纤毛是从真核细胞表面延伸出来的被膜覆盖的细胞器。纤毛可以是运动的，如呼吸纤毛，也可以是不运动的(如初生纤毛)，并根据其微管为基础的轴突的结构来区分。轴突由9个外周双线微管组成，在许多运动纤毛的情况下，也可能包含一对中央的单微管。它们分别被称为9+0或9+2轴突。相对于它们的非运动纤毛，运动纤毛还包含有助于运动的额外结构，包括内外动力蛋白臂、径向辐条和连接蛋白链接。人类纤毛有四种主要类型:9+2运动性纤毛(如呼吸纤毛)、9+0运动性纤毛(节状纤毛)、9+2非运动性纤毛(毛细胞的动纤毛)和9+0非运动性纤毛(初生纤毛和感光细胞)(Fliegauf et al .， 2007)。这条途径描述纤毛的形成，重点是初生纤毛。初生纤毛是一种感觉细胞器，需要传递大量的外部信号，如生长因子、激素和形态因子，完整的初生纤毛是Hh、WNT、钙、g蛋白偶联受体和受体酪氨酸激酶介导的信号通路所需要的。(Goetz and Anderson, 2010;Berbari等人，2009; Nachury, 2014). Unlike the motile cilia, which are generally present in large numbers on epithelial cells and are responsible for sensory function as well as wave-like beating motions, the primary cilium is a non-motile sensory organelle that is present in a single copy at the apical surface of most quiescent cells (reviewed in Hsiao et al, 2012). Cilium biogenesis involves the anchoring of the basal body, a centriole-derived organelle, near the plasma membrane and the subsequent polymerization of the microtubule-based axoneme and extension of the plasma membrane (reviewed in Ishikawa and Marshall, 2011; Reiter et al, 2012). Although the ciliary membrane is continuous with the plasma membrane, the protein and lipid content of the cilium and the ciliary membrane are distinct from those of the bulk cytoplasm and plasma membrane (reviewed in Emmer et al, 2010; Rohatgi and Snell, 2010). This specialized compartment is established and maintained during cilium biogenesis by the formation of a ciliary transition zone, a proteinaceous structure that, with the transition fibres, anchors the basal body to the plasma membrane and acts as a ciliary pore to limit free diffusion from the cytosol to the cilium (reviewed in Nachury et al, 2010; Reiter et al, 2012). Ciliary components are targeted from the secretory system to the ciliary base and subsequently transported to the ciliary tip, where extension of the axoneme occurs, by a motor-driven process called intraflagellar transport (IFT). Anterograde transport of cargo from the ciliary base to the tip of the cilium requires kinesin-2 type motors, while the dynein-2 motor is required for retrograde transport back to the ciliary base. In addition, both anterograde and retrograde transport depend on the IFT complex, a multiprotein assembly consisting of two subcomplexes, IFT A and IFT B. The primary cilium is a dynamic structure that undergoes continuous steady-state turnover of tubulin at the tip; as a consequence, the IFT machinery is required for cilium maintenance as well as biogenesis (reviewed in Bhogaraju et al, 2013; Hsiao et al, 2012; Li et al, 2012; Taschner et al, 2012; Sung and Leroux, 2013). The importance of the cilium in signaling and cell biology is highlighted by the wide range of defects and disorders, collectively known as ciliopathies, that arise as the result of mutations in genes encoding components of the ciliary machinery (reviewed in Goetz and Anderson, 2010; Madhivanan and Aguilar, 2014).