Anatomical Divisions and Main Functions
Anatmoically, the large intestine is divided into six segments:
- Caecum (caecum) — the short initial pouch featuring the vermiform appendix (appendix vermiformis).
- Ascending colon (colon ascendens).
- Transverse colon (colon transversum).
- Descending colon (colon descendens).
- Sigmoid colon (colon sigmoideum).
- Rectum (rectum).
Four key functions are carried out in this segment of the gastrointestinal tract. First, intensive reabsorption of water from chyme takes place. Second, fecal masses are formed, requiring active secretion of mucus to bind particulate matter and facilitate smooth propulsion. Third, the intestine performs an excretory role, eliminating heavy metal salts, phosphates, and calcium and magnesium ions from the blood into the lumen. Finally, owing to a rich bacterial flora, there is partial digestion of dietary fiber and synthesis of B-complex vitamins (specifically B12) and vitamin K.
Macroscopic Wall Architecture
The wall of the large intestine (excluding the rectum and appendix) exhibits distinct macroscopic features:
- Teniae coli (teniae): the outer longitudinal smooth muscle layer is not continuous, but gathered into three distinct longitudinal bands.
- Haustra (haustrae): transverse sacculated outpouchings. They form because the taeniae coli are shorter than the other layers, causing the gut wall to bunch up.
- Epiploic appendages (appendices epiploicae): finger-like projections of the serosa measuring 4–5 cm in length, filled with adipose tissue.
- Semilunar folds (plicae semilunares): crescentic folds on the internal surface. Unlike the stomach, all layers of the intestinal wall participate in their formation. These folds are absent along the lines of the taeniae coli.
Mucosal Structure
The principal feature of the large intestinal mucosa is its smoothness. There is no velvety texture because villi are entirely absent. The surface is formed exclusively by crypts — tubular invaginations extending into the lamina propria. Compared to the small intestine, these crypts are wider, deeper, and packed much more densely.
The epithelium is simple columnar. It contains the same five major cell types as the small intestine, but their proportions are drastically altered:
- Goblet cells — overwhelmingly dominant. These pale, mucus-secreting exocrinocytes give the epithelium a characteristic «pitted» appearance. Because of them, the crypts resemble simple, unbranched tubular glands.
- Columnar cells (colonocytes) — bear a microvillar brush border, though it is significantly shorter and sparser. The epithelium is referred to as "pseudo-brush-bordered".
At the base of the crypts reside undifferentiated epithelial cells, which ensure complete cellular renewal every 4–6 days. Sparse Paneth cells (containing acidophilic granules) and enteroendocrine cells (EC cells releasing serotonin, D cells releasing somatostatin, and D1 cells releasing VIP) are also located here.
Lamina Propria and Other Wall Layers
Beneath the epithelium lies the lamina propria (lamina propria) composed of loose fibrous connective tissue rich in fibroblasts, macrophages, and plasma cells. Its hallmark is an abundance of solitary lymphoid follicles (exceeding 20,000), which frequently extend deep into the submucosa. The muscularis mucosae consists of two smooth muscle layers (an inner circular and an outer longitudinal).
The submucosa (tela submucosa) is formed by loose connective tissue and contains blood vessels, adipocytes, and Meissner's nerve plexus.
The muscular layer (tunica muscularis) includes a continuous inner circular layer and an outer longitudinal layer gathered into three taeniae. Auerbach's nerve plexus lies between these layers. Externally, the intestine is covered by a serosa featuring characteristic epiploic appendages.
Microscopic Differential Diagnosis
To reliably differentiate the large intestine from the small intestine under a microscope, use the following criteria:
- Surface relief: lack of villi, presence of deep crypts only.
- Cellular composition: total dominance of goblet mucous cells over absorptive columnar cells.
- Muscularis externa: the outer longitudinal layer is not continuous, but split into three taeniae.
- External structures: presence of semilunar folds and haustra.