Lab-grown Retinas Explain Why People See Colors Dogs Can’t

Key Takeaways

  • Experiments with lab-grown retinas show how a form of Vitamin A (retinoic acid) generates red vs. green light-sensing cones in the retina.
  • The findings increase understanding of eye diseases that involve the loss of light-sensing photoreceptor cells.

Humans and closely related primates have three types of color-sensing cone cells in the retina, blue, red, and green, while other mammals have two, green and blue. This difference accounts for the ability of humans to see millions of colors, an ability that dogs, cats, and other mammals do not possess.  How humans with normal vision develop red sensors is a long-standing question.

With human retinas grown in a petri dish (retinal organoids), Johns Hopkins researchers have now discovered how an offshoot of vitamin A, called retinoic acid, generates red cone cells. By tweaking the cellular properties of the retinal organoids, the research team found that the level of retinoic acid determines whether a cone will specialize in sensing red or green light. The team also demonstrated how genes instruct the human retina to make specific color-sensing cone cells. Scientists previously thought this process was controlled either by thyroid hormones or through a coin toss mechanism where the cells haphazardly commit to sensing green or red wavelengths. Instead, the new research suggests red cones materialize through a specific sequence of events orchestrated by retinoic acid within the eye.

The findings, published in PLOS Biology, increase understanding of color blindness, age-related vision loss, and other diseases linked to photoreceptor cells – specialized cells in the retina that convert light into signals sent to the brain. To build understanding of diseases like macular degeneration, which causes loss of light-sensing cells near the center of the retina, the researchers are working with other Johns Hopkins labs. The goal is to deepen their understanding of how cones and other cells link to the nervous system.

Edited by Suditi Kedambadi and Miriam Kaplan.

Source:   Johns Hopkins University, ScienceDaily, January 11, 2024; see source article