3D viscoelastic drag forces contribute to cell shape changes during organogenesis in the zebrafish embryo.

The left-right organizer in zebrafish embryos, Kupffer's Vesicle (KV), is a simple organ that undergoes programmed asymmetric cell shape changes that are necessary to establish the left-right axis of the embryo. We use simulations and experiments to investigate whether 3D mechanical drag forces generated by the posteriorly-directed motion of the KV through the tailbud tissue are sufficient to drive such shape changes. We develop a fully 3D vertex-like (Voronoi) model for the tissue architecture, and demonstrate that the tissue can generate drag forces and drive cell shape changes. Furthermore, we find that tailbud tissue presents a shear-thinning, viscoelastic behavior consistent with those observed in published experiments. We then perform live imaging experiments and particle image velocimetry analysis to quantify the precise tissue velocity gradients around KV as a function of developmental time. We observe robust velocity gradients around the KV, indicating that mechanical drag forces must be exerted on the KV by the tailbud tissue. We demonstrate that experimentally observed velocity fields are consistent with the viscoelastic response seen in simulations. This work also suggests that 3D viscoelastic drag forces could be a generic mechanism for cell shape change in other biological processes.

Quantified Pore-Scale Nanoparticle Transport in Porous Media and the Implications for Colloid Filtration Theory.

This study evaluates the pore-scale distribution of silver nanoparticles during transport through a sandy porous medium via quantitative synchrotron X-ray computed microtomography (qSXCMT). The associated distributions of nanoparticle flow velocities and mass flow rates were obtained by coupling these images with computational fluid dynamic (CFD) simulations. This allowed, for the first time, the comparison of nanoparticle mass flow with that assumed by the standard colloid filtration theory (CFT) modeling approach. It was found that (i) 25% of the pore space was further from the grain than assumed by the CFT model; (ii) the average pore velocity agreed well between results of the coupled qSXCMT/CFD approach and the CFT model within the model fluid envelope, although the former were 2 times larger than the latter in the centers of the larger pores and individual velocities were upwards of 20 times those in the CFT model at identical distances from grain surfaces ; and (iii) approximately 30% of all nanoparticle mass and 38% of all nanoparticle mass flow occurred further away from the grain surface than expected by the CFT model. This work suggests that a significantly smaller fraction of nanoparticles than expected will contact a grain surface by diffusion via CFT models, likely contributing to inadequate CFT model nanoparticle transport predictions.