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Microvilli (sg.: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, [1] and are involved in a wide variety of functions, including absorption, secretion, cellular adhesion, and mechanotransduction.
Cells that absorb substances need a large surface area in contact with the substance to be efficient. [7] In intestinal cells, the microvilli are referred to as brush border and are protoplasmic extensions contrary to villi which are submucosal folds, while in the kidneys, microvilli are referred to as striated border. [8]
Intestinal villi (sg.: villus) are small, finger-like projections that extend into the lumen of the small intestine.Each villus is approximately 0.5–1.6 mm in length (in humans), and has many microvilli projecting from the enterocytes of its epithelium which collectively form the striated or brush border.
Microvilli on the apical surface increase its surface area. This facilitates transport of numerous small molecules into the enterocyte from the intestinal lumen . These include broken down proteins , fats , and sugars , as well as water, electrolytes , vitamins , and bile salts .
Other cellular extensions that protrude from the cell membrane are known as membrane protrusions or cell protrusions, also cell appendages, such as flagella, and microvilli. [ 8 ] [ 9 ] Microtentacles are cell protrusions attached to free-floating cells, associated with the spread of some cancer cells .
The terminal web is a filamentous structure found at the apical surface of epithelial cells that possess microvilli.It is composed primarily of actin filaments stabilized by spectrin, which also anchors the terminal web to the apical cell membrane.
In vertebrates, villin proteins help to support the microfilaments of the microvilli of the brush border. However, knockout mice appear to show ultra-structurally normal microvilli reminding us that the function of villin is not definitively known; it may play a role in cell plasticity through F-actin severing. [5]
The cell has the closest resemblance to the choanoflagellates which are the closest related single celled protists to the animal kingdom (metazoans). The flagellae beat regularly, creating a water flow across the microvilli which can then filter nutrients from the water taken from the collar of the sponge.