Bidirectional neuronal migration coordinates retinal morphogenesis by preventing spatial competition

While the design of industrial products is often optimized for the sequential assembly of single components, organismal development is hallmarked by the concomitant occurrence of tissue growth and organization. Often this means that proliferating and differentiating cells occur at the same time in a shared tissue environment that continuously changes. How cells adapt to architectural changes in order to prevent spatial interference remains unclear. To understand how cell movements important for growth and organization are orchestrated, we here study the emergence of photoreceptor neurons that occur during the peak of retinal growth using zebrafish, human tissue and human organoids. Quantitative imaging reveals that successful retinal morphogenesis depends on active bidirectional photoreceptor translocation. This leads to a transient transfer of the entire cell population away from the apical proliferative zone. This migration pattern is driven by distinct cytoskeletal machineries, depending on direction: microtubules are required for basal translocation, while actomyosin drives apical movement. Blocking photoreceptor translocation leads to apical overcrowding that hampers progenitor movements. Thus, photoreceptor migration is crucial to prevent competition for space and thereby allows concurrent tissue growth and lamination. This shows that neuronal migration, in addition to its canonical role in cell positioning, is involved in coordinating morphogenesis.