Microvillus

The microvilli (singular: microvillus) are structures that increase the surface area of cells by approximately 600 fold (human), thus facilitating absorption and secretion.



Locations
There are 2 microvilli present on the apical surface of a single cell in human small intestinal cells (intestinal villus).

Microvilli also occur in sensory cells of the inner ear (as stereocilia), in the cells of taste buds, and in olfactory receptor cells.

They 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 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 filaments, which serves as its structural core. 20 to 30 tightly bundled actin filaments are cross-linked by bundling proteins fimbrin and villin to form the core of the microvilli. The actin filaments render the microvilli capable of contracting motion, though the motion is limited, similar to flexing of fingers.

The structural core is attached to the plasma membrane along its length by lateral arms made of myosin Ia and Ca2+ binding protein calmodulin. Myosin Ia 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 collected in the tip of the microvillus, while the minus ends bind to a ‘terminal web’ composed of thin filaments, linked together by a complicated set of proteins including spectrin and myosin II.

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.

It is catalyzed by a complex made of two conserved actin related proteins (ARPs).

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 often have enzymes that aid their function present in them. For example, Lactase and other enzymes that can help hydrolyze carbohydrates are present on microvilli in intestinal epithelial cells.

These enzymes are localized in the amorphous dark staining tip of the microvilli. Thus, they are not only increasing the area for absorption, they are also increasing the area for enzymes involved in digestion to anchor on the cell surface and perform final stages of extracellular digestion, breaking down small peptides and disaccharides for transport across the membrane.

Glycocalyx
The microvilli are covered with glycocalyx, consisting of peripheral glycoproteins that can attach themselves to a membrane.

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, in Celiac 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.