For decades, scientists believed that the foveola — the tiny central region of the retina responsible for sharp color vision — achieves its specialized cone pattern because blue cones form and then migrate away. New research using lab-grown human retinal organoids shows this is not the case.

Instead, blue cone photoreceptors initially appear in the foveola during early fetal development but then transform into red and green cones through a precisely orchestrated process involving retinoic acid (a derivative of vitamin A) and thyroid hormones. Retinoic acid levels drop to halt new blue cone formation, while thyroid hormones drive the conversion of existing blue cones into red and green ones.