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Many microtubules in cells are stabilized through their association with other proteins and therefore do not show dynamic instability. Cells use such stable microtubules as stiff supports in the construction of a variety of polarized structures, including motile cilia and flagella.

Cilia are hairlike structures about 0.25 μm in diameter, covered by plasma membrane, that extend from the surface of many kinds of eukaryotic cells. Each cilium contains a core of stable microtubules, arranged in a bundle, that grow from a cytoplasmic basal body, which serves as an organizing center.

Motile cilia beat in a whiplike fashion, either to move fluid over the surface of a cell or to propel single cells through a fluid. Some protozoa, for example, use cilia to collect food particles, and others use them for locomotion. On the epithelial cells lining the human respiratory tract , huge numbers of beating cilia (more than a billion per square centimeter) sweep layers of mucus containing trapped dust particles and dead cells up toward the throat, to be swallowed and eventually eliminated from the body.

Similarly, beating cilia on the cells of the oviduct wall create a current that helps to carry eggs away from the ovary. Each cilium acts as a small oar, moving in a repeated cycle that generates the movement of fluid over the cell surface.

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