Transition zone migration: A mechanism for cytoplasmic ciliogenesis and postaxonemal centriole elongation

Abstract

The cilium is an elongated and continuous structure that spans two major subcellular domains. The cytoplasmic domain contains a short centriole, which serves to nucleate the mainprojection of the cilium.This projection,knownastheaxoneme,remains separated from the cytoplasm by a specialized gatekeeping complex within a ciliary subdomain called the transition zone. In this way, the axoneme is compartmentalized. Intriguingly, however, this general principle of cilium biology is altered in thespermcells ofmanyanimals,which instead contain a cytoplasmic axoneme domain. Here, we discuss the hypothesis that the formation of specialized sperm giant centrioles and cytoplasmic cilia is mediated by the migration of the transition zone from its typical location as part of a structure known as the annulus and examinethe intrinsic properties of the transitionzonethatmayfacilitate its migratorybehavior.