Retinal organoid at 291 days. Red and green cone cells are green in the photo, while blue cone cells are blue. Photo: Johns Hopkins University

Scientists were able to grow human retinas from stem cells for 1 year, allowing them to mimic human fetal development of retinas and closely observe how color-detecting cells form.

Why it matters: The information they gathered could be used to prevent or treat eye diseases and disorders like glaucoma, macular degeneration, color blindness and eye problems from premature births, Johns Hopkins University scientists say in a new study published in Science Thursday.

Background: Retinas are the part of the eye that detects light and determines colors. Humans have 3 types of "cone photoreceptors," which are color-detecting cells that sense red (long wavelength), green (medium wavelength) or blue (short wavelength) light.

What they did: The scientists took stem cells, which have the ability to become any type of cell, and directed them to grow into hundreds of "retinal organoids," so they could monitor the photoreceptor development. Organoids are simplified organs grown in a lab dish from stem cells.

  • Researchers checked their development over the one-year period to see when the different color cones formed.
  • They also used the gene editing technology, CRISPR, to knock out the thyroid hormone receptor in the cone cells at different times. This was done to determine the impact of the hormone as well as to see if the hormone's timing altered development.

What they found: Blue cones form first while red and green cones develop later, study author Kiara Eldred says. "We also found the timing of thyroid hormone exposure is really key to developing the red/green cones and the lack of thyroid hormone is important to developing the blue ones," says Eldred, who's a Ph.D candidate at Johns Hopkins.

  • The researchers hypothesize that the retina contains genes that help control the level of thyroid hormone received via the placenta.
  • Only low levels of thyroid are allowed to come into contact with the blue cones as they are being developed, followed by the activation of genes allowing high levels of the hormone to help develop the red and green cones.

What they're saying: Jay Neitz, professor of ophthalmology at the University of Washington who was not part of this study, says "this is an amazing study" that will open up new research. He notes that its aim is to unlock "the secrets of cellular differentiation and development." 

"[T]hese results open the way to taking a cell from a person with inherited eye disease, repairing the genetic mistake, growing a retina from the person's own repaired cell and transplanting the repaired cells back in the eye to cure a blinding disorder."
— Jay Neitz

What's next: Robert J. Johnston Jr., co-author of the study and assistant professor at Johns Hopkins, tells Axios he hopes the information can lead to multiple therapeutic uses — such as possibly helping premature babies receive enough thyroid hormone to fully develop their red and green cone cells.

  • Johnston also hopes the knowledge will eventually be used to prevent diseases like macular degeneration, which is the leading cause of blindness, as well as glaucoma, which degenerates the retinal ganglion cells.
  • Eldred says she hopes their findings can eventually be used to "inject and enrich the population of cells that need to be replaced" for disease treatment.

Go deeper: Watch Johns Hopkins' video.

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