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Cilia and Centrioles 27 Introduction Ciliary beating is an extensively studied form of cellular movement. Cilia are tiny hairlike appendages about 0.25 µm in diameter with a bundle of microtubules at their core; they extend from the surface of many kinds of cells and are found in most animal species, many protozoa, and some lower plants. The primary function of cilia is to move fluid over the surface of the cell or to propel single cells through a fluid. Protozoa, for example, use cilia both to collect food particles and for locomotion. On the epithelial cells lining the human respiratory tract, huge numbers of cilia (109/cm2 or more) sweep layers of mucus, together with trapped particles of dust and dead cells, up toward the mouth, where they are swallowed and eliminated. Cilia also help to sweep eggs along the oviduct, and a related structure, the flagellum, propels sperm. Cilia Move by the Bending of an Axoneme - a Complex Bundle of Microtubules 27Fields of cilia bend in coordinated unidirectional waves ( Figure 16-39). Each cilium moves with a whiplike motion: a forward active stroke, in which the cilium is fully extended and beating against the surrounding liquid, is followed by a recovery phase, in which the cilium returns to its original position with an unrolling movement that minimizes viscous drag ( Figure16-40A). The cycles of adjacent cilia are almost but not quite in synchrony, creating the wavelike patterns that can be seen in fields of beating cilia under the microscope. The simple flagella of sperm and of many protozoa are much like cilia in their internal structure, but they are usually very much longer. Instead of making whiplike movements, they propagate quasi-sinusoidal waves ( Figure16-40B). Nevertheless, the molecular basis for their movement is the same as that in cilia. It should be noted that the flagella of bacteria (described in Chapter 15) are completely different from the cilia and flagella of eucaryotic cells. The movement of a cilium or a flagellum is produced by the bending of its core, which is called the axoneme. The axoneme is composed entirely of microtubules and their associated proteins. The microtubules are modified and arranged in a pattern whose curious and distinctive appearance was one of the most striking revelations of early electron microscopy: nine special doublet microtubules are arranged in a ring around a pair of single microtubules ( Figure 16-41). This "9 + 2" array is characteristic of almost all forms of cilia and eucaryotic flagella - from those of protozoa to those found in humans. The microtubules extend continuously for the length of the axoneme, which is usually about 10 µ long but may be as long as 200 µ in some cells. While each member of the pair of single microtubules (the central pair) is a complete microtubule, each of the outer doublets is composed of one complete and one partial microtubule fused together so that they share a common tubule wall. In transverse sections each complete microtubule appears to be formed from a ring of 13 subunits, while the incomplete tubule of the outer doublet is formed from only 11. Dynein Drives the Movements of Cilia and Flagella 28The microtubules of an axoneme are associated with numerous proteins, which project at regular positions along the length of the microtubules. Some serve as cross-links that hold the bundle of microtubules together. Others generate the force that drives the bending motion, while still others form a mechanically activated relay system that controls the motion to produce the desired waveform. The most important of these accessory proteins is ciliary dynein, whose heads interact with adjacent microtubules to generate a sliding force between the microtubules. Because of the multiple links that hold adjacent microtubule doublets together, what would be a sliding movement between free microtubules ( Figure 16-42) is converted to a bending motion in the cilium ( Figure 16-43). Like cytoplasmic dynein, ciliary dynein has a motor domain, which hydrolyzes ATP to move along a microtubule toward its minus end, and a tail region that carries a cargo, which in this case is an adjacent microtubule. Ciliary dynein is considerably larger than cytoplasmic dynein, both in the size of its heavy chains and in the number and complexity of its polypeptide chains. In flagella of the unicellular green algae Chlamydomonas, for example, the dynein is composed of either 2 or 3 heavy chains (there are multiple forms of dynein in the flagellum) and 10 or more smaller polypeptides ( Figure16-44). Note that the tail of ciliary dynein binds only to the A tubule and not to the B tubule, which has a slightly different structure. The resulting asymmetry in the arrangement of the dynein molecules is required to prevent a fruitless tug-of-war between neighboring microtubules, which presumably explains why each of the nine outer microtubules is an A-B doublet. Flagella and Cilia Grow from Basal Bodies That Are Closely Related to Centrioles 29If the two flagella of the green alga Chlamydomonas are sheared from the cell, they rapidly re-form by elongating from structures called basal bodies. The basal bodies have the same structure as the centrioles that are found embedded in the center of animal centrosomes. Indeed, in some organisms, basal bodies and centrioles seem to be functionally interconvertible: during each mitosis in Chlamydomonas, for example, the flagella are resorbed and the basal bodies move into the cell interior and become embedded in the spindle poles. Centrioles and basal bodies are cylindrical structures about 0.2 µ wide and 0.4 µ long. Nine groups of three microtubules, fused into triplets, form the wall of the centriole, each triplet being tilted inward like the blades of a turbine ( Figure 16-45). Adjacent triplets are linked at intervals along their length, while faint protein spokes can often be seen in electron micrographs to radiate out to each triplet from a central core, forming a pattern like a cartwheel (see Figure 16-45A). During the formation or regeneration of a cilium, each doublet microtubule of the axoneme grows from two of the microtubules in the triplet microtubules of the basal body so that the ninefold symmetry of the basal body microtubules is preserved in the ciliary axoneme. Autoradiographic evidence suggests that the addition of tubulin and other proteins of the axoneme takes place at the distal tip of the structure, at the plus end of the microtubules. How the central pair of single microtubules forms in the axoneme is not known; there is no central pair in basal bodies or centrioles. It is not known how the length of flagella and cilia is determined. The length is constant for a given species of cell, and it is not limited by the availability of components or the kinetics of elongation. If one of the two flagella is removed in Chlamydomonas, for example, the remaining flagellum begins to shrink while the lost flagellum simultaneously regenerates. Once the shrinking flagellum and the regrowing flagellum reach the same length, they then both grow out together to reach their final characteristic length. This experiment suggests that flagellar length is constantly monitored in some way ( Figure16-46). Centrioles Usually Arise by the Duplication of Preexisting Centrioles 30The otherwise continuous increase in cell mass throughout the animal cell cycle is punctuated by two discrete duplication events: the replication of DNA and the doubling of the centrosome, which usually has a centriole pair at its center. The two centrioles of the pair are positioned at right angles to each other ( Figure16-47). In cultured fibroblasts centriole doubling begins at around the time that DNA synthesis begins: first the two members of a pair separate, and then a daughter centriole is formed perpendicular to each original centriole (see Figure 18-4). An immature centriole contains a ninefold symmetric array of single microtubules; each microtubule then presumably acts as a template for the assembly of the triplet microtubule of mature centrioles. The two centrioles of a pair are not identical: the daughter centriole not only has a distinct orientation but differs also in detailed morphology and function. In many vertebrate cells, for example, one of the two centrioles is distinguished by its ability to nucleate a so-called primary cilium - an isolated nonmotile cilium that has no known function. Parent/daughter differences also exist in basal bodies and can lead to asymmetries in the cytoskeleton. In ciliated protozoa, basal body replication is coordinated with cell division and the stereospecificity of the duplication process is thought to be important for maintaining the orientation of cilia on the cell surface. This was clearly demonstrated in a classic experiment performed in the 1960s on Paramecium, a large protozoan whose surface is covered with rows of motile cilia. Normally, all of the rows are aligned with the same polarity through the coordinated replication of basal bodies, which consistently produce daughter basal bodies with the same orientation relative to the cell surface. The array of cilia growing from these basal bodies enables the cell to swim with great efficiency. By grafting experiments, however, it is possible to disturb this pattern and produce some inverted rows of cilia that beat in the direction opposite to that of their neighbors ( Figure 16-48). Once established, such altered patterns are passed on from parent to daughter Parameciumfor more than 100 generations. This form of heredity has nothing to do with DNA: the modified cells inherit a particular pattern of ciliary rows through the stereospecific replication of their basal bodies. SummaryThe axoneme of a cilium and a eucaryotic flagellum contains a cylindrical bundle of nine outer doublet microtubules. Dynein side arms extend between adjacent microtubule doublets and hydrolyze ATP to generate a sliding force between the doublets. Accessory proteins bundle the ring of microtubule doublets together and convert the sliding force into the bending movement that underlies ciliary beating. The complex structure of the ciliary axoneme forms by the self-assembly of its component proteins and is nucleated by a centriole (basal body), which serves as a template for the distinct 9 + 2 pattern of microtubules that forms the core axoneme. The centriole duplicates in a highly controlled process in which a daughter centriole is nucleated from the side of a mother centriole and grows at right angles to it. Oriented replication of basal bodies underlies the heritable pattern of beating cilia on the surface of ciliated protozoa. |
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