Structure and function of vertebrate cilia, towards a new taxonomy

In this review, we propose a new classification of vertebrate cilia/flagella and discuss the evolution and prototype of cilia. Cilia/flagella are evolutionarily well-conserved membranous organelles in eukaryotes and serve a variety of functions, including motility and sensation. Vertebrate cilia have been traditionally classified into conventional motile cilia and sensory primary cilia. However, an avalanche of emerging evidence on the variations of cilia has made it almost impossible to classify them in a simple dichotomic manner. For example, conventional motile cilia are also involved in the sensation of bitter taste to facilitate the beating of cilia as a defense system of the respiratory system. On the other hand, the primary cilium, often regarded as a non-motile sensory organelle, has been revealed to be motile in vertebrate embryonic nodes, where they play a crucial role in the determination of left–right asymmetry of the body. Moreover, choroid plexus epithelial cells in the cerebral ventricular system exhibit multiple primary cilia on a single cell. Considering these lines of evidence on the diversity of cilia, we believe the classification of cilia should be based on their structure and function, and include more detailed criteria. Another intriguing issue is how in the evolution of cilia, their function and morphology are combined. For example, has motility been acquired from originally sensory cilia, or vice versa? Alternatively, were they originally hybrid in nature? These questions are inseparable from the classification of cilia per se. We would like to address these conundrums in this review article, principally from the standpoint of differentiation of the animal cell.