Microvillus

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Microvillus
Enterocytes with microvilli
Details
Identifiers
Latinmicrovillus
MeSHD008871
THH1.00.01.1.01011
FMA67296
Anatomical terms of microanatomy

Microvilli (sg.: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume,

cellular adhesion, and mechanotransduction
.

Structure

Microvilli are covered in plasma membrane, which encloses

organelles
present in the microvilli.

Each microvillus has a dense bundle of cross-linked

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 and Ca2+ binding protein calmodulin. 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 proteins,[2] while the minus ends are anchored in the terminal web composed of a complicated set of proteins including spectrin
and myosin II.

The space between microvilli at a cell's surface is called the intermicrovillous space. Intermicrovillous space increases with contractile activity of myosin II and tropomyosin, and decreases when contraction ceases.

Locations

Thousands of microvilli form a structure called the

intestinal villi
, which are made of many cells. Each of these cells has many microvilli.) 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. They are large objects that increase surface area for absorption.

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 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.

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.[3]

Function

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. Microvilli are also present on immune cells, allowing the immune cells to sense features on the surface of pathogens and other antigen-presenting cells.[4]

Glycocalyx

The microvilli are covered with

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.

Clinical significance

Destruction

The destruction of microvilli can occur in certain diseases because of the rearrangement of cytoskeleton in host cell. This is seen in infections caused by EPEC subgroup Escherichia coli, in celiac disease, and microvillus inclusion disease[5] (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.[6]

Congenital lack of microvilli in the intestinal tract causes

microvillus atrophy
, a rare, usually fatal condition found in new-born babies.

See also

References

  1. . Retrieved 25 November 2010.
  2. ^ The Cytoskeleton: Cytoskeleton in specialized tissues and in pathological states, edited by JE Hesketh and IF Pryme, Elsevier, 1996ISBN 1559386894, 9781559386890. Vol3, pp371, M1[permanent dead link]
  3. PMID 6035639. link Archived April 6, 2008, at the Wayback Machine
  4. .
  5. ^ Malathy Kapali, MD, Ronald Jaffe, MD and Rocco M Agostini Jr. B.Sc. Final Diagnosis: Microvillus Inclusion Disease. "Final Diagnosis -- Case 163". Archived from the original on 1999-10-06. Retrieved 2006-02-19.
  6. ^ Shattuck, T. (2004) Cells studied for immune function. "Cells studied for immune function". Archived from the original on 2016-08-20. Retrieved 2006-02-19..

External links