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

ABSTRACTIn 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.

Entrainment of mammalian motile cilia in the brain with hydrodynamic forces

Motile cilia are widespread across the animal and plant kingdoms, displaying complex collective dynamics central to their physiology. Their coordination mechanism is not generally understood, with previous work mainly focusing on algae and protists. We study here the entrainment of cilia beat in multiciliated cells from brain ventricles. The response to controlled oscillatory external flows shows that flows at a similar frequency to the actively beating cilia can entrain cilia oscillations. We find that the hydrodynamic forces required for this entrainment strongly depend on the number of cilia per cell. Cells with few cilia (up to five) can be entrained at flows comparable to cilia-driven flows, in contrast with what was recently observed in Chlamydomonas. Experimental trends are quantitatively described by a model that accounts for hydrodynamic screening of packed cilia and the chemomechanical energy efficiency of the flagellar beat. Simulations of a minimal model of cilia interacting hydrodynamically show the same trends observed in cilia.

Hydrodynamics and Semi-Flexible Filament Behavior

In this paper we simulate the effect of hydrodynamic interaction on the Brownian dynamics of semiflexible filaments. Semiflexible filaments are those whose entropy-driven bending fluctuations are resisted by the elastic bending stiffness. Semiflexible filaments make up the structural scaffold of cell and tissue matrix, and understanding their dynamic behavior is necessary for studying force transmission and remodeling in cells and tissue matrix. Hydrodynamic interaction refers to force on filament mediated through the local solvent flow around it. The local solvent flow is induced by the motion of the filament itself. Dynamic studies of semiflexible filaments tend to assume a uniform friction coefficient at every point on the filament. However Lagamarsino et al [1] showed that even for a filament in uniform translation, most of the drag is localized at the filament ends, which increases the tendency of the filament to bend and rotate even under a uniform driving force. In this paper we explore how the combined effect of non-uniform friction coefficient due to hydrodynamic screening and the non-uniform local solvent flow due to the filament fluctuations affects its Brownian dynamics.