Experiments and modeling of “irreversible” brain torsion in early chick embryos*

The rightward torsion of the chick embryonic brain tube is one of the earliest organ-level left-right asymmetry developmental events, often associated with birth defects such as situs inversus, but the biomechanics of this process remains incompletely understood. Previous works showed that vitelline membrane (VM) exerts a force on the brain that drives the torsion, and surface tension (ST) can replace the mechanical role of VM. However, our experiments showed when ST was removed the torsion does not fully reverse suggesting other overlooked mechanical factors. Here, we use a combination of experiments and modeling to reveal that the deformation during the early brain torsion can be path and stage dependent. With optical coherence tomography imaging, we tracked the twisting and untwisting in a step-wise manner. A computational model is employed to help interpret the findings, in particular the path-dependent, "irreversible" shape evolution. Results show that the body forces such as gravity and buoyancy also play an important role during this left-right asymmetric morphogenesis process, thus revealing the hidden mechanical factors in the normal and abnormal development of early embryos.