“Mini-Eyes” Show Scientists How Colour Vision Develops

Our world appears in vivid colours, thanks to the cone photoreceptors in our eyes, yet little was known about how they developed until researchers observed it by growing tiny blobs of eye tissue in a lab.

   Scientists at the Johns Hopkins University (JHU) have successfully grown organoids (miniature organs) that contained photoreceptors who responded to light and behaved like human cone cells. Intriguingly, these three colour-sensing cells appeared in the same order as they do naturally in a fetus: first the blue-light-sensing cone cells and then the ones who sense green light or red light.
   Though it’s already known that the blue cone cells develop first, it was unclear as to why. “We weren't sure what in a development context cued those cells to be different from each other,” said lead study author Kiara Eldred, a doctoral candidate in the Department of Biology at JHU. 

A retinal organoid — at days 43 of growth — doesn’t really look (get it?) like an eye but is allowing scientists to see (so sorry) how eye cells grow and interact.

Credit: Johns Hopkins University

 
   However, using prior research, the scientists hypothesized that the thyroid hormone helped trigger the development of cells linked to colour vision. To test this, they manipulated the cone cells' receptors for the hormone, and the results didn’t disappoint. The researchers reported that when the receptors for the hormone were disabled, the mini-eyes grew only blue-light-sensing cells. They also found that when the organoids were saturated with the hormone early in the growth process, all the colour cells developed into red or green light-sensing cells.
    “That told us that we understood the mechanism enough that we could grow human retinal cells in a dish, and we could tell them what kind of cells we wanted to make,” said study co-author Robert Johnston Jr., an assistant professor in the JHU Department of Biology, 
   Experimenting with these “mini-eye” cells provided our first glimpse at what produces our colour vision, and the benefits won’t stop there. Studying lab-grown eye tissue will be valuable for uncovering other aspects of our unique sight, as well as provide treatment insights for blindness and vision-related illnesses.

For more about this fascinating research, read the original articles: http://science.sciencemag.org/content/362/6411/eaau6348 and https://www.livescience.com/63810-eye-tissue-grown-in-lab.html!