An all-optical approach for probing microscopic flows in living embryos

Living systems rely on fluid dynamics from embryonic development to adulthood. To visualize biological fluid flow, devising the proper labeling method compatible with both normal biology and in vivo imaging remains a major experimental challenge. Here, we describe a simple strategy for probing microscopic fluid flows in vivo that meets this challenge. An all-optical procedure combining femtosecond laser ablation, fast confocal microscopy and 3D-particle tracking was devised to label, image and quantify the flow. This approach is illustrated by studying the flow generated within a micrometer scale ciliated vesicle located deep inside the zebrafish embryo and involved in breaking left-right embryonic symmetry. By mapping the velocity field within the vesicle and surrounding a single beating cilium, we show this method can address the dynamics of cilia-driven flows at multiple length scales, and can validate the flow features as predicted from previous simulations. This approach provides new experimental access to questions of microscopic fluid dynamics in vivo.