Digestive disorders

II./1.2.: The histology of the large intestine

II./1.2.1.: General characteristics

The characteristic histological layers of the gastrointestinal tract can be recognized in the colonic wall as well. However, due to the specific functions of the colon, its histological structure exhibits some fundamental differences when compared with the wall of the small intestine: the mucosa has a smooth surface (no Kerckring’s folds, no intestinal villi); the outer longitudinal smooth muscle layer is present only in the teniae; there are differences also in the cellular pattern of the epithelium etc. The plicae semilunares, as projections into the lumen are not identical with the folds of Kerckring in the small intestine since the semilunar folds are made by not only the tela submucosa, but all the layers of the colonic wall. The histological layers of the colonic wall, starting from the lumen contain the tunica mucosa (mucous membrane), the tela submucosa, the tunica muscularis propria, the tela subserosa and the tunica serosa (serous membrane covering).

II./1.2.2.: Tunica mucosa (mucous membrane)

This surface layer contains very densely packed crypts of Lieberkühn; these crypts are not only depressions of the surface but tubular glands with no ducts. These tubular glands are longer than the similar tubular glands of the small intestinal mucosa. The tunica mucosa consists of the epithelial covering (epithelium mucosae), the stroma of the mucous membrane (lamina propria mucosae) and the thin, own smooth muscle of the mucous membrane (lamina muscularis mucosae).

II./1.2.2.1.: Epithelium mucosae

This layer is formed by the cells covering the surface of the mucous membrane.

These cells contain

• - simple columnar epithelial cells with cuticle,

• - goblet cells,

• - mitotic (basal) cells and

• - enteroendocrine (enterochromaffin) cells.

The (i) simple columnar epithelial cells with cuticle are absorptive cells – their surface microvilli serve the absorption (these cells do not contain digestive enzymes, in contrast to the epithelial cells of the small intestine); in their basolateral membrane Na+ and water move out with active transport.

The (ii) goblet cells produce mucus which covers the mucosal surface of the colon. Mucus, as a lubricant helps the passage of the intestinal content, which –due to the fluid absorption- is rather dry, tough material in the colon.

These cells are also found in the wall of the crypts.

The crypts, in their lower portions contain (iii) mitotic (basal) cells as well: these cuboidal cells, due to their mitotic activity are responsible for the renewal of the surface epithelium. The newly generated cells after their differentiation replace the epithelial cells and goblet cells within 1-2 days.

Hormone producing (iv) enteroendocrine (enterochromaffin) cells also occur in the crypts.

II./1.2.2.2.: Lamina propria mucosae

Loose connective tissue, containing blood vessels and lymphatics (fluid absorption!) as well as numerous fibroblasts and reticular fibers. Normally, a great number of immunocompetent cells (T and B lymphocytes, plasma cells, macrophages, mast cells, eosinophilic granulocytes) infiltrate this layer; in case of inflammation (eg. ulcerative colitis) this immune cell invasion becomes even more evident. Soliter lymphatic follicles are also found here since the colonic mucous membrane is an important immunological zone (MALT: mucosa-associated lymphoid tissue, GALT: gut-associated lymphoid tissue). The mucosa of the colon has a large surface which is colonized with normal bacterial flora; there are strong antigen stimulations occurring through the enormous surface of the gastrointestinal tract.

II./1.2.2.3.: Lamina muscularis mucosae

This is the thin, own smooth muscle layer of the mucous membrane.

II./1.2.3.: Tela submucosa

Loose connective tissue with soliter lymphatic follicles, blood vessels, nerves (submucosal plexus of Meissner).

II./1.2.4.: Tunica muscularis propria

In the colon it is also composed of two layers, although the external longitudinal layer is rudimental. Its layers:

- stratum circulare (inner, circular smooth muscle layer),

- stratum longitudinale (outer, longitudinal smooth muscle): arranged in teniae: the typical 3 longitudinal ribbons on the outer side of the colon are clearly visible with naked eye. The areas between the teniae exhibit only a minimal amount of longitudinally arranged smooth muscle. Exceptions – ie. having a complete outer smooth muscle layer – are the appendix, rectum and anal canal.

As mentioned above, the tunica muscularis participates in the formation of the crescent-shaped invaginations, the semilunar folds.

II./1.2.5.: Tela subserosa

A fatty connective tissue layer situated under the peritoneal covering. The intraperitoneal parts of the large intestine exhibit fatty appendages called appendices epiploicae – these are formed by the fatty connective tissue of tela subserosa and the peritoneal covering of tunica serosa together.

II./1.2.6.: Tunica serosa

Peritoneal covering, lined by simple squamous epithelium (mesothelial cells) on its external surface. Those parts of the colon, which are attached to the posterior abdominal wall don’t exhibit the tela subserosa and tunica serosa – instead of them are surrounded by the tunica adventitita (loose connective tissue), being the outermost layer.