Cilia density and flow velocity affect alignment of motile cilia from brain cells

Abstract

In many organs, thousands of microscopic ‘motile cilia’ beat in a coordinated fashion generating fluid flow. Physiologically, these flows are important in both development and homeostasis of ciliated tissues. Combining experiments and simulations, we studied how cilia from brain tissue align their beating direction. We subjected cilia to a broad range of shear stresses, similar to the fluid flow that cilia themselves generate, in a microfluidic setup. In contrast to previous studies, we found that cilia from mouse ependyma respond and align to these physiological shear stress at all maturation stages. Cilia align more easily earlier in maturation, and we correlated this property with the increase in multiciliated cell density during maturation. Our numerical simulations show that cilia in densely packed clusters are hydrodynamically screened from the external flow, in agreement with our experimental observation. Cilia carpets create a hydrodynamic screening that reduces the susceptibility of individual cilia to external flows.

Formation and function of multiciliated cells

“…In Xenopus and mouse MCCs, the external fluid flow can redirect the BB orientation (Guirao et al, 2010;Mitchell et al, 2007). The alignment of BB can be more easily achieved in immature mouse ependymal MCCs at early stages than those at the later fully matured stage (Pellicciotta et al, 2020). Thus, the intracellular BB orientation requires proper cilia motility and external fluid flow, although the corresponding mechanism remains unclear.…”

Flow Physics Explains Morphological Diversity of Ciliated Organs

“…Existing studies have focused on exposed ciliated surfaces and externally ciliated organisms because it is difficult to measure ciliary beat and fluid flow in intact internal ducts. Cilia oscillations, metachronal coordination, and microscale flows have been observed in microdissected ex-vivo epithelia [13], engineered in-vitro tissues [14, 15], and, more recently, in organ-on-chip models [16, 17]. Additionally, leveraging the remarkable conservation of the ultrastructure of motile cilia among eukaryotes, diverse protist and animal model systems have emerged for probing the functional spectrum of cilia, from locomotion and feeding [18, 19] to symbiotic host-microbe partnerships [20, 21].…”

“…Additionally, leveraging the remarkable conservation of the ultrastructure of motile cilia among eukaryotes, diverse protist and animal model systems have emerged for probing the functional spectrum of cilia, from locomotion and feeding [18,19] to symbiotic host-microbe partnerships [20,21]. Current consensus from experimental and theoretical evidence is that coordinated metachronal waves are prominent features of ciliary carpets [14,[22][23][24], emerge spontaneously through hydrodynamic and substrate interactions [24][25][26][27][28] and dominate mechanisms of directional fluid transport [24,[29][30][31][32]. Ciliary carpets with no coordinated waves appear to be the exception and have been associated with fluid mixing as opposed to directional fluid pumping [4,20].…”

Coordination of Cilia Movements in Multi-Ciliated Cells

“…The more ependymal cells mature, the greater the density of cilia. As a result, each cilium is sheltered from the fluid, thereby weakening the effect of fluid on cilia [ 66 , 67 ]. Of note, mature ependymal cells do not completely lose their responsiveness to this flow.…”