When a mammal’s central nervous system nerves are cut or destroyed, they typically don’t regrow. That conventional understanding may be challenged by a study of regrowth of retinal ganglion cell axons in mice. The potential long-term ramifications of nerve cell regrowth point to the wondrous and still barely understood capabilities of mammalian brains.
A team of scientists led by a Stanford University School of Medicine investigator in collaboration with researchers from Univesity of California-San Diego, Harvard University, and Utah State University have had success in restoring aspects of vision in mammals. The study was published in Nature Neuroscience earlier this month. Ganglion cells from the back of the retina “project” wire-like axons that are bundled along the optic nerve and then disperse to connect with the corresponding visual nerve cells in the brain. This is the essential process that has to occur to regain visual function. Experimenting with mice, the team found that completely severed optic nerves were able to regenerate and, equally impressive, retrace the neural pathways to connect to the correct parts of the brain to allow partially regained vision.
In the study, adult mice with the optic nerve for one eye damaged were treated in one or both of two regimens. Some had “intensive daily exposure” to constantly moving black and white images. A second group was treated with biochemicals known to stimulate the retinal ganglion cell area. The third group received both treatments. With each of the treatments — visual stimulation and biochemicals — some growth of the axons was noted, but not far enough down the optic nerve. The mice who received both treatments, and who also had their healthy eyes blocked to encourage the use of the damaged eyes, not only had more axonal growth but growth past the dispersal point and onward to find the proper parts of the brain.
The study’s senior author, Andrew Huberman, PhD, a Stanford associate professor of neurobiology said, “Somehow these retinal ganglion cells’ axons retain their own GPS systems. They went to the right places and did not go to the wrong places.” The mice did not have full, fine detail vision restored, but were able to see enough to move away from visual threats. Much more research and study is necessary. The ramifications for people who have previously been sighted but then had what has been considered irreversible damage, as with glaucoma, are exciting and raise hope for help in restoring sight in humans.