Brush border microvilli and enterocyte absorption

The small intestine increases its absorptive surface at three levels. The plicae circularis are large circular folds of the lining. Villi are finger-like projections of the lining into the lumen. The finest level is the brush border, made of microvilli: microscopic projections of the cell membrane on the apical surface (the side facing the lumen) of each enterocyte, an absorptive epithelial cell. Together the three levels increase the absorptive surface more than 600-fold.

Microvilli enlarge the membrane where absorption happens, and enzymes are fixed to their surface. The membrane holds the transporters that carry nutrients into the cell: SGLT1 carries glucose and galactose, GLUT5 carries fructose and PepT1 carries short peptides. Brush border enzymes, for example lactase and sucrase, finish the digestion of sugars at the cell surface. Enterocytes are responsible for the majority of absorption.

There is a range of epithelial cells, including the enterocytes. Goblet cells produce mucus, Paneth cells produce antimicrobial peptides (substances that help defend against microbes), and stem cells replace the other cell types. Enteroendocrine cells (hormone-producing cells) are present too. Tight junctions, the seals between neighbouring cells, make the epithelium a selective barrier but add no membrane for absorption.

When microvilli are lost, the absorptive area falls and fewer enzymes and transporters are available, so food is poorly absorbed. This is malabsorption (poor uptake of nutrients), and it produces diarrhoea and weight loss. Intact villi with missing microvilli place the fault at the level of the cell surface and not in the villus as a whole. In coeliac disease, the immune system reacts to gluten (a protein found in wheat, barley and rye) and damages the lining of the small intestine, so the villi flatten and the microvilli are blunted, and both reduce the area available for absorption.