The hitchhiker's guide to quantitative diffusion measurements.

Quantifying diffusional motion of submicron tracer particles with high spatiotemporal resolution is often key to assess the dynamics of physico-chemical systems, e.g. in complex fluids, colloidal suspensions, or living cells. A variety of methods has been developed over the past decades, but often their quantitative comparability has remained poorly explored on an experimental basis. Yet, knowing their experimental benefits and limitations can be a crucial piece of information when designing experiments on new and unexplored specimen. Therefore, we have implemented three very widespread techniques for quantifying diffusional motion (single-particle tracking, fluorescence correlation spectroscopy, and differential dynamic microscopy) in a light sheet microscope and performed a quantitative comparison on standardized samples of varying concentrations. Light sheet microscopy is particularly suited for imaging-based diffusion measurements because of its high spatiotemporal resolution in combination with an improved contrast and signal-to-noise-ratio at low excitation powers. As a result, all three methods are found to yield good quantitative estimates with respect to the theoretically predicted diffusion constant, yet their accuracy and bias varies markedly, raising specific caveats for the individual methods' range of applicability.

Setting the Clock for Fail-Safe Early Embryogenesis.

The embryogenesis of the small nematode Caenorhabditis elegans is a remarkably robust self-organization phenomenon. Cell migration trajectories in the early embryo, for example, are well explained by mechanical cues that push cells into positions where they experience the least repulsive forces. Yet, how this mechanically guided progress in development is properly timed has remained elusive so far. Here, we show that cell volumes and division times are strongly anticorrelated during the early embryogenesis of C. elegans with significant differences between somatic cells and precursors of the germline. Our experimental findings are explained by a simple model that in conjunction with mechanical guidance can account for the fail-safe early embryogenesis of C. elegans.

Mechanical cues in the early embryogenesis of Caenorhabditis elegans.

Biochemical signaling pathways in developmental processes have been extensively studied, yet the role of mechanical cues during embryogenesis is much less explored. Here we have used selective plane illumination microscopy in combination with a simple mechanical model to quantify and rationalize cell motion during early embryogenesis of the small nematode Caenorhabditis elegans. As a result, we find that cell organization in the embryo until gastrulation is well described by a purely mechanical model that predicts cells to assume positions in which they face the least repulsive interactions from other cells and the embryo's egg shell. Our findings therefore suggest that mechanical interactions are key for a rapid and robust cellular arrangement during early embryogenesis of C. elegans.