The primary cilium, which was first so named by Sergei Sorokin (Sorokin, 1968), is a solitary organelle that emanates from the cell surface of most mammalian cell types during growth arrest. Increasing evidence suggests that primary cilia are key coordinators of signaling pathways during development and in tissue homeostasis and, when defective, are a major cause of human diseases and developmental disorders (now commonly referred to as ciliopathies).

Primary cilia consist of an axoneme of nine doublet microtubules that extends from a basal body [which is derived from the older (mother) centriole of the centrosome, surrounded by the ciliary membrane (a specialized domain extension of the cell membrane)]. The microtubule pattern of the ciliary axoneme is traditionally abbreviated by referring to the numbers of peripheral doublets and single central microtubules as 9+ 2, 9+ 0 etc. In contrast to those of motile 9+ 2 cilia, axonemes of non-motile primary cilia lack key elements involved in ciliary motility, including the central pair of microtubules and the proteins that surround them, most if not all radial spokes and, importantly, outer and inner dynein arms that power microtubule sliding to produce motility (Satir and Christensen, 2007). Single 9+ 0 primary cilia are found on a large number of cells in the mammalian body, including stem, epithelial, endothelial, connective-tissue and muscle cells as well as neurons [for a more detailed list, please see Wheatley (Wheatley, 1982) and the website http://www. bowserlab. org/primarycilia/cilia3. htm]. The 9+ 0 pattern of the primary cilium is often lost towards the cilium tip, where doublet microtubules end or change position.