Researchers have grown human retinas in a laboratory setting to reveal the process by which a derivative of vitamin A creates unique cells that enable humans to perceive a vast spectrum of colors. Dogs, cats, and other mammals do not possess this visual ability.
"These retinal organisms allow us to study this very human-specific trait for the first time," said author Robert Johnston, associate professor of biology. "It's an important question of what makes us human and what makes us unique."
The research results, published in "PLOS Biology", deepen people's understanding of color blindness, age-related vision loss and other diseases related to photoreceptor cells. They also demonstrate how genes instruct the human retina to make specific color vision cells, a process scientists believe is controlled by thyroid hormones.
By adjusting the organism's cellular properties, the team found that a molecule called retinoic acid determines whether the cones specifically sense red or green light. Only humans and closely related primates with normal vision develop red sensors.
For decades, scientists have thought that red cones formed through a coin-flip-like mechanism, in which cells chaotically work to sense green or red wavelengths -- a process that recent research from Johnston's team suggests may be controlled by thyroid hormone levels. But new research shows that red cones are formed through a specific chain of events orchestrated by retinoic acid in the eye.
The team found that during an organism's early development, higher levels of retinoic acid were associated with a higher proportion of green cones. Likewise, low concentrations of retinoic acid alter the genetic instructions of the retina to produce red cones later in development.
"There may still be some randomness in this, but our big discovery is that retinoic acid is produced very early in development. That timing is really important for learning and understanding how these cones are produced," Johnston said.
Green cones are very similar to red cones, except for a protein called opsin, which detects light and tells the brain what color a person sees. Different opsins determine whether a cone becomes a green or red sensor, although the genes for each sensor are 96 percent identical. Using a breakthrough technique, the team discovered these subtle genetic differences in the organisms and tracked changes in cone proportions over 200 days.
"Because we can control the number of green and red cells in an organism, we can push the pool of cells to become greener or redder, which has important implications for understanding how retinoic acid acts on genes," said author Sarah Hadyniak, a doctoral student in Johnston's lab and now at Duke University.
The researchers also mapped different proportions of these cells in the retinas of 700 adults. Hardiniak said seeing how the ratio of green to red cones changes in humans was one of the most surprising findings of the new study.
Scientists still don't fully understand why the ratio of green to red cones can vary so much without affecting a person's vision. If these cells determine the length of a human arm, Johnston said, then different ratios would produce "astonishing differences" in arm length.
To understand diseases like macular degeneration, which causes the loss of photoreceptor cells near the center of the retina, researchers are collaborating with other Johns Hopkins labs. The goal is to deepen their understanding of how cones and other cells communicate with the nervous system.
"The hope for the future is to help people solve these vision problems," Johnston said. "It's going to take a while to get there, but just knowing that we can make these different types of cells is very, very promising."
Compiled source: ScitechDaily