**2.1 Cilium structure**

The cilia are organelles extending from the eukaryotic cell surface, similar to hair. The cilia are short and flagella are long, yet they share the same structure in all eukaryotes [5,6]. The primary cilium emerges from one of the centrioles, a *basal body* (a modified centriole) formed by 9 triplets of microtubules. The basal body is the microtubule organizing center derived from the oldest of the pair of centrioles and controls the movement of cilia and flagella. During interphase the distal end of the older centriole in the centrosome attaches to and becomes enclosed by a membrane vesicle. The microtubule core of the cilium (the axoneme) then assembles directly onto the microtubules of the centriole. As the axoneme lengthens, the primary ciliary vesicle enlarges and becomes a sheath. With time, the sheath (ciliary membrane) fuses with the cytoplasmic membrane and the primary cilium protrudes from the cell surface.

Microtubules are double and parallel (A complete, B incomplete), and are linked by nexin bridges. Some cilia also contain a pair of central single microtubules, called the central pair, involved in regulating motility, surrounded by a sheath and linked by bridges. Thus the types of cilia are classified as follows (TABLE 1):


Defects in any of these structures can lead to ciliary diseases, and symptoms depend on the target structure.

Ciliary motility is accomplished by dynein motor activity, which slides the microtubule doublets relative to one another. At its base, the ciliary axoneme extends from the nine triplet microtubules of the basal body. Motility is provided by two sets of dynein arms, internal and external, emerging from a doublet of the *A microtubule* and going to the *B microtubule* of the following doublet. It is regulated by a 9-spoke (radial spoke) interacting with the central pair from the *A microtubule*. [7].

Hundreds of proteins are required for cilia formation and function, which is essential for Hh signaling in mammals, therefore these proteins could influence the Hh signal transduction. If this is proven, the challenge will be to understand the physical relationships between components and the Hh pathway that allow efficient ciliary Hh signaling [8, 9]. Ciliary abnormalities fall into four major categories: 1) cilia with abnormal axial microtubules; 2) compound cilia; 3) swollen cilia; and 4) cilia with dynein arm defects.
