Cilia are tiny hairlike structures that project from the surface of a cell and, like the coordinated action of rowers in a boat, stroke in unison. Cilia in the human body are found on the apical surface (the surface facing the lumen, or cavity) of stationary epithelial cells in the respiratory and female reproductive tracts. In the respiratory system, the cilia transport strands of mucus to the pharynx (throat), where the mucus can either be swallowed or expectorated. In the female reproductive tract, ciliary movements in the epithelial lining of the uterine tube draw the ovum (egg) into the tube and move it toward the uterus.
Fox: Human Physiology, I 3. Cell Structure and I Text I I © The McGraw-Hill
Eighth Edition Genetic Control Companies, 2003
Cell Structure and Genetic Control 55
■ Figure 3.5 Electron micrographs of cilia. The cilia can be seen in (a) a scanning electron micrograph and (b) cross sections in a transmission electron micrograph. Notice the characteristic "9 + 2" arrangement of microtubules in the cross sections.
Sperm cells are the only cells in the human body that have flagella. The flagellum is a single whiplike structure that propels the sperm cell through its environment. Both cilia and flagella are composed of microtubules (thin cylinders formed from proteins) arranged in a characteristic way. One pair of micro-tubules in the center of a cilium or flagellum is surrounded by nine other pairs of microtubules, to produce what is often described as a "9 + 2" arrangement (fig. 3.5).
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