Ciliary Beating Compartmentalizes Cerebrospinal Fluid Flow in the Brain and Regulates Ventricular Development

Abstract

SummaryMotile cilia are miniature, propeller-like extensions, emanating from many cell types across the body. Their coordinated beating generates a directional fluid flow, which is essential for various biological processes, from respiration to reproduction. In the nervous system, ependymal cells extend their motile cilia into the brain ventricles and contribute to cerebrospinal fluid (CSF) flow. Although motile cilia are not the only contributors to CSF flow, their functioning is crucial, as patients with motile cilia defects develop clinical features, like hydrocephalus and scoliosis. CSF flow was suggested to primarily deliver nutrients and remove waste, but recent studies emphasized its role in brain development and function. Nevertheless, it remains poorly understood how ciliary beating generates and organizes CSF flow to fulfill these roles. Here, we study motile cilia and CSF flow in the brain ventricles of larval zebrafish. We identified that different populations of motile ciliated cells are spatially organized and generate a directional CSF flow powered by ciliary beating. Our investigations revealed that CSF flow is confined within individual ventricular cavities, with little exchange of fluid between ventricles, despite a pulsatile CSF displacement caused by the heartbeat. Interestingly, our results showed that the ventricular boundaries supporting this compartmentalized CSF flow are abolished during bodily movement, highlighting that multiple physiological processes regulate the hydrodynamics of CSF flow. Finally, we showed that perturbing cilia reduces hydrodynamic coupling between the brain ventricles and disrupts ventricular development. We propose that motile-cilia-generated flow is crucial in regulating the distribution of CSF within and across brain ventricles.

Origin of the bidirectionality of cerebrospinal fluid flow and impact on long-range transport between brain and spinal cord

“…The caudal tilt is consistent with previous reports 39,42 . The measured frequencies are higher than the 1215 Hz previously reported in zebrafish central canal 17 (see Methods explaining the discrepancies), but are consistent with beating frequencies of ependymal cilia located in the brain ventricles of rats 43 and zebrafish larvae 20,43 .…”

“…Historically, it has been generally assumed that CSF flow along the walls of all cavities must be driven by motile cilia. In the brain ventricles, the direction of beating cilia correlates indeed with the direction of CSF flow 20,24 . However, in the long cylinder of the central canal, the dynamics of CSF flow is strikingly different than in the brain ventricles.…”

“…Ciliadriven flows can also be observed in the pronephros 16,17 , in the respiratory tract 18 , and in the nasal cavity 19 . The motility of cilia in brain ventricles is also correlated with the cerebrospinal fluid (CSF) flow in zebrafish 17,20,21 , African clawed frog 22,23 and rodents 24 .…”

“…Interestingly, bodyaxis defects observed in mutants with defective cilia appear by 30 hpf, i.e. few hours before cilia paving the walls of the brain ventricles become motile and a directed CSF flow emerges in the brain ventricles 20,40 . It suggests that proper dynamics of CSF flow in the central canal, not in the brain ventricles, is critical for embryonic body axis formation.…”

Brain Ventricular System and Cerebrospinal Fluid Development and Function: Light at the End of the Tube

“…In the brain ventricular system, ependymal cells and choroid plexus epithelial cells are multi‐ciliated. The motile cilia beat has been popularly considered a mechanism by which CSF flows . However, in zebrafish, slow directed CSF flow is present before multi‐cilated cells are present, indicating that cilia do not drive early embryonic flow .…”