Dev Growth Differ 2017 Jun 01;595:434-443. doi: 10.1111/dgd.12372.

Measurement of surface topography and stiffness distribution on cross-section of Xenopus laevis tailbud for estimation of mechanical environment in embryo.

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The stress distribution inside a Xenopus laevis tailbud embryo was estimated to examine the cause of the straightening and elongation. The embryos were cut in the middle, yielding a cross-section perpendicular to the body axis. The section was not flat, owing to the residual stress relief. The stress needed to restore the flatness corresponded to the stress inside the embryo and was calculated using the surface topography and Young's-moduli in the section. We found the areas of the notochord (Nc), neural tube (NT), and abdominal tissue (AT) bulged in the cross-section, which revealed that compressive forces acted in these tissues. The moduli of the Nc, NT, and AT were in the order of several thousand, hundred, and tens of pascals, respectively. In the Nc, the compressive force was largest and increased with the development, suggesting Nc playing a central role in the elongation. The bending moment generated by the AT was 10 times higher than that by the Nc in the early stages of the tailbud formation, and the two were similar in the latter stages, suggesting that the compressive force in the AT was the major cause of the straightening during the early stage. The straightening and elongation could be orchestrated by changes in the compressive forces acting on the Nc, NT, and AT over time. For the sake of simplicity, we calculated the compressive force only and neglected the tensile force. Thus, it should be noted that the amount of the compressive force was somewhat overestimated.

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