Left-right asymmetries in the nervous system are common among animals, and their importance is obvious in phenomena such as human language processing,which occurs mainly in the brain's left hemisphere. Now, Myriam Roussigné and colleagues reveal that Nodal signalling regulates asymmetric neurogenesis in zebrafish(). The elaboration of asymmetries between the bilateral habenular nuclei, a group of nuclei located in the diencephalon, largely depends on the parapineal, a nucleus that sits on the brain's left side owing to unilateral, left-sided Nodal signalling. Here, the authors identify the chemokine receptor gene cxcr4b as an early marker of habenular neurons. They show that neurogenesis begins earlier in the left than in the right habenula, and that this asymmetry is independent of the parapineal. Disrupting the asymmetric activity of Nodal, however, leads to the symmetric onset of habenular neurogenesis. These results indicate that asymmetric Nodal signalling acts not just to bias the laterality of the parapineal but also has an early role in asymmetric habenular neurogenesis.
