In the mature retina, retinal inhibitory neurons (RINs) are arranged in three distinct layers composed of horizontal cells (HCs), inner nuclear layer amacrine cells (iACs) and displaced amacrine cells (dACs), respectively. How do such interneurons reach their specific laminar positions during development? To explore this question, William Harris and co-workers (p. ) quantify cell behaviour in the developing retina of several transgenic zebrafish lines over long periods of time. They first show that all RIN types show a bipolar morphology and migrate to the centre of the retina, near the region where the inner plexiform layer (IPL) later forms. RINs then adopt a multipolar morphology and can migrate tangentially, frequently changing direction. Interestingly, multipolar RINs are highly dynamic and do not just pile up in the centre of the retina according to their time of arrival, as previously thought. Moreover, RINs undergo cell type-specific behaviours that fine-tune their position. Contrary to previous belief, dACs actively migrate to their respective layer through the proto-IPL rather than being trapped in their layer by the future IPL. This study offers a valuable framework for further dissecting the molecular mechanisms of retina lamination.
