The complexity of the cilium: spatiotemporal diversity of an ancient organelle.

Cilia are microtubule-based appendages present on almost all vertebrate cell types where they mediate a myriad of cellular processes critical for development and homeostasis. In humans, impaired ciliary function is associated with an ever-expanding repertoire of phenotypically-overlapping yet highly variable genetic disorders, the ciliopathies. Extensive work to elucidate the structure, function, and composition of the cilium is offering hints that the `static' representation of the cilium is a gross oversimplification of a highly dynamic organelle whose functions are choreographed dynamically across cell types, developmental, and homeostatic contexts. Understanding this diversity will require discerning ciliary versus non-ciliary roles for classically-defined `ciliary' proteins; defining ciliary protein-protein interaction networks within and beyond the cilium; and resolving the spatiotemporal diversity of ciliary structure and function. Here, focusing on one evolutionarily conserved ciliary module, the intraflagellar transport system, we explore these ideas and propose potential future studies that will improve our knowledge gaps of the oversimplified cilium and, by extension, inform the reasons that underscore the striking range of clinical pathologies associated with ciliary dysfunction.