Microvilli9 (MY-cro-VIL-eye; singular, microvillus) are extensions of the plasma membrane that serve primarily to increase a cell's surface area (figs. 3.10 and 3.11a-b). They are best developed in cells specialized for absorption, such as the epithelial cells of the intestines and kidney tubules. They give such cells 15 to 40 times as much absorptive surface area as they would have if their apical surfaces were flat. On many cells, microvilli are little more than tiny bumps on the plasma membrane. On cells of the micro = small + villi = hairs
Table 3.2 Functions of the Glycocalyx
Defense Against Cancer
Cell Adhesion Fertilization
Cushions the plasma membrane and protects it from physical and chemical injury
Enables the immune system to recognize and selectively attack foreign organisms Changes in the glycocalyx of cancerous cells enable the immune system to recognize and destroy them Forms the basis for compatibility of blood transfusions, tissue grafts, and organ transplants Binds cells together so that tissues do not fall apart Enables sperm to recognize and bind to eggs
Guides embryonic cells to their destinations in the body taste buds and inner ear, they are well developed but serve sensory rather than absorptive functions.
Individual microvilli cannot be distinguished very well with the light microscope because they are only 1 to 2 ^m long. On some cells, they are very dense and appear as a fringe called the brush border at the apical cell surface. With the scanning electron microscope, they resemble a deep-pile carpet. With the transmission electron microscope, microvilli typically look like finger-shaped projections of the cell surface. They show little internal structure, but some have a bundle of stiff filaments of a protein called actin. Actin filaments attach to the inside of the plasma membrane at the tip of the microvillus, and at its base they extend a little way into the cell and anchor the microvillus to a protein mesh called the terminal web. When tugged by another protein in the cytoplasm, actin can shorten a microvillus to "milk" its absorbed contents downward into the cell.
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