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Histology: Epithelial Tissues & Glands

Terms in this set (65)

(a) A diagram of a cilium with the axoneme consisting of two central microtubules surrounded by nine peripheral microtubular doublets associated with other proteins. In the doublets, microtubule A is complete, consisting of 13 protofilaments, whereas microtubule B shares some of A's protofilament heterodimers. The axoneme is elastic but relatively stiff, with its structure maintained by nexins linking the peripheral doublets and other protein complexes forming a sheath and radial spokes between the doublets and the central microtubules.
The axoneme is continuous with a basal body located in the apical cytoplasm. Basal bodies are structurally very similar to centrioles, consisting of nine relatively short microtubular triplets linked together in a pinwheel-like arrangement. A dynamic pool of tubulin and other proteins exists distally in cilia, and proteins are transported into and out of the structure by kinesin and cytoplasmic dynein motors moving along the peripheral doublets of microtubules.
(b) Ciliary movement involves a rapid series of changes in the shape of the axoneme. Along the length of each doublet, a series of paired "arms" with axonemal dynein is bound to microtubule A, with each pair extended toward microtubule B of the next doublet. When activated by ATP, the dynein arms briefly bind the neighboring microtubule and the doublets slide past each other slightly. The sliding motion is restricted by nexin cross-links between the doublets, causing the axoneme to bend. A rapid succession of this movement along the axoneme produces ciliary motion.