Centrioles, Cilia, Flagella, and Microtubules

Many cells contain centrioles, little cylinders about 0.15 µm in diameter and 0.5 µm long, which are not enclosed by membranes. Each centriole contains a series of fine microtubules of 25 nm diameter. A pair of centrioles are present near the nucleus in most animal cells and play an important role in cell division. Together with surrounding materials they form the centrosome. However, centrioles have never been
observed in plant cells. Related in structure to centrioles are the long flagella and shorter cilia (the two words are virtually synonymous) which are commonly present as organ- elles of locomotion in eukaryotic cells. Stationary cells of our own bodies also often have cilia. For example, there are 109 cilia/cm2 in bronchial epithelium. Modified flagella form the receptors of light in our eyes and of taste in our tongues. Flagella and cilia have a diameter of about 0.2 µm and a characteristic internal structure. Eleven hollow microtubules of ~24 nm diameter are usually arranged in a “9 + 2” pattern with nine pairs of fused tubules surrounding a pair of single tubules. Each microtubule resembles a bacterial flagellum in appearance, but there are distinct and significant chemical differences. The basal body of the flagellum, the kinetosome, resembles a centriole in structure, dimensions, and mode of replication. Recently a small 6–9 megabase pair DNA has been found in basal bodies of the protozoan Chlamydomonas. Microtubules similar to those found in flagella are also present in the cytoplasm. Together with thinner microfilaments of several kinds they form an internal
cytoskeleton that provides rigidity to cells. Microtubules also form the “spindle” of dividing cells. In nerve axons (Chapter 30) the microtubules run parallel to the length of the axons and are part of a mechanical transport system for cell constituents.

Structure of cilia and flagella of eukaryotes. After P. Satir