Cilia, Flagella and Centrioles

Cilia and flagella are specialised, hair-like or whip-like extensions of the cell surface, found projecting outward from the plasma membrane of many eukaryotic cells, and both are chiefly associated with movement -- either propelling the cell itself through a fluid medium, or moving fluid and particles across the surface of a stationary cell.

Structurally, cilia and flagella are built on the same underlying plan and differ mainly in their length, their number per cell, and the pattern in which they move. Cilia are typically short and are present in large numbers over the cell surface; they beat with a rhythmic, coordinated, oar-like stroke, often in synchronised waves that sweep fluid or particles in a particular direction across the cell surface, as in the cells lining the human respiratory tract, which use coordinated ciliary beating to move mucus and trapped particles upward and out of the airway. Flagella, by contrast, are considerably longer and generally occur singly or in small numbers on a cell; rather than beating like an oar, a flagellum moves with an undulating, wave-like motion that propels the cell forward through its fluid environment, as seen in the tail of a sperm cell or in many free-swimming protists and algae.

Despite these differences in length, number and pattern of beating, the internal structure of both cilia and flagella is remarkably uniform, built around a core structure called the axoneme, itself covered by an extension of the cell's own plasma membrane. The axoneme is built from microtubules arranged in a highly characteristic and consistent pattern universally referred to as the "9 + 2" arrangement: nine pairs (doublets) of microtubules are arranged evenly around the circumference of the axoneme, enclosing, at its very centre, a single further pair of individual (singlet) microtubules, known as the central pair. The nine peripheral doublets are linked to one another, and to the central pair, by a system of protein cross-links, including radial spokes and dynein arms; it is the coordinated, ATP-powered sliding of adjacent doublets against one another, driven by the motor protein dynein, that produces the bending movement characteristic of ciliary and flagellar beating.

At the base of every cilium and flagellum, embedded within the cytoplasm just beneath the plasma membrane, lies a short, cylindrical structure called the basal body, from which the axoneme develops and grows. The basal body is structurally very similar to -- and, in most cells, is in fact derived from -- a centriole, though it characteristically shows a "9 + 0" arrangement of microtubules (nine peripheral triplets, with no central pair), distinguishing it from the "9 + 2" arrangement of the axoneme it gives rise to. …