Eye can see a rainbow

Have you ever wondered why colours are what they are? Why aren’t there new colours to discover? And what about colour blindness? Well, researchers at Johns Hopkins University have been trying to unravel these mysteries by growing their own sets of eyes.

The scientists have been specifically growing human retinas, the part of the eye that turns light into the electrical signals sent to the brain. Lots of different cells are found in the retina, including cone cells which are the best at detecting different colours. In humans, there are three types of cone: red, blue and green (no points for guessing why). A lot of other mammals only have two cones, for example dogs have blue and yellow cones, yellow being between red and green on the light spectrum. So if dogs can’t see the difference between red and green, why can humans?

Red cone cells have only been found in humans and our closest primate cousins. For a long time scientists thought that red/green cones underwent a “coin toss mechanism” that would randomly fixate them on red or green light, and a newer theory suggested it was down to certain hormone levels. But now, a team led by biology professor Robert Johnston have found evidence that the molecule retinoic acid is what controls the growth of these cells. High levels of retinoic acid early in development create lots of green cones. The trick is that the amount of retinoic acid drops over time, changing the retina’s growth pattern so it can make more red cones. Prof Johnston says “this timing really matters for learning and understanding how these cone cells are made.”

Retinoic acid is made with vitamin A, which explains why you were always told it’s good for your eyesight. The only difference between green and red cones is a type of protein called opsin, with different opsins pushing the cone to detect green or red light.

The team of researchers aren’t sure that randomness doesn’t still play some part in the cell growth. They looked at red/green cones in the eyes of 700 adults, and found that the ratio of red to green was wildly different among them. Despite this, their eyesight wasn’t affected by how many red vs green cones they had and it looks like retinoic acid is the key player in this process.

In the future, the researchers hope to understand how cone cells are connected to our nervous system and improve understanding of eyesight diseases like macular degeneration.

by Louis Davies

@louis.on.air

Source:

Hadyniak, S.E. et al (2024) Retinoic acid signaling regulates spatiotemporal specification of human green and red cones. PLoS Biol 22(1): e3002464. https://doi.org/10.1371/journal.pbio.3002464