Cilia and flagella

FIGURE 17.1 Micrographs and electron micrographs of cytoskeletal elements, cilia, and flagella (a) microtnbnles, (b) rat sperm tail microtnbnles (cross-section),... 

Microtubules Are the Fundamental Structural Units of Cilia and Flagella... 

As already noted, microtubules are also the fundamental building blocks of cilia and flagella. Cilia are short, cylindrical, hairlike projections on the surfaces of the cells of many animals and lower plants. The beating motion of cilia functions either to move cells from place to place or to facilitate the movement of extracellular fluid over the cell surface. Flagella are much longer structures found singly or a few at a time on certain cells (such as sperm cells). They pro-... 

In general, movement is an intrinsic property of living creatures. It occurs at different structural levels, including ion transfer through membranes, separation of replicated chromosomes, beating of cilia and flagella or, the most common, contraction of muscles. These contractions enable... 

When tubulin heterodimers are assembled into microtubules, they form linear protofilaments with the P-tubulin subunit of one tubulin molecule linking covalently with the a-subunit of the next. Direct examination by electron microscopy of tannic acid-treated specimens has shown that micrombules in neurons and the A-microtubules of cilia and flagella have 13 protofilaments arranged side to side to form a cylinder around what appears to be an empty lumen. 

The force-producing MAPs (kinesin, dynein, and dynamin) function as energy-transducing ATPases to provide the motive force for cilia and flagella by means of... 

Parallel arrays of microtubules are found in the axoneme of cilia and flagella of eukaryotic cells, and these are of constant pattern throughout the phylogenetic scale. 

Other lateral structures that are bound to the microtubules of cilia and flagella are filamentous interdoublet links composed of nexin, radial spokes directed to the central pair of microtubules, and a central sheath that is attached to the walls of the central pair (Figure 1). 

Microtubules, an integral component of the cellular cy-toskeleton, consist of cytoplasmic tubes 25 nm in diameter and often of extreme length. Microtubules are necessary for the formation and function of the mitotic spindle and thus are present in all eukaryotic cells. They are also involved in the intracellular movement of endocytic and exocytic vesicles and form the major structural components of cilia and flagella. Microtubules are a major component of axons and dendrites, in which they maintain structure and participate in the axoplasmic flow of material along these neuronal processes. 

An absence of dynein in cilia and flagella results in immotile cilia and flagella, leading to male sterility and chronic respiratory infection, a condition known as Kartagener syndrome. 

Naitoh, Y., Eckert, R. Control of Ciliary Activity in Protozoa, in Cilia and Flagella (Sleigh, M.A., ed.), pp. 305-352. London Academic Press 1974... 

Sleigh, M.A. (ed.) Cilia and Flagella. London Academic Press 1974... 

Microtubule a hollow rod formed from the protein tubulin. Microtubules form part of the cytoskeleton of cells as well as cilia and flagella. 

Figure 1-8 Structure of cilia and flagella of eukaryotes. After P. Satir.5...

In addition to the highly organized arrangement of actin and myosin filaments in muscle (discussed in detail in Squire et al., 2005), there are also highly organized arrays of microtubules in the cilia and flagella of eukaryotes that either pass fluid across the cell surface or act as the propellers for spermatazoa (Fig. IB). Ordered arrays of actin also occur in microvilli on the surfaces of cells from the brush border of intestinal epithelia. Here, the highly cross-linked actin arrays appear to be tensioned by interaction with myosin at the bases of the microvilli in the terminal web (Fig. 2B). 

As the number of systems that are resistant to gradualist explanation mounts, the need for a new kind of explanation grows more apparent. Cilia and flagella are far from the only problems for Darwinism. In the next chapter I will look at the biochemical complexity underlying the apparent simplicity of blood clotting. 

Cilia and flagella are stable microtubule-based structures which project from the plasma membranes of particular eukaryotic cells. The energy-dependent oscillations of these structures can drive material over the surface of a cell or propel the cell along. For example, the whip-like motions of cilia on the cells at the head of the fallopian tube draw newly released ova from the ovaries into and along the oviduct. The snake-like movements of the flagellum on a sperm provide these cells with movement. 

Although the movements of cilia and flagella are somewhat different, the microtubule-based axoneme is common to them both. The axoneme is cylindrical (—200 nm in diameter) and built from the 9 + 2 arrangement of microtubules [Fig. 5-36(6)]. Note that the two central microtubules are complete while each of the nine doublets of outer microtubules contains one complete (A tubule) and one incomplete (B tubule) fused together. 

Each axoneme doublet grows from two of the tubules in each triplet of the basal body. It is not known how the growth of the two central microtubules of the axoneme is controlled, or how the carefully regulated length of the cilia and flagella is monitored. 

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