Differential Sensitivity of Midline Patterning to Mitosis during and after Primitive Streak Extension

Abstract

Midline establishment is a fundamental process during early embryogenesis for Bilaterians. Midline patterning in nonamniotes can occur without mitosis, through Planar Cell Polarity (PCP) signaling. By contrast, amniotes utilize both cell proliferation and PCP signaling for patterning early midline landmark, the primitive streak (PS). This study examined their roles for midline patterning at post PS-extension stages. In contrast to 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 and laterality was lost. Remarkably, no or background level of expression was detected for the majority of PCP core component in the NC-HN-PS area at post PS extension stages, except for robustly detected Prickle-1. Morpholino knockdown of Prickle-1 showed little influence on midline patterning, but suppressed embryonic growth. Lastly, associated with mitotic arrest-induced size reduction, midline tissue cells displayed hypertrophy. Thus, the study has identified at least two distinct mitosis sensitivity phases during early midline patterning. One is PS extension that requires both mitosis and PCP, and the other is mitotic arrest-resistant midline pattering with little influence by PCP at post PS-extension stages.