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They are small finger-shaped projections located in the small intestine that protrude from the epithelial lining of its walls.
They extend into the lumen of the small intestine , and are approximately 0.5 to 1.6 mm in length in humans, with each villus having many microvilli.
The villi increase the area of the inner surface of the intestinal walls, which means that there is a greater surface area for the absorption of digested foods, such as amino acids, simple sugars (glucose), proteins, nutrients, among others.
The surface area augmented by the villi has an approximate range of 30 to 600 times.
In other words, increasing the surface area (in contact with the fluid in the light) decreases the average distance traveled by the nutrient molecules, thereby increasing the effectiveness of diffusion.
The villi also help the intestines move food along the digestive tract.
Structure of the alimentary canal
The alimentary canal is made up of the 7 m long wavy small intestine and its heavier 1.5 m companion, the large intestine. So the surface area for absorption is much higher in the small intestine.
After the digested food has passed through the stomach, it enters the small intestine for absorption into the blood.
The small intestine also releases digestive enzymes to ensure complete hydrolysis of food molecules.
When chyme, which is partially digested food mixed with gastric juices, just leaves the stomach, its first stop is the duodenum, a short, wide stretch of the small intestine.
At only 25 cm in length, the duodenum is the shortest segment of the small intestine. Here, the chyme mixes with digestive enzymes from the liver and pancreas.
Upon relaxation of the sphincter of Oddi, bile secreted by the liver and pancreatic juices secreted by the pancreas enter the duodenum to aid in chemical digestion. These secretions help reduce the acidity of strong gastric juices.
On the journey of food through the gastrointestinal tract, the next stop is the jejunum. This has a diameter of about 4 cm and a length of 2.5 m. Here, there are also large circular folds of submucosa called circular folds.
Finally, on the journey through the small intestine, food reaches the ileum. This is the longest segment of the small intestine, reaching about 3.5 m. The ileum is slightly less vascular and lighter in color than the jejunum.
Structure of the small intestine
The small intestine is made up of layers. The outermost layer is the serosa, which is continuous with the mesentery (tissues that connect the intestines with the abdominal wall).
The serosa contains blood vessels and nerves, and secretes fluid to lubricate the small intestine, protecting it from damage caused by friction.
Below the serosa is the longitudinal muscle layer. This muscle, along with the circular muscle (located just below the submucosa), contracts in peristaltic waves, moving food through the intestines.
The longitudinal muscle shortens the tract to facilitate movement of the chyme, while the circular muscle prevents the chyme from traveling backward.
The submucosa is made up of dense connective tissue to support the mucosa and connect it to the muscle layers. It also contains blood vessels, lymphatic vessels, and nerves to supply the mucosa.
Goblet cells, located in the epithelium, secrete mucus into the duct or lumen of the intestines.
The lamina is composed of loose areolar connective tissue that extends into the villi. It contains blood vessels and alkaline glands that secrete mucus that empty into the crypts (intestinal glands) to counteract the acid secretions of the stomach.
The mucosa is the innermost layer, it surrounds the lumen through which the chyme passes. It has three main functions: protection of the internal environment, secretion and absorption. The mucosa is incredibly folded, full of villi.
Villi
The architecture of the inner epithelial lining of the small and large intestine is unique, with extensive folding of epithelial cells to form crypts and finger-like projections called villi.
The villi, these finger-like projections have, as we already mentioned, the microvilli have their own projections.
Such an arrangement considerably increases the total surface area of the available intestinal lumen to allow maximum interaction of ingested nutrients with epithelial cells for efficient absorption of material.
The villi and microvilli create such a large absorption surface that the surface area of the small intestine is approximately 250 m 2.
Characteristics of the villi
Villi and microvilli exist on the surface of the mucosa along circular folds that slow down the passage of chyme through the intestines, which also increases the surface area.
In both the small and large intestines, villi are composed of enterocytes, mucus-secreting goblet cells, and peptide hormone-secreting enteroendocrine cells.
The villi are connected to the blood vessels, so the circulating blood carries away these nutrients. If there were no villi, absorption would be around 30 to 600 times slower.
An additional cell called a Paneth cell is present in the villi of the small intestine. Paneth cells participate in the gut’s innate immune defense by detecting pathogenic microbes and secreting microbicidal peptides.
Characteristics of microvilli and nutrient absorption capacity
The structural characteristics of the epithelial lining of the villi optimize its ability to absorb digested materials.
Tight joints create associations between the plasma membrane of two adjacent cells, creating an impermeable barrier.
They also keep digestive fluids separate from tissues and maintain a concentration gradient by ensuring one-way movement.
The borders of the microvilli significantly increase the surface area of the plasma membrane greater than 100 ×, which allows greater absorption to occur.
The membrane will be embedded with immobilized digestive enzymes and channel proteins to aid in the absorption of material.
The epithelial cells of the intestinal villi possess large amounts of mitochondria to provide ATP for active transport mechanisms.
ATP may be required for primary active transport (counter-gradient), secondary active transport (co-transport), or pinocytosis.
Pinocytotic vesicles are responsible for the non-specific absorption of dissolved liquids and solutes. These materials are ingested through the rupture and reform of the membrane and, therefore, contained within a vesicle.
Epithelial stem cells
The intestinal lining where stem cells reside consists of valley-like crypts between projections called villi.
These contain many types of cells, including mucus-secreting goblet cells, nutrient-absorbing enterocytes, and the mysterious cells of Paneth.
The skin, hair follicles, and intestinal villi are examples of tissues that must undergo rapid and continuous renewal.
Intestinal linings wear out quickly, so it makes sense that the intestinal epithelium is the fastest-renewing tissue in adult mammals.
Generally, the self-renewal process is believed to be supported by a layer of cells called myofibroblasts located at the base of the crypts.
As in the epidermis, the newly formed cells move in a polarized fashion as a column of cells from the bottom of the crypt upward to an adjacent villi, where migration occurs along the villi until the cells they come off at the tips of the villi.
The absorption functions of the cells are operative during migration from the base to the tip of the villi. The entire migratory passage from the newly formed cell in the crypt to the shed cell at the tip of the villi is believed to occur within 5 to 7 days.
The outer layer of the human epidermis is shed, with continuous replacement by differentiation of cells moving upward from a basal layer in the dermis.
Cells in the basal layer show slow but persistent cell division. Within this layer are epidermal stem cells and transient amplifier (progenitor) cells.
Because there is a complete renewal of the human epidermal layer completely within a few weeks and transient amplifying basal cells divide only three to six times before differentiating into epidermal cells, the self-renewal capacity of epidermal stem cells is astonishing.
