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Lab-grown retinas show why we can see colour and dogs can’t

  • March 13, 2024
  • 2 min read
Lab-grown retinas show why we can see colour and dogs can’t

With human retinas grown in a petri dish, researchers have discovered how an offshoot of vitamin A can generate the specialised cells that allow humans to see colours while dogs and other mammals can’t.

“These retinal organoids allowed us for the first time to study this very human-specific trait,” said author Robert Johnston, an associate professor of biology. “It’s a huge question about what makes us human, what makes us different.”

The findings were published in PLOS Biology and increase our understanding of colour vision as well as colour blindness, age-related sight loss, and other diseases linked to photoreceptor cells. They also show how genes instruct our retinas to make specific colour-sensing cells. This was a process that scientists thought was controlled by thyroid hormones.

Tweaking the cellular properties allowed the researchers to discover that a molecule known as retinoic acid determines whether a cone will specialise in sensing red or green light. Only humans with normal vision and closely related primates develop the red sensor.

For a long time, it was thought that red cones formed through a mechanism where cells “haphazardly” commit to sensing green or red wavelengths. Research from Johnston’s team hinted recently that the process could be controlled by thyroid hormone levels. But this new research suggests that red cones form through a specific sequence of events via retinoic acid within the eye. High levels of the acid in early development of the organoids correlated with a higher ratio of green cones while low levels changed the retina’s genetic instructions, generating the red cones later in development.

Scientists still don’t completely understand how the ratio of green and red cones can vary so much without affecting someone’s vision. Johnston added that if these type of cells determined arm lengths, the different ratios would lead to “amazingly different” lengths.

To better understand diseases including macular degeneration which causes loss of light-sensing cells, the researchers are working with other labs in Johns Hopkins and hope to deepen their understanding of how cones and other cells link to the nervous system.

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