P05. Mechanical force generated by leading edge mesoderm modulates collective cell polarization in axial mesoderm during Xenopus gastrulation

Abstract

Gastrulation is one of the most important processes during the morphogenesis of early embryo, involving dynamic cell shape change and migration. In chordates, notochord, a rod-shape structure that contributes to body extension and neural tube formation, is formed by collective cell movements of the axial mesoderm (AM) through gastrulation. During notochord formation, AM cells become narrow along mediolateral (M–L) axis and such polarized cells intercalate each other to extend the tissue along anterior–posterior (A–P) axis. This process, called convergent extension (CE), requires the establishment of cell polarity at the same time and in the same direction in AM. Although planar cell polarity (PCP) pathway is essential for cell polarization and regulation of CE, little is known about how the polarity of AM cells is established collectively at the beginning of CE. During Xenopus gastrulation, leading edge mesoderm (LEM), positioned in front of the AM, attach to the blastocoel roof and migrate towards the animal pole. The LEM contributes to the directional migration of involuting marginal zone cells. Therefore, we hypothesized that anterior migration of the LEM generate the pulling force along A–P direction, and the generated force stretch AM cells to establish M–L cell polarity. To test this hypothesis, we first performed mesoderm migration assay in vitro and found that the LEM migrated faster than the AM. We measured the force generated by the LEM in vitro using thin glass pipette. The estimated force varied but ranged from tens to hundreds of nano-newtons. These results suggest that the LEM generate the pulling force to the AM in a significant order of force magnitude. Next, to test whether the pulling force induces collective cell polarization in the AM, we developed stretch assay system using a silicone chamber. To determine the magnitude of stretch force applied to the activin-induced AM on the scale same as we observed, we measured the Young’s modulus of the AM using glass pipette and calculated the force by formula of Hooke’s law. Using these methods, we estimated that 5–15% stretch is probably equal to LEM-generated force. We are currently examining the effect of such stretch on AM with comparable magnitudes of force generated by LEM, and determine if the stretching affects polarity formation process. This study might contribute to better understanding of the distribution of force fields and the function of mechanical factors in animal development.