Intestinal villi

There are roughly 20–40 villi per mm² of mucosa. They are broad and leaf-like in the duodenum and slender and finger-like in the ileum. Each villus has a core of lamina propria containing an arteriole that breaks into a dense subepithelial capillary network draining to a venule; a blind-ended lymphatic, the central lacteal; strands of smooth muscle from the muscularis mucosae; and immune cells (plasma cells secreting IgA, macrophages, lymphocytes).

The surface is covered by simple columnar epithelium. Most of its cells are absorptive enterocytes with a microvillous brush border. Among them are mucus-secreting goblet cells, enteroendocrine cells, tuft cells and intraepithelial lymphocytes.

Villi and crypts form a single renewing unit. Stem cells at the crypt base divide, the daughter cells migrate up the villus as they differentiate, and they are shed at the tip after about 3–5 days. This rapid turnover makes the villus very sensitive to chemotherapy, radiation and immune injury. Absorbed sugars, amino acids and short-chain fatty acids enter the capillaries and travel to the liver in the portal blood. Chylomicrons are too large for the capillaries and enter the lacteal. Villous smooth-muscle contractions, driven by the submucosal plexus and the hormone villikinin, help pump lymph onward.