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Review
. 2008 Feb;20(1):48-52.
doi: 10.1016/j.ceb.2007.11.009. Epub 2008 Jan 14.

Cilia orientation and the fluid mechanics of development

Wallace F Marshall  1 , Christopher Kintner
Affiliations
Review

Cilia orientation and the fluid mechanics of development

Wallace F Marshall et al. Curr Opin Cell Biol. 2008 Feb.

Abstract

Motile cilia produce large-scale fluid flows crucial for development and physiology. Defects in ciliary motility cause a range of disease symptoms including bronchiectasis, hydrocephalus, and situs inversus. However, it is not enough for cilia to be motile and generate a flow -- the flow must be driven in the proper direction. Generation of properly directed coherent flow requires that the cilia are properly oriented relative to tissue axes. Genetic, molecular, and ultrastructural studies have begun to suggest pathways linking cilia orientation to planar cell polarity (PCP) and other long-range positional cues and also suggest that cilia-driven flow can itself play a causal role in orienting the cilia that create it. Errors in cilia orientation have been observed in human ciliary disease patients, suggesting that orientation defects may constitute a novel class of ciliopathies with a distinct etiology at the cell biological level.

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Figures

Figure 1
Figure 1
Degrees of freedom of ciliary orientation. Cilia orientation is specified by three coordinates of position (X,Y, and Z), two angles of tilt (θ and ϕ) defined relative to the plane of the epithelium, and one angle of rotation (ψ) about the long axis of the cilium.
Figure 2
Figure 2
Ciliary orientation must be aligned across multiple cells in a tissue in order to produce coherent overall flow of fluid. (A) when cilia are aligned, locally produced flows (red arrows) can sum to produce a coherent flow. (B) when cilia are randomly aligned, local flows run in different directions leading to overall chaotic motion of the fluid that resembles turbulence.
Figure 3
Figure 3
Cilia orientation is acquired gradually via several processes. When cilia first form, the orientation of the basal body (arrows) is very imprecise (top diagram). Tissue patterning perhaps involving PCP signaling helps to align the orientation of cilia along a common tissue axis. Ciliary flow is then used as a self-organizing cue to refine orientation further, thus producing laminar, directed flow.
See this image and copyright information in PMC

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