What are those cilia doing in the neural tube?

Since being functionally linked to the Sonic Hedgehog (Shh) signaling pathway, primary cilia have sparked enormous interest. The initial connection came from an unbiased forward genetic mouse screen in which a number of the mutations disrupted genes important for cilia; the resulting mutant embryos showed abnormal patterning of the neural tube [1,2]. Shh signaling controls neural tube patterning [3,4], and double mutant analysis showed cilia are critical for Shh signal transduction [1,2]. Previously, the focus of most research was on the function of motile cilia, which have a 9？+？2 microtubule structure, but the past decade has witnessed an explosion of interest in primary cilia, which lack the inner doublet and instead have a 9？+？0 axonemal arrangement. These cilia have now been implicated in many biological processes, from obesity to cancer to learning and memory [5-8]. Indeed, the widespread role of cilia in various systems is made clear by the range of phenotypes present in the ciliopathies, which are human diseases that arise from mutations in cilia genes [9-13]. In this review, we return to the beginning and the source of all the excitement - the embryonic neural tube.Core progress has been made towards understanding the mechanistic details behind the abnormal neural patterning of mouse mutants with disrupted cilia [1,2,14,15], but other roles for cilia in the neural tube have yet to be explored. Proper positioning of cilia in several developmental contexts is linked to the planar cell polarity (PCP) pathway, raising the possibility that the placement of cilia in the neural tube may be critical [16-21]. Additionally, specialized ependymal cilia control circulation and mechanosensation of cerebral spinal fluid (CSF) in the ventricles of the brain [22-25], which is derived from the anterior neural tube. Taken together, these data give us a glimpse of what the cilia in the neural tube are really doing there.Following the specification of the germ layers, the neur