Motile cilia and left-right symmetry breaking: from images to biological insights

In vertebrate embryos, cilia-driven fluid flows generated within the left-right organizer (LRO) are guiding the establishment of the left-right body asymmetry. To study such complex and dynamic biomechanical process and to investigate the generation and sensing of biological flows, it is required to quantify biophysical features of motile cilia in 3D and in vivo. In the zebrafish embryo, the LRO is called the Kupffer?s vesicle and is a spheroid shape cavity, which is covered with motile cilia oriented in all directions of space. This dynamic and transient structure varies in size and shape during development and from one embryo to the other. In addition, micrometric size cilia are beating too fast and are located too deep inside the embryo to be able to image their 3D motion pattern using fluorescence microscopy. As a consequence, the experimental investigation of motile cilia properties is challenging. In this talk, we will present how we circumvented these limitations by combining live 3D imaging using multiphoton microscopy and image registration, processing and analysis. We quantified cilia biophysical features, such as density, motility, 3D orientation, beating frequency, or length without resolving their motion. We combined the results from different embryos in order to perform statistical analyses and compare experimental conditions. We integrated such experimental features obtained in vivo into a fluid dynamics model and a multiscale physical study of flow generation and detection. Finally, this strategy enabled us to demonstrate how cilia orientation pattern generate the asymmetric flow within the LRO. In addition, we investigated the physical limits of flow detection to clarify which mechanisms could be reliably used for body axis symmetry breaking. We also identified a novel type of asymmetry in the left-right organizer. Together, this work based on quantitative image analysis of motile cilia sheds light on the complexity of left-right symmetry breaking and chirality genesis in developing tissues.