Lab-Grown Human Retinas Illuminate How Eyes Develop Color Vision
The mini organs may help scientists develop therapies for eye disorders such as color blindness and macular degeneration
Sight begins when light bounces off surfaces and enters our eyes. The muscles of our pupils control how much light passes through, and the clear cornea and lens bend the light and focus it onto the retina, a thin strip of tissue covered in millions of light-sensitive neurons, or photoreceptors.
These nerve cells, named for the way they are shaped—like rods and cones—are where light is converted into electrical signals then sent via the optic nerve to the visual centers of the brain. A paper published October 11 in Science uses a retina grown outside the body to show how cones develop into the eyes’ color sensors.
Our daytime vision depends on the cones because they respond best to bright light (as opposed to the rods, which are sensitive to dim illumination). The pyramid-shaped cells come in three types: blue, green and red—each named after the colors of light they are able to detect. We need all three to perceive the many hues in our surroundings. The most common cause of color blindness—which affects approximately 8 percent of males and 0.5 percent of females of northern European descent—is caused by an inherited defect in red or green cones, which leads to reduced or complete loss in ability to see the two colors those cells detect.
Robert Johnston, a developmental biologist at Johns Hopkins University, and his colleagues wanted to understand how, exactly, developing cells in the human eye decide to become blue, green or red. Prior research had provided some big clues, showing this process occurs in a stepwise manner—blue cells come first, then red and green ones follow—and that thyroid hormone, a molecule secreted by the thyroid gland in the neck, is a critical player in this process. But many of these studies had been conducted on animals such as fish, chicken and mice because of the obvious ethical challenge of experimenting with human tissue. Although researchers can study donated retinas from deceased fetuses, it is nearly impossible to obtain samples for some periods of early development.
Read more at Scientific American