ENTEROCYTES: CHARACTERISTICS, STRUCTURE, FUNCTIONS, DISEASES - BIOLOGY - 2025

The enterocytes are epithelial cells of the small intestine whose main function nutrient absorption and transport to other tissues of the body. They also participate as part of the intestinal immunological barrier against the entry of toxins and pathogens, since it is the area of ​​the body most exposed to the outside.

These cells make up about 80% of the epithelium in the small intestine. They are polarized cells, with numerous microvilli (brush border) towards the apical end.

Scheme of an enterocyte. Boumphreyfrderivative work: Miguelferig, via Wikimedia Commons

They originate from stem cells in the intestinal crypts. They are located in the villi of the small intestine and are short-lived. In humans, the intestinal epithelium is completely renewed every four to five days.

When there are defects in enterocytes, various congenital diseases can arise. They are the consequence of problems in the transport of proteins and in the mobilization and metabolism of lipids. Likewise, errors can occur in the immune system of the intestinal barrier.

Structure

The term enterocyte means "absorption cell" and was first used by Booth in 1968.

Enterocytes are formed as an almost continuous layer interspersed with other less abundant cell types. This layer constitutes the intestinal epithelium.

Morphology

Differentiated enterocytes are columnar cells that have an ellipsoidal nucleus in the basal half of the cytoplasm. Towards the apical end of the cell, numerous dictyosomes occur.

They present abundant mitochondria, which occupy approximately 13% of the cytoplasmic volume.

The most prominent characteristic of enterocytes is the evaginations of the plasma membrane towards the apical end. It has a large number of projections known as microvilli. They have a cylindrical shape and are arranged in parallel. The set of microvilli forms the so-called brush border.

The brush border microvilli increase the surface area of ​​the membrane 15 to 40 times. Digestive enzymes and those responsible for the transport of substances are located in the microvilli.

Characteristics

-Polarity of cells

Enterocytes, like many epithelial cells, are polarized. The cellular components are distributed among the different domains. The composition of the plasma membrane is different in these areas.

Cells generally have three domains: apical, lateral, and basal. In each of these, there are particular lipids and proteins. Each of these zones fulfills a particular function.

Two domains have been differentiated in the enterocyte:

• Apical domain: it is located towards the lumen of the intestine. Microvilli are featured and specialize in nutrient absorption.
• Basolateral domain: located towards the internal tissues. The plasma membrane is specialized in the transport of substances to and from the enterocyte.

-Features of the border or brush edging

The brush border has the typical structure of plasma membranes. It is made up of a lipid bilayer associated with very specific proteins.

The enzymes responsible for the digestion of carbohydrates and proteins are anchored to the brush border. Likewise, in this area are the enzymes specialized in the transport of substances.

Each of the microvilli is approximately 1-2 µm long and 100 µm in diameter. They have a particular structure formed by:

Nucleus of microvilli

Each microvillus contains a bundle of twenty actin filaments. The basal part of the bundle of filaments forms the root, which connects with the terminal network. In addition, the nucleus contains two types of polypeptides (fimbrin and vilin).

Terminal network

It is formed by a ring of actin filaments that intervene in the anchoring junctions between neighboring enterocytes. In addition, vinculin (cytoskeletal protein) and myosin are present among other proteins. It forms the so-called fibrillar plaque.

Glycocalyx

It is a layer that covers the microvilli. It is composed of mucopolysaccharides produced by the enterocyte. They form microfilaments that are attached to the outermost part of the microvilli.

The glycocalyx is considered to participate in the terminal digestion of nutrients, associated with the presence of hydrolases. It also participates in the immune barrier function of the intestinal epithelium.

-Unions between enterocytes

The cells that make up the intestinal epithelium (consisting mainly of enterocytes) are linked together. These junctions occur through protein complexes and provide the epithelium with structural integrity.

Junctions have been classified into three functional groups:

Tight junctions

They are intracellular junctions in the apical part. Its function is to maintain the integrity of the epithelial barrier as well as its polarity. They limit the movement of ions and luminal antigens towards the basolateral domain.

They are made up of four families of proteins: occludins, claudins, tricellulin, and adhesion molecules.

Anchor joints

They connect the cytoskeleton of neighboring cells as well as the extracellular matrix. They generate very resistant structural units.

The union between adjacent cells is carried out by adhesion molecules from the group of cadherins and catenins.

Communicating unions

They allow communication between the cytoplasms of neighboring cells, which occurs through the formation of channels that cross the membranes.

These channels are made up of six transmembrane proteins from the group of connexins.

Lifecycle

Enterocytes last approximately five days in humans. In the case of mice, the life cycle can be two to five days.

These cells form in the so-called Lieberkün crypts. Here are presented the stem cells of the different cell types that make up the intestinal epithelium.

Stem cells divide four to six times. Subsequently, the cells begin to move under pressure from the other cells in formation.

In its movement from the crypt to the apical zone of the villi, the enterocyte gradually differentiates. Contact with other cells, interaction with hormones, and diet composition have been shown to influence differentiation.

The process of differentiation as well as movement to the intestinal villi takes approximately two days.

Subsequently, the enterocytes begin to be exfoliated. Cells lose the different types of junctions. In addition, they are subjected to mechanical pressure until they detach, being replaced by new cells.

Features

Enterocytes have as their main function the absorption and transport of nutrients to different parts of the body. Likewise, they actively participate in the immune protection functions that occur in the intestine.

Nutrient absorption and transport

The nutrients absorbed by enterocytes come mainly from stomach breakdown. However, these cells can digest peptides and disaccharides due to the presence of specific enzymes.

Most of the nutrients in the digestive tract pass through the membrane of the enterocytes. Some molecules such as water, ethanol, and simple lipids move through concentration gradients. Others such as glucose and more complex lipids are mobilized by transporter proteins.

In the enterocytes the different lipoproteins are formed that transport triglycerides and cholesterol to different tissues. Among these we have chylomicrons, HDL and VDL.

The iron required for the synthesis of various proteins such as hemoglobin is taken up by the enterocytes. Iron enters cells through a membrane transporter. Later it joins other transporters that carry it to the blood where it will be used.

Gut immune barrier

The intestinal epithelium forms a barrier between the internal and external environment, due to the structure formed by the different cell junctions. This barrier prevents the passage of potentially harmful substances such as antigens, toxins and various pathogens.

Enterocytes must fulfill the dual function of absorbing nutrients and preventing the passage of harmful substances and organisms. For this, the apical area is covered by a layer of carbohydrates produced by other epithelial cells, called goblets. It lets small molecules pass, but not large ones.

On the other hand, the glycocalyx that lines the brush border has many negative charges that prevent direct contact of pathogens with the enterocyte membrane.

They also have the ability to produce an immune response in the presence of certain antigens.

Enterocytes have been shown to produce vesicles in the apical domain that are high in alkaline phosphatase. This compound inhibits bacterial growth and decreases the ability of bacteria to bind to the enterocyte.

Diseases

When errors occur in the formation or structure of enterocytes, various congenital pathologies may occur. Among these we have:

Microvillus inclusion disease

It occurs when in the differentiation of the enterocyte there is atrophy in the formation of the brush border.

Symptoms are persistent diarrhea, nutrient absorption problems, and developmental failure. In 95% of cases, symptoms appear in the first days after birth.

Trichohepatoenteric syndrome

This disease is associated with problems in the development of the villi of the intestine and affects the structure of the epithelial layer.

Symptoms are untreatable diarrhea in the first month of life. In addition, there are failures in nutrient absorption and development. Facial dysmorphism, hair and skin abnormalities may occur. The immune system is also affected.

Chylomicron retention disease

Chylomicrons (lipoproteins responsible for the transport of lipids) are not produced. Large lipid vacuoles are seen in enterocytes. In addition, chylomicron-like particles are present that do not emerge from the edges of the membrane.

Patients present with chronic diarrhea, severe lipid absorption problems, developmental failure, and hypocholesterolemia.

Congenital tuft enteropathy

It is associated with atrophy in the development of the intestinal villi, disorganization of the enterocytes and the presence of a kind of tufts at the apex of the villi.

Symptoms are persistent diarrhea immediately after birth. The intestine does not have the capacity to absorb nutrients, which must be given to the patient intravenously. Hair has a woolly appearance and development as well as the immune system are affected.

Enterocytes and HIV

In HIV-infected patients, problems may occur in the absorption of nutrients. In these cases the most obvious symptom is steatorrhea (diarrhea with lipids in the stool).

The HIV virus has been found to infect crypt stem cells in these patients. Therefore, the differentiation of enterocytes that are not capable of fulfilling their function is affected.

References

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