| dc.description.abstract | Cilia are hairlike cellular organelles that are responsible for generating fluid flow for various physiological purposes in a wide range of eukaryotic species. Many of these systems have thousands of cilia, which display a coordination pattern known as a metachronal wave. Such waves have been reported in a number of species, with a wide range of parameters. To produce this sustained coordination pattern, it is necessary for the multiple cilia of the array to interact. The mechanism of hydrodynamic interaction has received considerable attention, and is sufficient to produce metachronal coordination in theoretical and computational models. However, cilia in many biological systems have been shown to coordinate via other methods, and hydrodynamic models are not yet able to explain the origin of the variety of experimentally observed wave parameters.
Experimental measurements are necessary in order to discover the full range of interaction mechanisms present in these systems, and how they operate to produce large scale, sustained coordination patterns. Previous studies have been performed in unicellular eukaryotes or in arrays of multicilliated vertebrate epitheliate tissue. In this thesis, we study ciliary coordination and dynamics in ciliated marine invertebrate larvae, which are a valuable system of `intermediate' complexity.
We first study the larvae of the ecologically important coral species Acropora millepora, which use cilia primarily for swimming and navigation. We perform experiments at multiple scales, studying the overall swimming behaviour, the detailed ciliary coordination dynamics, and the ciliary response to a photostimulus. We use a simple squirmer model to describe the average propulsion generated by the beating cilia, and investigate how the measured wave parameters may contribute to physiological function.
We then study an early larval stage of Platynereis dumerilii, a marine annelid worm, which we propose as a model system for studying coordination dynamics. The larval ciliary band has a simple geometry, but we show that the metachronal coordination is surprisingly complex. The wave shows both spatial and temporal features, which we link to the underlying biological structure and control of the cilia. We use physical perturbations to investigate the nature of ciliary coupling and find that the primary mechanism is an external physical effect. The coupling is very short range and can be interrupted by gaps in the ciliary band of widths less than one quarter of the cilium length.
We have also used this system to make a novel experimental measurement of the timescale for coordination to emerge in a ciliary array. The establishment of coordination is rapid, and is spatially patterned around the band. Various intrinsic properties of the ciliary band show directional asymmetry, which likely helps to set the consistent observed wave direction. We also study the ability of the larvae to sense and respond to flow. We measure the mechanical properties of a `hybrid' cilium, which acts as a sensory cilium but also shows structural similarities to motile cilia.
Finally, we build an artificial robotic model of a cilium. Our minimal control implementation can be used to implement various coupling mechanisms, and study the resulting coordination.
This work shows how underlying biological complexity generates features in the coordination observed in previously unstudied systems. Using multiscale experimental measurements, various perturbations, and computational and robotic models, we investigate the mechanisms and function of ciliary coordination. | en_GB |
