Differential sensitivity of midline development to mitosis during and after primitive streak extension

Background: Midline establishment is a fundamental process during early embryogenesis for Bilaterians. Midline morphogenesis in non-amniotes can occur without mitosis, through Planar Cell Polarity (PCP) signaling. By contrast, amniotes utilize both cellular processes for developing the early midline landmark, the primitive streak (PS). This study focused on the role of cell proliferation for midline development at pre- and post-PS-extension stages and analyzed PCP signaling components at post-PS-extension stages. Results: In contrast to pre-PS-extension stages, embryos under mitotic arrest during the post-PS-extension preserved notochord (NC) extension and Hensen's node (HN)/PS regression judged by both morphology and marker genes; although they became shorter, their lengths remained proportional to the embryo length. Laterality and segmentation of paraxial mesoderm were lost upon mitotic arrest. Accompanied by mitotic arrest-induced embryonic size reduction, cells including midline tissue displayed hypertrophy. Conclusion: This study has identified at least two distinct mitosis sensitivity phases during early midline development: One is PS extension that requires both mitosis and PCP, and the other is mitotic arrest-resistant midline development at post-PS-extension stages, with a still undefined influence by PCP signaling components.