BIOLOGY Volume 1 - A Guide to General Biology - 2004

5. CELLS

5.10. Cell Structures

5.10.7. Microtubules

The Electron microscope has revealed a distinct Structure within the "ground substance" of the Cytoplasm, which was previously thought to be structureless. A network of fine protein filaments has been discovered in all Eukaryotic Cells, collectively forming what is known as the Cytoskeleton. At least Three types of such structures are distinguished: microtubules, microfilaments, and Intermediate filaments. Their Functions are associated with intracellular movement, the ability of cells to maintain their shape, and several other cellular activities such as Endocytosis and Exocytosis. Here, we will focus solely on microtubules.

Microtubules are found in almost all eukaryotic cells (Fig. 5.33). They are hollow, extremely thin, unbranched tubules with a diameter of approximately 24 nm, and walls about 5 nm thick built from helically packed tubulin protein subunits (Fig. 5.34). Fig. 5.30 provides an idea of what microtubules look like in electron micrographs. Microtubules grow from one end by The addition of tubulin subunits. Growth apparently can only begin in the presence of a template; there is reason to believe that this role is played by very small ring-like structures isolated from cells, which have been found to consist of tubulin subunits. In intact cells, centrioles perform the same function, which is why they are sometimes called microtubule-organizing centers (MTOCs). Centrioles are composed of short microtubules.

Microtubules participate in various intracellular processes; some of these will be mentioned here.

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Fig. 5.33. Distribution of microtubules within a Cell. Microtubules radiate from the microtubule-organizing center (MTOC) located near The Nucleus. The MTOC contains a centriole. Microtubules are visible in this micrograph through The Use of fluorescent Antibodies specifically binding to their protein. The Cell shown here is a fibroblast; fibroblasts are typically found in Connective Tissue, where they synthesize Collagen.

Fig. 5.34. Arrangement of tubulin subunits within a microtubule.

Centrioles and Nuclear Division

Centrioles are small, hollow cylinders (0.3–0.5 µm in length and about 0.2 µm in diameter) found as paired structures in almost all animal cells. Each centriole is built from nine triplets of microtubules. At the onset of nuclear division, centrioles replicate, and the two new pairs migrate to the poles of the spindle—the structure across whose equator Chromosomes align before separating. The spindle itself consists of microtubules ("spindle fibers"), during the assembly of which centrioles act as organizing centers. Microtubules regulate the segregation of chromatids or chromosomes. This is achieved through the sliding of microtubules (Ch. 23). Centrioles are absent in higher plant cells, although a spindle still forms during nuclear division. It is possible that these cells contain very small microtubule-organizing centers that cannot be detected even with an electron microscope.

Basal Bodies, Cilia, and Flagella

Cilia and flagella are identical in structure, though flagella are longer than cilia. Both of these Organelles are cellular outgrowths. They move either in a unidirectional manner (ciliary beating) or in a wave-like fashion (flagellar movement). Cilia and flagella serve both for the locomotion of individual cells and for driving fluid along cell surfaces (such as the beating of cilia moving mucus in the respiratory tract). A basal body is always found at the base of every cilium and flagellum. In structure, basal bodies are identical to centrioles, and it is believed that they form through the duplication of centrioles. They likely also act as microtubule-organizing centers, since cilia and flagella exhibit the characteristic "9 + 2" microtubule arrangement (Ch. 18).

In cilia and flagella, movement is accomplished by the sliding of microtubules. These processes are described in more detail in Ch. 18. It should be noted that bacterial flagella have a simpler structure than eukaryotic flagella and lack basal bodies.

Intracellular Transport

Microtubules are also involved in The transport of various cellular organelles, such as the movement of Golgi vesicles toward the forming cell plate (Fig. 5.30). Continuous transport takes place within the cell: Golgi vesicles move, vesicles budding off from the ER are directed toward the Golgi apparatus, and Lysosomes, Mitochondria, and other organelles move about. All of this movement halts if the microtubule system is damaged.

The Cytoskeleton

Microtubules also perform a structural role in cells: these long, tubular, fairly rigid structures form the cell's supporting framework as part of the cytoskeleton. They help determine cell shape during differentiation and maintain the shape of differentiated cells; they are often located in the region immediately adjacent to The Plasma Membrane (Figs. 5.10 and 5.11). Animal cells in which the microtubule system is disrupted assume a spherical shape. In plant cells, the arrangement of microtubules precisely corresponds to the orientation of Cellulose microfibrils deposited during Cell wall formation; thus, microtubules indirectly determine cell shape.