Chapter 3 - The Cellular Level of Organization - Figure 3.15 - Page 75: 1

The three most important cytoskeletal elements are microfilaments, intermediate filaments, and microtubules.The largest of these, the microtubules, are essential components of the structure of centrosomes, cilia and flagella Centrosomes are are organelles that are to be found near the nucleus of dividing cells, Each centrosome comprises two centrioles and a mass of pericentriolar proteins of various types. The centrioles are cylindrical structures made up of microtubules arranged in a circle. The number of microtubules per centriole is 20, They are arranged in nine sets of fused pairs( doublets) in a circle around the periphery and one pair of singlets( unfused tubes)) at the center. The microtubules of the centriole, as part of the centrosome, play important roles in cell division, They help to organize dyads on the metaphase plate. They participate in the formation, and function of the spindle which separates chromatids at anaphase. Centrosomes and the centriolar microtubules are essential for cell replication in higher mammals. Cilia are fine filamentous outgrowth from the surface of cells. These may be single (primary cilia) or multiple (secondary. Primary cilia function as sensory detectors ( eg in retina cells and in kidney tubules). Secondary cilta serve to move fluids over cell surfaces. Ciliary microtubules are arranged as nine groups of fused pairs (doublets) in a peripheral circle; in the center of the circle is a non-fused pair of single microtubules (singlets) . This is the well-studied 9+2 pattern. The microtubules of a secondary cilium are anchored in a basal body which, like the centriole of centrosomes, has its microtubules arranged in a circular pattern of nine sets of triplets. Secondary cilia function mainly to move fluids over cell surfaces. They effect this by a rowing or wave-like motion which includes a forward power stroke followed by a flexible recovery stroke. This function is exemplified by the cilia of the respiratory tract which move mucus, with debris and micro-organisms, away from the lungs toward the the mouth to be expectorated.. Microtubules are also an important part of the structure of flagella. The only examples of flagella in humans are the tails of spermatozoa. The axoneme or the central part of the sperm tail has a structure similar to that of a secondary cilium: the sperm tail has nine pairs of fused microtubules ( doublets) with a central pair of unfused singlets-- just as in secondary cilia. In humans, the only function of the sperm tail is to move the cell through the fluid on the walls of the fallopian tubes so that it can meet and fertilize an oocyte. The flagellum moves in a more or less meandering serpentine motion. This propels the cell forward rather than move fluid over the cell. Both microtubular motor proteins(dyneins) and ATP promote this action.

Microtubules are small cylinders of the globular protein tubulin. In centrioles of centrosomes they are associated with several other kinds of peri-centriolar proteins. Some of these proteins form the microtubule organizing center (MTOC). The centrosome comprises the pair of centrioles and the MTOC. These structures produce the mitotic spindle which is very important in mitosis and meiosis. The basal bodies of cilia and flagella are essentially centrioles in their microtubular structure and arrangement -- nine sets of triplets (9+0). The arrangement in the motile cilia and axoneme of flagella is similar(9+2). This organization displays nine pairs of doublets in a circle with two central unfused single tubules (singlets ) in the center of the cilia or flagellum., The movement of cilia and flagella are complex. They are produced by the actions of microtubular motor proteins ( like dyneins) powered by the energy of ATP. While ciliary motion is wave-like and moves fluid over cells, the wiggling , serpentine motion of flagella move cells through fluids.