Humans develop sharp vision during early fetal development thanks to an interplay between a vitamin A derivative and thyroid hormones in the retina, Johns Hopkins University scientists have found. The findings could upend decades of conventional understanding of how the eye grows light-sensing cells and could inform new research into treatments for macular degeneration, glaucoma, and other age-related vision disorders. Details of the study, which used lab-grown retinal tissue, are published today in Proceedings of the National Academy of Sciences.