Fig. 4: Self-organization process in sea urchin twins.

a Schematic summary of halved H. pulcherrimus development. Grey bars indicate the time frame that each event which we argued in this article occurs. b Two crucial events in sea urchin self-organization which we discussed. (Upper row) Making sphere shape: At the timing for shape transition from flat to cup, actomyosin activity at the basal side of the cells (red) contribute to cells elongate along apicobasal axis direction and make cells cone chape. Apex of each cell is bundled and becomes the center of the embryo. Simultaneously, septate junction (blue) gives each cell the ability to adhere each other on the lateral side. As a result, the free edge of the cup can bind each other and form sphere. (Lower row) A-P axis re-organization: At the cup shape stage, presumptive anterior pole (magenta) and posterior pole (green) are in their original positions. When sphere shape is formed, presumptive anterior and posterior region are temporarily placed very close together and anterior-posterior (A-P) axis is temporarily disturbed. Triggered by anterior and posterior pole contact, Wnt/ß-catenin signal is newly activated at adjacent region of original posterior pole, and the signal makes the anterior-most region shift to the opposite side. Finally proper A-P axis is formed.