En-Face Preparations of the Medial Hypothalamus and Cilia-Driven Flow Across its Ventricular Wall

Abstract

The posterior basal third ventricle is situated in the midline, between the cerebral hemispheres, adjacent to hypothalamic nuclei pertinent to metabolic homeostasis. The neuroepithelium lining the ventricle in this region is composed of ependymal cells and tanycytes. Ependymal (E1) cells extend multiple motile cilia from their apical surface, while tanycytes extend two motile cilia (E2) or a single non-motile cilium (E3) from their apical surface. This unique arrangement of neuronal nuclei lined by a mixed neuroepithelium in contact with the ventricle is suggested to provide neurons in this region with privileged access to signaling factors and other molecules in the cerebrospinal fluid (CSF). However, CSF flow in this region has not been studied. Here, we characterized motile cilia-generated flow along the walls of the posterior basal third ventricle and found an unexpected asymmetry between the left and right sides. Flow generated by E1 cells on the left hemisphere was directed anterior and ventral, away from the tanycyte domain, while flow on the right hemisphere was directed posterior and ventral, towards the tanycyte domain. To explore cellular planar polarity—that is, polarity orthogonal to the apical-basal axis of an epithelium—that may underlie this difference in flow between the two hemispheres, we quantified “translational” planar polarity in E1 cells of this region. Interestingly, E1 translational polarity reflected the flow of CSF across the epithelium in the right hemisphere, but not in the left hemisphere. Altogether, this work provides a framework for understanding how CSF flow in this ventricular region regulates the exposure of CSF-circulating factors to the adjacent brain.