| Summary: | Although the body of bilaterians is largely symmetric with respect to the midline, some visceral organs and the central nervous system show substantial asymmetries that are essential for the correct functioning of the organism. Brain lateralisation is a conserved feature across vertebrates enabling a more complex pattern of neuronal circuits and function at an individual level, as well as coordinated behaviour at population level. However, the underlying developmental mechanisms that establish asymmetric neuroanatomy in vertebrates remain enigmatic. Here, I have used zebrafish (Danio rerio) and chick (Gallus gallus) epithalamus as models to look at the development of brain asymmetries during embryogenesis. By laser-ablation and transplantation experiments, I show that in zebrafish the parapineal – a left-sided group of cells in the pineal complex – is able to induce left-side fate in both left and right habenula. The parapineal regulates habenular asymmetry at several stages of zebrafish development, first during neurogenesis and later in the establishment of asymmetric connectivity. In chick, however, no apparent parapineal has been described to date and concomitantly, I find that the habenulae do not exhibit molecular asymmetries similar to those described in zebrafish or other species with a parapineal-like organ. These results indicate that the development of a parapineal nucleus as part of the pineal complex is essential for the elaboration of overt molecular and structural asymmetries in the vertebrate epithalamus.
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