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The cytoplasm of all eukaryote cells contains hollow fibrillar structures called microtubules.

Cytoplasmic microtubules show a tendency to form bundles or sheets. The bundles may consist of as few as two or three to several hundred microtubules. Microtubules are essentially rigid structures, although they can bend and form aches. This rigidness enables them to form a structural support (cytoskeleton) in various cell types.

Microtubules in cilia and flagella. The axis of a cilium or flagellum consists of microtubules arranged in a nine-fold radial symmetry, and is called the axoneme. It consists of 11 longitudinal fibrils, 2 central and 9 peripheral. The two central microtubules are similar in structure to cytoplasmic microtubules. Each microtubule is a hollow cylinder composed of 13 longitudinal protofilaments.

Microtubules in centrioles and basal bodies. Centrioles are morphologically identical to the basal body of cilia and flagella. The wall of each centriole consists of 9 peripheral fibrils. Each fibril is made up of 3 subfibres or microtubules (designated as A, B and C). The A subfibre is complete and tubular, while the B and C subfibres are incomplete and appear C-shaped in transverse sections.

Microtubules in nerve cells. Passing along the length of the axons of nerve cells are neurofilaments and neurotubules. Neurotubules are simply microtubules of nerve cells. They consists of 13 parallel filaments, each about 50A in diameter, arranged in a ring to form a 250A tube. They are made up of tubulin. The microtubules provide structural support to the axons and are also believed to function in the fast transport of proteins and other substances down the axon.

Microtubules in axopodia of Protozoa. Microtubules involved in motility are found in axopodia of Protozoa and in sperm tails. In the protozoan Actinospaerium the axopodium is supported by two interlocking coils of microtubules forming several layers and arranged in a 12-fold symmetry.

Microtubules in thrombocytes. In human and rat thrombocytes there appears to be an equatorial ring of 8-24 microtubules situated close to the cell membrane. This is probably made up of one long, coiled microtubule.

Microtubules in the mitotic spindle. The mitotic spindle consists of spindle fibres which are bundles of microtubules. Kinetochore microtubules (KMTs) make up the chromosomal fibres which extend from the kinetochores in the chromsomes to the poles.

Microtubules in sensory cells. Cilia-like structures have been found in sensory cells of the inner ear, the rods and cones of the retina and in ocelli. Many receptor cells contain a cilium on which terminates a nerve fibre. Such cilia have the typical 9 peripheral doublets, but the two central tubules are usually short or absent.

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