On "irreversible" torsion in early chick embryonic brain development*

The rightward torsion of the chick embryonic brain tube is one of the earliest organ-level left-right asymmetry developmental events. Previous studies have shown that vitelline membrane (VM) exerts the necessary force on the chicken embryo brain that drives the torsion, and surface tension can replace the mechanical role of VM resulting in a similar degree of torsion at a comparable stage. However, recent experiments show that when the surface tension is removed the torsion does not fully reverse suggesting that there are other overlooked mechanical factors in this process. Here, we show through a combination of in vivo experiments and a physical model of the embryonic morphology that the twisting of the chick embryonic brain tube is partially reversed when the surface tension is removed and the deformation of the early brain can be path dependent. We also studied the effect of embryonic curvature and shape on the degree of torsion, and identified how buoyancy may play a mechanical role in in this “irreversible” brain torsion process.