There is a network of light-sensitive cells in the immature retina, indicating a greater role in brain development
Even before the fetus can recognize objects, light-sensitive cells active in the retina may play a greater role in the developing eye and brain than previously thought.
Intrinsically photosensitive retinal ganglion cells appear to help establish the supply of blood to the retina, circadian rhythms and pupillary light reflex. Researchers have now discovered that in a network that can detect light intensity, these cells are electrically connected, suggesting a greater role in development.
Long before a baby’s eyes can see images, they can detect light through the second trimester.
However, the light-sensitive cells in the developing retina the thin sheet of brain-like tissue at the back of the eye were thought to be simple on – off switches, presumably there to set up the parents ‘ 24-hour, day-night rhythms hope their baby will follow.
Scientists at the University of California, Berkeley, have now found evidence that these simple cells actually talk to each other as part of an interconnected network which provides more light sensitivity to the retina than previously thought, and can increase the effect of light on behavior and brain development in unsuspected ways.
In the developing eye, maybe 3% of ganglion cells the cells in the retina that send messages into the brain through the optic nerve are sensitive to light and, to date, researchers have found around six distinct subtypes which interact with different brain places. Some talk to the suprachiasmatic nucleus in order to adjust our internal clock to the day-night cycle. Others send signals to the area that will limit our pupils in bright light.
But other are connected to unexpected areas: the perihabenula which regulates mood, and the amygdala which deals with emotions.
Recent evidence indicates in mice and monkeys that these ganglion cells often talk to each other via electrical connections called gap junctions, implying much more complexity in immature rodent and primate eyes than imagined.
“Given the variety of these ganglion cells and that they project to many different parts of the brain, it makes me wonder whether they play a role in how the retina connects up to the brain,” Marla Feller, professor of molecular and cell biology at the University of Berkeley and senior author of a paper published in Current Biology this month, said.
“Maybe not for visual circuits, but for non-vision behaviors. Not only the pupillary light reflex and circadian rhythms, but possibly explaining problems like light-induced migraines, or why light therapy works for depression. “he added.
The researchers also found evidence that the circuit tunes itself in a way that could adapt to the intensity of light, which probably has an important role in development, Feller said.
she said, “In the past, people demonstrated that these light-sensitive cells are important for things like the development of the blood vessels in the retina and light entrainment of circadian rhythms, but those were kind of a light on/light off response, where you need some light or no light,”.
“This seems to argue that they are actually trying to code for many different intensities of light, encoding much more information than people had previously thought.”she add