Abstract

The establishment of asymmetry in the embryonic zebrafish brain begins with the movement of the parapineal organ (PpO) to the left side, an event required for the development of asymmetries in neighbouring diencephalic nuclei. Previous studies have shown that the laterality of PpO asymmetry requires Nodal signalling but the morphogenetic events that are controlled by Nodal remain unclear. Here we developed and applied computational image processing tools for in vivo 3D microscopy datasets, and combined them with indirect immunofluorescence to unravel the cellular morphogenetic behaviours underlying PpO formation in Tg(flh::EGFP) zebrafish embryos. We found that precursor cells of the PpO display early left-right differences in cellular morphogenetic behaviours during the process of nucleogenesis. Left-sided precursors contract their axis of elongation while right-sided precursors move across the midline to assemble with left-sided precursors and form a left-sided compact cluster organised in a 3-dimensional rosette. In embryos with affected Nodal signaling, however, left and right-sided PpO precursors show similar cellular behaviours and remain positioned at the dorsal midline, thus experiencing a delay in the asymmetric positioning within the brain. Thus, Nodal signaling directs early events of asymmetric morphogenesis in the PpO that leads to the formation of an asymmetric brain nucleus in the zebrafish embryo. Funding: ICM P09-015-F, Fondecyt 1161274 & 1151029, Conicyt PIA ACT1402, Fondap 15150012.