What Are Villi and Microvilli and Their Function?

Villi are tiny, finger-like projections that line the inside of the small intestine, and microvilli are even smaller hair-like extensions covering the surface of each villus. Together, they dramatically expand the intestine’s absorptive surface, turning what would otherwise be a smooth tube into a deeply folded, almost shaggy landscape optimized for pulling nutrients out of digested food. The relationship between the two is nested: each villus is covered in cells, and each of those cells sprouts hundreds of microvilli. This stacked architecture is one of the body’s most elegant engineering feats, but what happens inside these structures, how they respond to disease and fasting, and where else they show up in the body are all worth exploring in detail.

The Basic Architecture of a Villus

A villus is roughly 0.5 to 1.5 millimeters tall, visible to the naked eye if you know what you’re looking for. Picture a field of densely packed fingers sticking up from the intestinal wall. Each one has a core of connective tissue called the lamina propria, which houses blood capillaries and at least one lymphatic vessel known as a lacteal. Covering the outside of each villus is a single layer of epithelial cells, predominantly absorptive cells called enterocytes, along with mucus-secreting goblet cells and hormone-releasing enteroendocrine cells.¹

The blood capillary network inside each villus forms a mesh just beneath the epithelial layer, positioned to pick up absorbed sugars, amino acids, and other water-soluble nutrients and shuttle them into the bloodstream. The central lacteal handles a different cargo: dietary fats. Lacteals are lymphatic vessels that absorb lipids packaged into particles called chylomicrons and route them into the lymphatic system rather than directly into the blood. In the upper portions of the small intestine, where villi tend to be broader, a single villus can contain two to five lacteals, while narrower villi farther down typically have just one.² In some species the number can be even higher, with individual villi housing three to ten or more lacteals depending on their width.³

Microvilli and the Brush Border

If villi are the fingers, microvilli are the fuzz on those fingers. Each enterocyte on a villus sprouts roughly a thousand microvilli from its top surface, creating a dense fringe that pathologists long ago nicknamed the “brush border” because of how it looks under a microscope. Each microvillus is only about one micrometer long, far too small to see without magnification. But the collective effect of millions upon millions of them is enormous: they multiply the absorptive surface of the intestine many times over on top of the amplification already provided by the villi themselves.⁴

Inside each microvillus is a bundle of actin filaments, the same structural protein that gives muscle cells their contractile ability. These filament bundles are not rigid rods. Research using electron tomography has shown that the actin filaments inside a microvillus twist gently in a clockwise coil and that this coiling is driven partly by the tension of the surrounding cell membrane. When the membrane is experimentally stripped away, the coil relaxes, suggesting the bundle behaves more like an elastic spring than a stiff pillar.⁵ This flexibility likely helps microvilli bend and recover as food slurry (chyme) flows past them.

Coating the outside of the microvilli is a fuzzy carbohydrate-rich layer called the glycocalyx. This molecular mesh acts as a protective barrier and creates a microenvironment close to the cell surface where digestive enzymes can do their final work on nutrients before absorption.

How Microvilli Organize Themselves

For decades, scientists assumed microvilli simply grew out of the cell surface and stayed put. But research over the past ten years has revealed that microvilli actively link to one another through protein bridges at their tips. A complex called the intermicrovillar adhesion complex, or IMAC, was identified in 2014. It uses specialized adhesion proteins to connect the tips of neighboring microvilli, keeping them packed in a tight, orderly array. When one of the key IMAC components is deleted in mice, the microvilli splay apart in a disorganized fan, a pattern also observed in patients with inflammatory bowel disease.⁶

More recently, a second adhesion complex was found near the base of the microvilli rather than at the tips. This complex relies on a protein called TMIGD1, and when it is knocked out in mice, microvilli form membrane blebs, lose their adhesion to neighbors, and fail to assemble a proper brush border.⁷ The picture that emerges is that microvilli are not passive outgrowths; they are held together by at least two distinct sets of molecular glue, one at the top and one at the bottom, and losing either one compromises the whole structure.

Nutrient Absorption at Work

The villi and microvilli divide the labor of absorption between them. The microvilli provide the final surface where nutrients actually cross the cell membrane, while the villus interior provides the vascular and lymphatic plumbing to carry those nutrients away.

Water-soluble nutrients, including glucose, amino acids, and many vitamins, pass through transporter proteins embedded in the microvillar membranes and are picked up by the capillary network inside the villus. Dietary fats follow a different path. After being broken down by bile salts and pancreatic enzymes, lipids are absorbed by the enterocytes, repackaged internally, and then secreted into the lacteals. The integrity of those lacteals, particularly the junctions between the cells that form their walls, is critical for efficient fat absorption. Research has shown that lacteals maintain a balance between two types of cell-cell junctions, and disruptions to that balance can impair lipid uptake.⁸

The brush border also hosts its own suite of digestive enzymes embedded directly in the microvillar membranes. These enzymes handle the final breakdown of carbohydrates and proteins that pancreatic enzymes have already partially digested. Disaccharidases like lactase, sucrase, and maltase split double sugars into single sugars that transporters can grab, while peptidases cleave short protein fragments into individual amino acids. These enzymes only become active once the enterocyte has fully matured and migrated from the crypt (the valley between villi where new cells are born) up onto the villus surface.⁹

What Happens When Villi Are Destroyed

Celiac disease is the most widely known condition involving villus damage. In people with celiac disease, ingesting gluten triggers an immune response that sends inflammatory cells, predominantly certain types of immune cells, flooding into the lining of the small intestine. Over time, this inflammation flattens the villi, a process called villous atrophy, while simultaneously causing the crypts to deepen and hyperproliferate in an attempt to compensate. The result is a dramatically reduced absorptive surface, leading to malabsorption of nutrients, deficiencies in iron and calcium, diarrhea, and weight loss.¹⁰ The good news is that removing gluten from the diet allows the villi to regrow, often within months, though full recovery can take longer in adults than in children.

Infections can also target these structures. Enteropathogenic E. coli (EPEC), a common cause of infant diarrhea in developing countries, attaches to the enterocyte surface and locally destroys the microvilli beneath it. The bacterium creates what pathologists call an “attaching and effacing” lesion: it wipes away the brush border in its immediate vicinity, pushes the cell membrane into a pedestal shape, and hijacks the cell’s own actin to anchor itself in place.¹¹¹² This destruction of microvilli impairs nutrient absorption and contributes to the watery diarrhea that characterizes the infection.

Microvillus Inclusion Disease

While celiac disease and infections damage structures that were once healthy, some infants are born with microvilli that never form properly in the first place. Microvillus inclusion disease (MVID) is a rare genetic condition that typically shows up within days to months after birth as severe, persistent watery diarrhea that does not respond to standard treatments. The underlying cause, in most cases, involves mutations in a gene called MYO5B, which encodes a molecular motor protein responsible for shuttling cargo to the cell’s apical surface, the side facing the intestinal lumen.¹³

Without functional myosin Vb, the enterocyte cannot properly build or maintain its microvilli. Electron microscopy of affected tissue shows an almost complete absence of the normal brush border, along with abnormal inclusions, small pockets of microvilli trapped inside the cell rather than displayed on the surface. Mutations in at least one other gene, STX3, have also been identified in MVID patients.¹⁴ Because the disease prevents normal absorption, most affected infants require long-term intravenous nutrition, and small bowel transplantation is sometimes the only definitive treatment.

Villi Are Not Static Structures

One of the more surprising things about intestinal villi is how quickly they respond to changes in diet and feeding status. During fasting, villi shrink. Studies in birds have shown that within the first 24 hours of fasting, villi in the upper small intestine already show a remarkable reduction in height. After several days without food, villus height drops progressively throughout the intestine, though the upper regions are affected earliest and most dramatically. Refeeding reverses the shrinkage rapidly: even after 20 days of fasting, the duodenal villi in hens began recovering within a single day of eating again.¹⁵

Research in rodents has added detail to this picture. During prolonged fasting, the lacteals inside the villi retract, and the basement membrane at the villus tips begins to detach. Interestingly, though, the microvilli themselves appear to be preserved through fasting, and the expression of certain nutrient transporters on their surface actually increases, as if the body is priming the brush border to absorb as efficiently as possible the moment food returns. Refeeding triggers a rapid influx of lipids into the enterocytes and a swift expansion of the lacteals.¹⁶

Diet composition matters too. In one controlled experiment, rats fed a plant-based diet developed longer and heavier intestines with wider villi and deeper crypts compared to controls. Meanwhile, rats that exercised regularly had shorter and lighter intestines with shorter villi, suggesting the gut actively remodels itself in response to both what you eat and how much energy you expend.¹⁷

Villi and Microvilli Outside the Gut

The small intestine gets most of the attention, but villus-like and microvillar structures appear elsewhere in the body, serving analogous surface-area-expanding purposes in completely different contexts.

The most prominent example outside the gut is the placenta. During pregnancy, the outer layer of the placenta differentiates into branching, tree-like projections called chorionic villi. These are not identical to intestinal villi in their cell types, but they serve a strikingly similar geometric purpose: maximizing the surface area available for exchange between two fluid compartments, in this case maternal and fetal blood.¹⁸ The architecture of the placental villous tree matters for efficiency. Modeling studies have shown that oxygen exchange is sensitive to villous branch length and volume, and that the tree’s geometry must balance maximizing surface area against allowing enough maternal blood flow to reach the exchange surfaces.¹⁹ When this geometry is disrupted, as occurs in some cases of growth restriction during pregnancy, oxygen transfer to the fetus suffers.

Microvilli also appear in the kidneys, where the proximal tubule cells sport a dense brush border much like the one in the intestine. In the kidney, these microvilli dramatically increase the surface area available for reclaiming sodium, water, and other filtered substances from the urine before they are lost. A sodium-hydrogen exchanger protein called NHE3, located specifically in the apical microvilli of proximal tubule cells, plays a central role in this reabsorption.²⁰ Beyond the kidney, microvilli are found in the inner ear (where they are closely related to sensory stereocilia), in the respiratory tract, and on numerous other epithelial surfaces throughout the body.

The Gut Barrier and Microbes

Villi and microvilli are not just absorptive structures. They form part of the body’s physical barrier against the enormous microbial population living in the gut. The single-cell-thick epithelial lining covering each villus must simultaneously absorb nutrients and keep bacteria, toxins, and other threats from penetrating deeper tissues. This balancing act depends on tight junctions between enterocytes, the mucus layer secreted by goblet cells, and the glycocalyx coating the microvilli themselves.

The relationship between intestinal structure and microbes runs both ways. The microbial community in the gut influences how well the epithelial barrier develops and how quickly it renews itself. Microbial colonization promotes competent immune responses, epithelial turnover, and even the development of the blood vessel network within the villi. The epithelial cells lining each villus are constantly being replaced; new cells are born in the crypts, migrate up the villus surface over the course of a few days, and are shed from the tip. This migration is not just passive. It depends on active signaling pathways, and heat stress or other insults can slow cell migration and reduce the number of differentiated cell types on the villus surface.²¹

Why the Two-Tier System Matters

You might wonder why the intestine bothers with both villi and microvilli rather than just making one or the other larger. The answer comes down to physics and biology pulling in different directions. Villi increase surface area at the tissue level, creating folds visible to the naked eye. But there is a limit to how tall or densely packed villi can be before they start interfering with the flow of chyme through the intestine. Microvilli solve this by adding amplification at a scale so small it does not obstruct flow at all. A single enterocyte is only about ten micrometers across, yet its microvillar fringe can multiply its apical surface area by roughly twenty-fold. Stack that on top of the roughly tenfold increase from the villi and the additional folds of the intestinal wall itself, and the total absorptive surface of the human small intestine is vastly larger than the exterior surface of the body.

This two-tier design also allows each level to be regulated somewhat independently. As the fasting studies show, villi can shrink and regrow on a timescale of days without necessarily changing the microvilli. And the microvilli can adjust their enzyme and transporter profiles without the underlying villus architecture needing to change. The gut can tune absorption at both the macro and the micro scale, which is useful for an organ that must deal with wildly different meals, periods of starvation, illness, and everything in between.