Introduction
In 1675, Antony Van Leeuwenhoek described a living protozoan "with divers incredibly thin feet, or little legs, which were moved very nimbly" [1]. Today, these "thin feet, or little legs" are referred to as cilia. Due to Van Leeuwenhoek’s report, cilia are the oldest known cell organelles. They represent tiny cytoplasmic protrusions with a length of 1-15 μm. A distinction can be drawn between motile and immotile cilia. By their beating, motile cilia act as the driving force of several dynamic processes. Apart from enabling protozoa to move, they generate the nodal flow, create the movement of sperms and transport mucus through the trachea or oocytes through the Fallopian tube [2]. For a long time, immotile cilia were considered to be non-functional vestiges originally derived from motile cilia [3]. In the 1970s, a sensory function of immotile cilia which are also known as primary cilia was suggested. Around the turn of the millennium, several studies confirmed this suggestion [4]. In addition to the sensation of external stimuli such as fluid flow, light or odors, primary cilia mediate numerous signalling pathways which are essential for vertebrate development and homeostasis, such as hedgehog (HH) signalling, Wingless-Int (WNT) signalling, platelet-derived growth factor receptor-α (PDGFRα) signalling and transforming growth factor-β (TGF-β) signalling [5]. The dysfunction of cilia results in severe, often deadly human diseases collectively termed ciliopathies. According to recent estimates, ciliopathies affect between 1:700 and 1:2000 people in the general population worldwide [6]. Importantly, faulty primary cilia cause more varied sensory, physiological and developmental abnormalities than defective motile cilia [7]. In the past, ciliopathies were considered to be rare diseases comprising disorders such as polycystic kidney disease, Meckel–Gruber syndrome, Joubert syndrome, Bardet–Biedl syndrome, Leber congenital amaurosis, Senior–Løken syndrome, orofaciodigital syndrome type 1, Alström syndrome, Jeune asphyxiating thoracic dystrophy, Ellis–van Creveld syndrome and Sensenbrenner syndrome. But since the number of diseases identified as or assumed to be ciliopathies is permanently rising, this point of view has changed. Even common diseases such as cancer or neurodegenerative diseases are associated to primary cilia [5, 8].
In the main, primary cilia are composed of three different compartments – the axoneme, the basal body (BB) and the transition zone (TZ) (Figure 1).
