Enterocyte

The enterocytes are intestinal epithelial cells responsible for the absorption of various essential nutrients, the transport of water and electrolytes within the body, and the secretion of protein into the intestinal lumen. They have an abundant number of microvilli at their apical pole, which increase the area for both digestion and intestinal transport.
Enterocytes fulfill the fundamental function of forming the intestinal barrier, which presents biomechanical, biochemical, and immunological characteristics.
They originate in the Lieberkühn crypts and migrate towards the intestinal villi, from whose end they fall off, after five to seven days, when they finish their life cycle.

Structure

Enterocytes are the absorptive cells of the intestine, they have dimensions of 20-25 µm in height and 7-8 µm in width and are characterized by their apical striated plate.

Polarization

Schematic drawing of an Enterocito showing its apical pole with microvellosities (microvilli) towards the intestinal light.

Enterocytes have a very marked cellular polarization; there is an apical absorptive and digestive pole, oriented towards the lumen (lumen) in contact with the intestinal content, and a basal secretory pole, which is in contact with the connective tissue, lymphatic vessels, and capillaries of the mucous layer of each villi..

Microscopic Anatomy

Enterocytes in a thin intestine cut. They limit the light of the intestine (above), are elongated prismatic. Cytoplasm is dyed in light pink and the nuclei in blue are at the same level in the basal part. In the apical pole of these cells the edge of the brush is seen, as a narrow area more contrasted. Optical micrograph with HyE staining.

Light Microscopy The enterocyte is a prismatic cell, elongated with its major apical-basal axis. Its apical border presents a continuous refractive structure called edging or brush border.
The nucleus is large and oval, it is located in the lower two thirds and occupies almost the entire diameter of the cell. The chromatin appearance is clear with one or two highly visible nucleoli.
chemical stains The dyes are necessary to highlight the structures, since they react in different sectors of the cells and reveal specific areas that can be observed with light microscopes.
With H&E staining, the enterocyte appears in two colors: the nucleus is dark blue (basophilic), and the cytoplasm appears pink (weakly acidophilic). The brush border is visible as a thin refractive band in sections treated with hematoxylin and eosin.
With PAS staining the brush border appears red due to the presence of mucopolysaccharides and mucus in the glycocalyx.

Electron microscopy Ultrastructure of the enterocyte shows densely packed organelles: numerous mitochondria, an extensive endoplasmic reticulum, a large Golgi apparatus, and abundant vacuoles and vesicles.
The brush border of the apical pole is formed by microvilli, which are clearly visible only at high magnification. Microvilli are specializations of the apical membrane of the enterocyte.

Microvellosities of the enterocyte, projecting from the apical pole to the light of the intestine (above). They form the edge in the brush, it is observed its homogeneous size and the insertion of its internal filaments in the fibrillary terminal network. Electronic transmission microscope micrograph.

With the electron microscope, microvilli are observed as parallel cylinders 1 to 2 micrometers (µm) in height and 0.1µm in width; Through its central axis passes a bundle of filaments that form its skeleton.
Microvilli project from the surface of their cell membrane, a filamentous molecular structure, called a glycocalyx, which performs several functions. This extracellular structure shows a great enzymatic activity, of specific hydrolases destined to the terminal digestion of nutrients.
The cytoplasm under the luminal surface of the enterocyte contains a terminal filamentous veil, called the terminal network or terminal frame, where the filaments of the microvillous skeleton are inserted.
The apical cytoplasm contains numerous smooth endoplasmic reticulum (SER) cisterns, which are necessary for triglyceride synthesis. The rough endoplasmic reticulum (RER) is extensive and lies deeper within the cell. The Golgi complex is large, showing a supranuclear position in close relation to the RER. Pinocytic vesicles and vacuoles are abundant below the apical membrane and basolateral membrane.

Function

Hexagonal structure and loclization of Caderina (in green, in intercellular unions) and Actina (in red, with diffuse and cortical intracellular distribution).

Enterocytes are the major and most numerous cell type of the intestinal epithelium and line the entire inner surface of the intestine. They fulfill biomechanical, biochemical and immunological barrier functions in symbiosis with the normal microbiota that limit them by their apical pole, located in the intestinal lumen.
These cells form a physical barrier that, together with mucous secretions, form the body's first defense against the invasion of agents that could be pathogens.

Biomechanical barrier

Enterocytes form a semi-permeable barrier that allows the selective passage of certain substances, while preventing the access of others.

Biochemical barrier

Enterocytes are chemical factories, which excrete and secrete a large number of substances.

Immune barrier

Enterocytes prevent the passage of microbial antigens, maintain the immune system, and facilitate the acquisition of tolerance to food antigens and intestinal microbiota.

Polarization and domains

Enterocytes (asterisks) and their apical-basal polarization. Right intestinal light in white. Left below basal domain.

Epithelial cells display a polarity, which can be related to morphological, biochemical, and functional domains also defined in the enterocyte. As a consequence of their cell differentiation, two regions or domains are described in the enterocyte.

Apical domain

It is anatomically limited to the end of the enterocyte in contact with the contents of the intestinal lumen. It includes the microvilli and the glycocalyx, the cytoplasmic structural apical veil, and the intercellular tight junctions.

Basolateral domain

It has anatomically broader and more diffuse boundaries in the lower half of the enterocyte. This domain is in contact with the basement membrane, and through it with myofibroblasts, fibroblasts with their extracellular matrix, and blood capillaries.

Renewal

The enterocytes that form the monolayer of the epithelium are highly specialized and therefore incapable of reproducing. They are short-lived, dying after three to five days of life and shed from the tip of the villi into the lumen of the intestine.

Complete physiological renewal of the intestinal epithelium occurs in approximately one week and is driven by a group of Intestinal Stem Cells (IESCs) at the base of each intestinal crypt. This impressive turnover rate is tightly controlled in homeostasis.