Serotonin in the morphogenesis of ascidian nervous system

The monoamine serotonin (5-HT) exerts key neuromodulatory activities in all animal phyla and it is supposed to play a morphogenetic role during the development. Nevertheless this aspect of serotonin activity is still poorly understood. By immunofluorescence and confocal laser scanning microscopy we studied the serotonergic nervous system of ascidian larvae belonging to the three orders of Phlebobranchia, Stolidobranchia and Aplousobranchia. A basic homology of the serotonergic nervous system was observed in all the species studied as most serotonergic components of nervous system are localized in the brain. However a lot of differences are shown from the fine cellular localization. Our results revealed that divergent developmental strategies exist during terminal differentiation, probably following different adaptation constraint.