Microvillus

Microvillus

Overview
Microvilli are microscopic cellular membrane protrusions that increase the surface area of cells, and are involved in a wide variety of functions, including absorption, secretion
Secretion
Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...

, cellular adhesion, and mechanotransduction
Mechanotransduction
Mechanotransduction refers to the many mechanisms by which cells convert mechanical stimulus into chemical activity.- Tendon :The process of is explained for the lay-reader at http://bjsm.bmj.com/content/43/4/247.full- Ear :...

.

Thousands of microvilli form a structure called the brush border
Brush border
A brush border is the name for the microvilli-covered surface of simple cuboidal epithelium and simple columnar epithelium cells found in certain locations of the body. Microvilli are approximately 100 nanometers in diameter and their length varies from approximately 100 to 2,000 nanometers in...

 that is found on the apical surface of some epithelial cells, such as the small intestinal
Microvilli are observed on the plasma surface of eggs, aiding in the anchoring of sperm cells that have penetrated the extracellular coat of egg cells.
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Encyclopedia
Microvilli are microscopic cellular membrane protrusions that increase the surface area of cells, and are involved in a wide variety of functions, including absorption, secretion
Secretion
Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...

, cellular adhesion, and mechanotransduction
Mechanotransduction
Mechanotransduction refers to the many mechanisms by which cells convert mechanical stimulus into chemical activity.- Tendon :The process of is explained for the lay-reader at http://bjsm.bmj.com/content/43/4/247.full- Ear :...

.

Locations


Thousands of microvilli form a structure called the brush border
Brush border
A brush border is the name for the microvilli-covered surface of simple cuboidal epithelium and simple columnar epithelium cells found in certain locations of the body. Microvilli are approximately 100 nanometers in diameter and their length varies from approximately 100 to 2,000 nanometers in...

 that is found on the apical surface of some epithelial cells, such as the small intestinal
Microvilli are observed on the plasma surface of eggs, aiding in the anchoring of sperm cells that have penetrated the extracellular coat of egg cells. Clustering of elongated microtubules around a sperm allows for it to be drawn closer and held firmly so fusion can occur.

Microvilli are also of importance on the cell surface of white blood cells, as they aid in the migration of white blood cells.

Structure


Microvilli are covered in plasma membrane, which encloses cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

 and microfilaments. Though these are cellular extensions, there are little or no cellular organelles present in the microvilli.

Each microvillus has a dense bundle of cross-linked actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...

 filaments, which serves as its structural core. 20 to 30 tightly bundled actin filaments are cross-linked by bundling proteins fimbrin
Fimbrin
-Structure:Fimbrin belongs to the calponin homology domain superfamily of actin cross-linking proteins. Like other members of this superfamily, which include α-actinin, β-spectrin, dystrophin, ABP-120 and filamin, it has a conserved 27 kDa actin-binding domain that contains a tandem duplication of...

 and villin
Villin
Villin is a 92.5 kDa tissue-specific actin-binding protein associated with the actin core bundle of the brush border. Villin contains multiple gelsolin-like domains capped by a small "headpiece" at the C-terminus consisting of a fast and independently-folding three-helix bundle that is stabilized...

 to form the core of the microvilli.

In the enterocyte microvillus, the structural core is attached to the plasma membrane along its length by lateral arms made of myosin 1a
MYO1A
Myosin-Ia is a protein that in humans is encoded by the MYO1A gene.The protein encoded by this gene belongs to the myosin superfamily. Myosins are molecular motors that, upon interaction with actin filaments, utilize energy from ATP hydrolysis to generate mechanical force. Each myosin has a...

 and Ca2+ binding protein calmodulin
Calmodulin
Calmodulin is a calcium-binding protein expressed in all eukaryotic cells...

. Myosin 1a functions through a binding site for filamentous actin on one end and a lipid binding domain on the other. The plus ends of the actin filaments are located at the tip of the microvillus and are capped, possibly by capZ
CapZ
CapZ is a protein that caps the barbed end of actin filaments in muscle cells. It is located in the muscle Z band....

 proteins, while the minus ends are anchored in the ‘terminal web’ composed of a complicated set of proteins including spectrin
Spectrin
Spectrin is a cytoskeletal protein that lines the intracellular side of the plasma membrane of many cell types in pentagonal or hexagonal arrangements, forming a scaffolding and playing an important role in maintenance of plasma membrane integrity and cytoskeletal structure...

 and myosin II.

Function


Microvilli can act as mechanosensors in kidney proximal tubule. They sense the fluid flow in the tubule lumen and convert this information via biochemical responses into reabsorption.
Microvilli are also of importance on the cell surface of white blood cells, as they aid in the migration of white blood cells.

Relationship to cell


As mentioned, microvilli are formed as cell extensions from the plasma membrane surface.

Actin filaments, present in the cytosol, are most abundant near the cell surface. These filaments are thought to determine the shape and movement of the plasma membrane.

The nucleation
Nucleation
Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...

 of actin fibers occurs as a response to external stimuli, allowing a cell to alter its shape to suit a particular situation.

This could account for the uniformity of the microvilli, which are observed to be of equal length and diameter. This nucleation process occurs from the minus end, allowing rapid growth from the plus end.

Interestingly, though the length and composition of microvilli is consistent within a certain group of homogenous cells, it can differ slightly in a different part of the same organism.

For example, the microvilli in the small and large intestines in mice are slightly different in length and amount of surface coat covering.

Enzymes


Microvilli function as the primary surface of nutrient absorption in the gastrointestinal tract. Because of this vital function, the microvillar membrane is packed with enzymes that aid in the breakdown of complex nutrients into simpler compounds that are more easily absorbed. For example, enzymes that digest carbohydrates called glycosidases are present at high concentrations on the surface of enterocyte microvilli. Thus, microvilli not only increase the cellular surface area for absorption, they also increase the number of digestive enzymes that can be present on the cell surface.

Glycocalyx


The microvilli are covered with glycocalyx
Glycocalyx
Glycocalyx is a general term referring to extracellular polymeric material produced by some bacteria, epithelia and other cells. The slime on the outside of a fish is considered a glycocalyx. The term was initially applied to the polysaccharide matrix excreted by epithelial cells forming a...

, consisting of peripheral glycoproteins that can attach themselves to a plasma membrane via transmembrane proteins.

This layer may be used to aid binding of substances needed for uptake, to adhere nutrients or as protection against harmful elements.

It can be another location for functional enzymes to be localized.

Destruction of microvilli


The destruction of microvilli can occur in certain diseases because of the rearrangement of cytoskeleton in host cells. This can lead to malabsorption of nutrients and persistent osmotic diarrhea, often accompanied by fever.

This is seen in infections caused by EPEC subgroup Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

, in Celiac
Celiac
Celiac may refer to:* Coeliac disease* Celiac artery* Celiac lymph nodes* Celiac plexus...

 disease, and Microvillus Inclusion Disease (an inherited disease characterized by defective microvilli and presence of cytoplasmic inclusions of the cell membrane other than the apical surface).

The destruction of microvilli can actually be beneficial sometimes, as in the case of elimination of microvilli on white blood cells which can be used to combat auto immune diseases.

Congenital lack of microvilli in the intestinal tract causes microvillous atrophy, a rare, usually fatal condition found in new-born babies.

See also

  • Villus
    Villus
    Intestinal villi are tiny, finger-like projections that protrude from the epithelial lining of the intestinal wall. Each villus is approximately 0.5-1.6 mm in length and has many microvilli , each of which are much smaller than a single villus...

  • Flagellum
    Flagellum
    A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...

  • Brush border
    Brush border
    A brush border is the name for the microvilli-covered surface of simple cuboidal epithelium and simple columnar epithelium cells found in certain locations of the body. Microvilli are approximately 100 nanometers in diameter and their length varies from approximately 100 to 2,000 nanometers in...

  • Intestinal villus
  • Cilia
  • Stereocilia
    Stereocilia
    In the inner ear, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance. They are about 10–50 micrometers in length and share some similar features of microvilli...


External links

- "Ultrastructure of the Cell: microvilli and basal enfoldings, endocytic vesicles" - "Ultrastructure of the Cell: microvillous border and Junctional Complex, oblique section"