophysiology of the anal part
The anal part is divided into three zones, which differ in their relief and type of epithelium. A common feature for all three zones is the complete absence of intestinal crypts.
1. Columnar zone The relief is represented by 8–10 longitudinal folds — anal columns. Between them are 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 zigzag anorectal (dentate) line. The surface is smooth. The epithelium changes to stratified squamous non-keratinizing. The muscularis mucosae disappears. The lamina propria contains many lymphocytes, mast cells, and elastic fibers. The hemorrhoidal venous plexus forms in the submucosa.
3. Cutaneous zone Covered with typical skin. The epithelium becomes stratified squamous keratinizing. The lamina propria and submucosa seamlessly transition into the dermis. Hairs, sebaceous, and apocrine sweat glands appear.
Sphincters and vascular structures
The muscular apparatus of the anal canal is represented by two sphincters:
- Internal sphincter: formed by a thickening of the circular layer of smooth muscle tissue.
- External sphincter: located under the skin and composed of striated (skeletal) muscle tissue, ensuring its voluntary control.
Specific vascular structures are distinguished in the wall of the anal canal: thin-walled venous lacunae in the columnar zone and the hemorrhoidal plexus in the intermediate zone. Blood congestion in these areas leads to varicose veins, mucosal protrusion, and the formation of hemorrhoidal nodes. The outer coat of the anal part is represented exclusively by connective tissue (adventitia).
Briefly: answers to topical questions
What types of cells are part of the epithelium of the intestinal crypts of the rectal ampulla?
The epithelium of the intestinal crypts of the rectal ampulla predominantly comprises mucous cells, as well as other elements characteristic of the large intestine mucosa. The epithelial lining of the ampulla is simple columnar.
The cellular composition is represented by the following types:
- Mucous cells — predominate directly in the ampullary part.
- Columnar cells — tall and narrow cells providing the absorption function.
- Goblet cells — produce mucus to facilitate the movement of feces.
- Undifferentiated cells — located deep in the crypts and serve as stem/cambial elements.
- Endocrine cells — located at the bottom of the crypts.
Where do the excretory ducts of the rudimentary anal glands open?
The excretory ducts of the rudimentary anal glands of the rectum open into the anal sinuses (sinus anales). The anal glands themselves are specific rudimentary formations in the form of blind epithelial tubules, numbering 6–8. They are localized predominantly in the well-developed submucosa of the anal part, although in some places these structures can penetrate deeper, reaching the muscular coat of the intestinal wall.
Class and exam questions
What is the difference between permanent and inconstant folds of the rectum?
Permanent transverse folds are formed by three layers (mucosa, submucosa, and circular muscle) and do not disappear. Inconstant longitudinal folds are formed only by the mucous membrane and are easily smoothed out when the organ fills.
How does the epithelium change throughout the rectum?
In the ampulla, the epithelium is simple columnar. In the columnar zone, it becomes stratified cuboidal, in the hemorrhoidal zone — stratified squamous non-keratinizing, and in the cutaneous zone — stratified squamous keratinizing.
In which zones of the rectum are intestinal crypts absent?
Intestinal crypts (tubular glands) are completely absent in all three zones of the anal part (columnar, intermediate, and cutaneous).
What forms the internal anal sphincter?
It is the result of a thickening of the circular layer of the smooth muscle coat of the intestinal wall.
What else to study on this topic
- Epithelial cell composition: the role of ECL cells in the columnar zone
- Structure and functions of submucosal baroreceptors
- Histology of rudimentary anal glands
- Pathophysiology of hemorrhoid formation
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