Ultrastructural basis for ciliary motility

The axoneme of a cilium or a sperm tail is a machinery of great complexity. The mechano-chemical work is exerted by the dynein arms and brings about a sliding of the microtubular doublets relative to each other. Nexin links hold the doublets together and restrict the sliding; this makes the cilium bend rather than slide apart. The spokes have several functions and one is to prevent the cilium from becoming buckled when bending. There is a structure called the ciliary necklace in the proximal region of the cilium; this is believed to influence the activity of the cilium by regulating the calcium influx. The two principal pathways, which can be used to explore the ciliary machinery and its many components are (a) "chemical dissection" of the cilium by selectively dissolving the various parts of the machinery and (b) examination of ciliary mutants which can be obtained from experiments with protists or at the investigation of the population attending lung clinics or male fertility clinics as patients. Such human ciliary mutants usually belong to the immotile-cilia syndrome.