villus

A villus (plural villi) is a small, finger-like projection that extends into the lumen of a hollow organ. In humans, the term most commonly refers to the villi of the small intestine, where they markedly increase the surface area for nutrient absorption. The word can also describe the chorionic villi of the placenta, which participate in fetal-maternal exchange.

Intestinal villi are lined by simple columnar epithelium with tight junctions and a brush border formed by microvilli on enterocytes. Each villus contains a core of loose connective tissue called lamina propria, which houses a network of capillaries and a central lacteal (vessel of the lymphatic system) involved in lipid transport. The villi rest on the muscularis mucosae and are interspersed with goblet cells that secrete mucus. At the base of each villus lie the crypts of Lieberkühn, where intestinal stem cells continuously regenerate the epithelium and give rise to other cell types such as Paneth and enterocytes.

Functionally, villi increase the absorptive surface area of the small intestine, allowing efficient uptake of monosaccharides, amino acids, fats, vitamins, and minerals. Nutrients are transported from enterocytes into the bloodstream via capillaries or into the lymphatic system via the lacteals.

Clinical relevance includes conditions that affect villus structure. Villous atrophy or blunting, seen in celiac disease and other enteropathies, impairs absorption and can be detected on biopsy by measuring villus height relative to crypt depth.

Placental villi are another type, consisting of chorionic projections containing fetal blood vessels and surrounded by trophoblastic tissue, facilitating exchange between mother and fetus. They form the placental tree, including floating and anchoring villi, and mature to support fetal development.