A cilium, plural cilia, is a finger-like projection from the surface of specially adapted cells. They are found in large numbers on epithelial cells of the respiratory tract and female genital tract where they function to move particles along the surface of the cell.
In cross-section, a cilium is membrane-bound and contains a cylinder composed of 9 pairs of microtubules; each pair consists of 2 microtubules which are fused. At the centre of this cylinder are 2 microtubules which do not associate with each other. The 9+2 arrangement of microtubules runs from the tip of the cilium to the base. At the base, only the outer microtubules join a modified centriole termed the basal body. The basal body governs the formation of cilia early in the cell's life - centrioles divide and provide an organizing centre for microtubule assembly beneath the cell membrane.
One of each member of a pair has bridging processes to an adjacent pair. These processes contain a protein, dynein, and ATPase activity. ATP breakdown at the site results in one microtubule being displaced relative to its neighbours. The overall effect for the cilium of numerous such interactions per second is a wafting motion that is cyclical. The cycle has a phase in which the cilium is relatively straight, and pushes surface matter along, followed by a phase of being bent during return to its start position, when surface matter is not propelled.
By local membrane interactions, the cycle of wafting is slightly out of phase between adjacent cilia in one plane along the cell surface. This results in a co-ordinated sweeping motion that is more efficient at directing particles, such as mucus in the respiratory tree, along the cell surface.
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