Unraveling the regulative development and molecular mechanisms of identical sea urchin twins.

Since Hans Driesch's pioneering work in 1891, it has been known that animal embryos can develop into complete individuals even when divided. However, the developmental processes and molecular mechanisms enabling this self-organization remain poorly understood. In this study, we revisit Driesch's experiments by examining the development of isolated 2-cell stage blastomeres in the sea urchin, Hemicentrotus pulcherrimus. Contrary to intact embryos, these isolated blastomeres initially form a flat, single layer of dividing cells that eventually round up to be a blastula. Live imaging and knockdown experiments reveal that actomyosin activity at the basal side of the cells and septate junctions drives this process. Intriguingly, we observed temporal disorganization of the anterior-posterior (A-P) and dorsal-ventral (D-V) axes, where the original A-P poles come into contact after sphere shape formation. The disrupted A-P axis is subsequently corrected as the embryos employ the Wnt/β-catenin signaling mechanisms assumed to be used in intact embryos to re-establish a normal axis. These findings suggest that axis re-organization through pre-existing developmental mechanisms is essential for the successful regulative development of divided embryos.