Nodal: master and commander of the dorsal–ventral and left–right axes in the sea urchin embryo

Recent studies suggest that specification of the dorsal–ventral and left–right axes of the sea urchin embryo relies on Nodal-expressing signalling centres located in the ventral ectoderm and in the archenteron that share striking similarities with vertebrate organising centres. Nodal and its downstream target BMP2/4 pattern all three germ layers along the dorsal–ventral axis, repress neural fates and control morphogenesis of the larva. Moreover, Nodal establishes left–right asymmetry by repressing formation of the adult rudiment and inhibiting germline cells differentiation on the right side, while BMP2/4 promotes expression of mesodermal genes on the left side. These findings provide a framework for future studies and raise new questions regarding the events upstream and downstream of Nodal and BMP signalling during axis formation.