Intestinal villi

The intestinal villi (villi intestinales) are numerous finger-like projections of the mucosa that extend into the lumen of the small intestine, giving its internal surface a characteristic velvety appearance. They are present throughout the entire length of the small intestine and greatly increase the absorptive surface area available for digestion and nutrient absorption.

Intestinal villi are largest, tallest, and most densely packed in the duodenum and jejunum, where absorption is most active. Towards the ileum, they gradually become shorter, narrower, and less numerous.

Structure of an intestinal villus (histological features)

Each intestinal villus consists of a central core of lamina propria containing lymphatic, vascular, muscular, and immune components, covered by a specialized absorptive epithelium. Microscopically, an intestinal villus is identified by the following features:

• Finger-like projection of the mucosa

• Simple columnar epithelium with goblet cells

• Central core of lamina propria

• Prominent central lacteal

• Dense capillary network

• Smooth muscle fibres extending from the muscularis mucosae

1. Surface Epithelium (enterocytes and goblet cells)

Each villus is covered by a simple columnar epithelium consisting predominantly of enterocytes, which are absorptive cells bearing numerous apical microvilli that collectively form the brush border. Interspersed among the enterocytes are goblet cells, which appear pale in histological sections and secrete mucus that lubricates and protects the intestinal lining.

The epithelium rests upon a thin basement membrane, which separates it from the underlying lamina propria.

2. Lacteal or lymphatic channels

The central feature of each villus is the lacteal, a blind-ended lymphatic capillary that occupies the core of the villus and forms its principal axis.

The wall of the lacteal consists of a single layer of endothelial cells. It is responsible for the absorption of dietary lipids, which enter the lacteal as chylomicrons and are subsequently transported through the lymphatic system before reaching the bloodstream.

3. Blood vessels

A rich vascular network surrounds the central lacteal. An arteriole enters the intestinal villus and branches into an extensive capillary plexus immediately beneath the epithelial basement membrane. These capillaries absorb monosaccharides, amino acids, water-soluble vitamins, electrolytes, and water.

The capillary network drains into a venule at the base of the intestinal villus and ultimately into the hepatic portal venous system. The close anatomical relationship between the capillaries and enterocytes facilitates efficient nutrient absorption and transport.

4. Smooth muscle fibres

Smooth muscle fibres derived from the muscularis mucosae extend into the core of each villus. These fibres are arranged predominantly as longitudinal bundles surrounding the central lacteal and extend from the base towards the apex of the villus.

Fine muscle fibres branch laterally within the lamina propria and attach to the surrounding connective tissue. Rhythmic contraction of these fibres produces villous movements, which facilitate lymphatic drainage through the lacteal.

5. Lamina propria (reticular connective tissue)

The supporting framework of the villus is formed by the lamina propria, a specialized loose connective tissue rich in reticular fibres. The lamina propria encloses and supports the blood vessels, lacteals, smooth muscle fibres, and numerous immune cells including lymphocytes, plasma cells, macrophages, and eosinophils. These immune cells contribute significantly to mucosal immune defence system by providing continuous surveillance against ingested pathogens and antigens.

Functions of intestinal villi

Intestinal villi are highly specialised structures that maximise the efficiency of digestion and absorption by:

• Greatly increasing the absorptive surface area of the small intestine

• Facilitating rapid transfer of nutrients into blood and lymphatic vessels

• Enabling absorption and transport of dietary lipids via lacteals

• Supporting continuous epithelial renewal through stem cells located in the intestinal crypts

• Providing immune surveillance through abundant lymphoid cells within the lamina propria

Together with the plicae circulares and microvilli, the intestinal villi increase the absorptive surface area of the small intestine many-fold, thereby optimising nutrient absorption.

Clinical correlates

1. Coeliac disease

Coeliac disease is an autoimmune disorder in which exposure to gluten triggers inflammation and intestinal villus atrophy. Loss of villous height reduces the absorptive surface area of the small intestine, resulting in malabsorption, chronic diarrhoea, weight loss, and nutritional deficiencies

2. Tropical sprue

Tropical sprue is characterize by inflammatory injury to the small intestinal mucosa, leading to shortening and blunting of villi with consequent impairment of nutrient absorption.

3. Intestinal ischaemia

Because intestinal villi possess a dense capillary network and high metabolic activity, interruption of their blood supply can rapidly impair absorptive function and lead to mucosal injury.