Heterogeneity in ciliary structure. (A) Nodal cilia with either a 9+0 or 9+2 microtubule arrangement (inset), (B) primary cilia with a 9+0 microtubule arrangement (inset) and (C) airway cilia with a 9+2 microtubule structure (inset). (D) The detailed structure of a primary cilia, schematizing four main components: the basal body, transition zone, axoneme, and ciliary membrane.