Exponential growth and neuronal yield in the vertebrate developing retina are mediated by Hedgehog signaling

The balance between self-renewal and terminal differentiation of neural progenitors is regulated by a complex interplay of signaling pathways that set the spatial and temporal cues that ultimately shape and organize neurogenic tissues. In this contribution, we combine in toto experiments with three-dimensional image analysis, computational modeling, and theoretical tools to provide a quantitative characterization of the dynamics of growth and differentiation of the developing vertebrate retina. We show that the tissue develops by gradually increasing the average rate of differentiation and reducing the average cell cycle length. Moreover, this balance between differentiation and cell cycle duration increases the yield of production of terminally differentiated neurons, and it is required to achieve a well-defined exponential growth. We also show that a potential regulator of this balance is Hedgehog (Hh), promoting simultaneously cell cycle progression and cell cycle exit of RPCs. Our results represent a detailed and accurate quantitative characterization of retinal neurogenesis and how signals regulate the balance between proliferation and differentiation during the development of the vertebrate retina.