The findings increase understanding of color blindness, age-related vision loss, and other diseases linked to photoreceptor cells. They also demonstrate how genes instruct the human retina to make specific color-sensing cells, a process scientists thought was controlled by thyroid hormones.
Scientists for decades thought red cones formed through a coin toss mechanism where the cells haphazardly commit to sensing green or red wavelengths -- and research from Johnston's team recently hinted that the process could be controlled by thyroid hormone levels. Instead, the new research suggests red cones materialize through a specific sequence of events orchestrated by retinoic acid within the eye.
Green and red cone cells are remarkably similar except for a protein called opsin, which detects light and tells the brain what colors people see. Different opsins determine whether a cone will become a green or a red sensor, though the genes of each sensor remain 96% identical. With a breakthrough technique that spotted those subtle genetic differences in the organoids, the team tracked cone ratio changes over 200 days.
Scientists still don't fully understand how the ratio of green and red cones can vary so greatly without affecting someone's vision. If these types of cells determined the length of a human arm, the different ratios would produce"amazingly different" arm lengths, Johnston said. Other Johns Hopkins authors include: Kiara C. Eldred, Boris Brenerman, Katarzyna A. Hussey, Joanna F. D. Hagen, Rajiv C. McCoy, Michael E. G. Sauria, and James Taylor; as well as James A. Kuchenbecker, Thomas Reh, Ian Glass, Maureen Neitz, Jay Neitz of the University of Washington.Sarah E. Hadyniak, Joanna F. D. Hagen, Kiara C. Eldred, Boris Brenerman, Katarzyna A. Hussey, Rajiv C. McCoy, Michael E. G. Sauria, James A.