Large intestine

Anatomical division and main functions

The large intestine is anatomically divided into six sections:

1. Caecum (caecum) — a short initial segment with the vermiform appendix (appendix vermiformis).
2. Ascending colon (colon ascendens).
3. Transverse colon (colon transversum).
4. Descending colon (colon descendens).
5. Sigmoid colon (colon sigmoideum).
6. Rectum (rectum).

In this section of the gastrointestinal tract, four key functions are carried out. First, intensive resorption (reabsorption) of water from the chyme takes place. Second, this is where fecal mass formation occurs, which requires active mucus secretion to bind particles together and facilitate their passage. Third, the intestine performs an excretory function, removing heavy metal salts, phosphates, as well as calcium and magnesium ions from the blood into the lumen. Finally, thanks to the rich bacterial flora, partial digestion of fiber and synthesis of B vitamins (particularly B12) and vitamin K occur.

Macroscopic wall relief

The wall of the large intestine (except for the rectum and appendix) has characteristic features that are easily identified macroscopically:

• Teniae coli (teniae): the outer longitudinal muscle layer here is not continuous, but gathered into three distinct bands.
• Haustra (haustrae): transverse sacculated outpouplings. They appear because the muscle bands are shorter than the other layers, and the intestine gathers like an "accordion".
• Epiploic appendages (appendices epiploicae): finger-like outgrowths of the serous membrane, 4–5 cm long, filled with adipose tissue.
• Semilunar folds (plicae semilunares): intermittent folds on the inner surface. Unlike the stomach, absolutely all layers of the wall participate in their formation. These folds are absent in the area of the muscle bands.

Structure of the mucous membrane

The main feature of the large intestine mucosa is its smoothness. There is no velvety texture here, as villi are completely absent. The surface is formed exclusively by crypts — tubular invaginations into the lamina propria. Compared to the small intestine, the crypts here are wider, deeper, and located much more densely.

The epithelium is simple columnar. It contains the same 5 cell types as the small intestine, but their proportions are drastically altered:

• Goblet cells — clearly dominant. These pale, mucus-forming exocrinocytes give the epithelium a specific "perforated" appearance. Because of them, crypts resemble simple unbranched tubular glands.
• Columnar cells (colonocytes) — have a microvillous brush border, but it is significantly thinner. The epithelium is called "conditionally brush-bordered".

At the bottom of the crypts are undifferentiated epithelial cells, ensuring complete renewal of the cell composition within 4–6 days. Sparse Paneth cells (with acidophilic granules) and endocrinocytes (EC cells secrete serotonin, D cells — somatostatin, D1 cells — VIP) are also found there.

Lamina propria and other wall layers

Beneath the epithelium lies the lamina propria (lamina propria) made of loose fibrous connective tissue. It is rich in fibroblasts, macrophages, and plasma cells. Its distinguishing feature is a huge number of solitary lymphatic follicles (more than 20 thousand), which often grow deeper, reaching the submucosa. The muscularis mucosae consists of two layers of smooth myocytes (inner circular and outer longitudinal).

Submucosa (tela submucosa) is formed by loose connective tissue, contains blood vessels, fat cells, and Meissner's nerve plexus.

Muscular coat (tunica muscularis) includes a continuous inner circular layer and an outer longitudinal layer gathered into three teniae. Auerbach's nerve plexus lies between them. Externally, the intestine is covered by a serous membrane with characteristic fat appendices.

Differential diagnosis of the specimen

To unmistakably distinguish the large intestine from the small intestine under a microscope, use the following criteria:

1. Relief: no villi, only deep crypts are present.
2. Cells: total predominance of goblet mucocytes over absorptive cells.
3. Muscular coat: the outer layer is not continuous, but divided into three bands.
4. External structures: presence of semilunar folds and haustra.

Class and exam questions

What else to study on this topic

• Cellular composition of crypts: endocrinocytes and Paneth cells
• Mechanisms of regeneration of the large intestinal epithelium
• Organization of the lymphoid apparatus: solitary follicles
• Innervation of the intestinal wall: Meissner's and Auerbach's plexuses

Other topics in the section "Digestive System: Large Intestine, Liver, and Pancreas"

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