Ciliary structure in health and disease.

The architecture of the cilium seemingly is very simple but the appearance is deceptive; its 'engine' actually is quite complex. There is a circle of nine microtubular doublets around two central singlet microtubules, with dynein arms and spokes extending from the doublets, and nexin links joining them. The inner microtubules are partly surrounded by a central sheath. Many uncertainties remain, for instance the role of the spokes, the nexin links and of the various categories of inner dynein arms. The complexity as seen today is largely the result of new methodologies being introduced, such as the deep-etch technique or image processing using various computer programs. The inborn disease named immotile-cilia syndrome is characterized by the cilia being defective. It is a highly heterogeneous disease in that more than a dozen subgroups characterized by different ciliary defects have been recognized.