Mucosa of small intestine

The mucosa of small intestine is the innermost layer of the intestinal wall and is highly specialised for digestion, absorption, secretion, and immune defence. It is structurally adapted to maximise the absorptive surface area through the presence of plicae circulares, intestinal villi, and microvilli. Histologically, the mucosa consists of three components: the surface epithelium, lamina propria, and muscularis mucosae.

Epithelium

The luminal surface of the mucosa is lined by a simple columnar epithelium composed predominantly of enterocytes and goblet cells. Enterocytes are absorptive cells with numerous apical microvilli that form the brush border, greatly increasing the surface area available for nutrient absorption. Interspersed among the enterocytes are goblet cells, which secrete mucus that lubricates and protects the intestinal lining.

Projecting from the mucosal surface are numerous intestinal villi, which are finger-like extensions of the mucosa that protrude into the intestinal lumen. These villi markedly increase the absorptive surface area and contain a central core of lamina propria. Within each villus are blood capillaries, a central lymphatic capillary known as a lacteal, smooth muscle fibres, and various immune cells. The villi are longest and most numerous in the duodenum and jejunum and gradually become shorter and less abundant in the ileum.

Between adjacent villi are the crypts of Lieberkühn, which are simple tubular intestinal glands formed by invaginations of the surface epithelium extending down to the muscularis mucosae. The crypts contain stem cells responsible for epithelial renewal, absorptive enterocytes, goblet cells, antimicrobial Paneth cells, hormone-secreting enteroendocrine cells, and occasional tuft cells. These glands play important roles in secretion, epithelial regeneration, and mucosal defence.

Lamina propria

The lamina propria is a layer of loose connective tissue situated beneath the epithelium and forms the central core of each villus. It contains an extensive network of blood vessels, lymphatic vessels, nerve fibres, smooth muscle fibres, and numerous immune cells, including lymphocytes, plasma cells, macrophages, and eosinophils. The lamina propria provides structural support for the mucosa, facilitates nutrient transport, and contributes significantly to immune surveillance and defence against pathogens.

Muscularis mucosae

The deepest layer of the mucosa is the muscularis mucosae, a thin layer of smooth muscle that separates the mucosa from the submucosa. Fine bundles of smooth muscle extend from the muscularis mucosae into the villi, where their rhythmic contraction produces villous movements that facilitate absorption and promote lymphatic drainage through the lacteals.

Regional variations along the length of the small intestine

• In the duodenum, the mucosa contains broad, leaf-shaped villi and is associated with Brunners glands in the submucosa.

• The jejunum possesses long, slender villi and prominent plicae circulares, providing the greatest absorptive surface area.

• In the ileum, the villi are shorter, goblet cells are more numerous, and large aggregates of lymphoid tissue known as Peyers patches are present within the lamina propria and submucosa.

Summary of functions of intestine mucosa

Functionally, the mucosa of small intestine is responsible for the digestion and absorption of nutrients, absorption of water and electrolytes, secretion of mucus and intestinal fluid, absorption and transport of dietary lipids through the lacteals, continuous epithelial renewal, and immune defence. Together with the plicae circulares, villi, and microvilli, it provides an exceptionally large surface area that enables efficient digestion and absorption of ingested nutrients.