By growing human retinal tissue from stem cells, researchers have determined how the various types of cells that enable people to see colors develop. The new study sets the stage for the development of therapies and treatments for eye diseases like color blindness or macular degeneration. What's more, the results - some of the first such vision research to be done on human tissue, versus that of mice or fish - further establish lab-grown organoids as a powerful model for studying mechanisms underlying human development. Three types of cone photoreceptors are present in the retina, which respond to different wavelengths of light according to their pigments to give humans color vision. Mutations that affect the expression or function of these light-sensitive proteins are known to cause forms of color blindness and other eye diseases. According to the authors, however, little is known about how the specialized red, blue and green cone cell subtypes develop in the human retina. In the lab, Kiara Eldred and colleagues grew stem cells into retinal tissue organoids that closely mirrored the developmental stages observed in in vivo retinal tissue. Eldred et al. found that blue-detecting cones developed first, followed by red- and green-detecting ones. Thyroid hormone signaling regulated which type of cells developed and when, they say. By varying the amount of thyroid hormone during specific stages of development, the authors were able to create organoids with only one type of cone. Eldred et al. note that their results provide insight into why pre-term infants with low levels of thyroid hormone often have a higher incidence of color vision defects.