What is the Optic Nerve?
The optic nerve is composed of approximately 1.5 million nerve fibers at the back of the eye that carry visual messages from the retina to the brain. When light hits the retina, the photoreceptors (light-sensitive cells) receive and transmit this information to other specialized cells, including the final cell type in the chain, called the retinal ganglion cells. These cells reside in the retina, but their output “cables” or “fibers,” called axons, extend from the optic nerve to very specific regions in the brain. Therefore, the retinal ganglion cell plays a critical role as the output nerve cell of the eye that transmits visual information to the brain. In the brain, the visual information is further processed to give us “sight.”
What Happens to the Optic Nerve in Glaucoma?
In glaucoma, the axons of these optic nerve cells degenerate, and eventually lead to cell death. Unfortunately, like other central nervous system regions such as the spinal cord, the regenerative capacity of the optic nerve is limited. There are many reasons for this, including the fact that retinal ganglion cells cannot repair or regenerate themselves after injury without significant help. Furthermore, the environment in which the optic nerve resides contains signals that inhibit regeneration and also lacks signals to stimulate regrowth. Therefore, glaucoma is currently an incurable disease. However, there are many scientists around the world studying ways in which to regenerate the optic nerve, not only to help glaucoma patients, but also because the optic nerve is particularly helpful in studying central nervous system regeneration.
Can the Optic Nerve Regenerate?
Regeneration of the optic nerve to restore vision requires a number of steps:
- The damaged retinal ganglion cells need to stay alive and not die.
- Surviving brain cells (neurons), which normally do not grow in the adult, need to go “backwards in time” and become more “immature” in order to regrow, as they had done during early development.
- Nerve fibers that re-grow need to overcome signals that inhibit growth.
- Regenerating axons (nerve fibers) have to connect to the appropriate location in the visual targets of the brain.
For the first step, researchers have made important progress in understanding the factors that help retinal ganglion cells survive and prevent degeneration. For example, in a U.S.-based clinical trial, ciliary neurotrophic factor-secreting implants have been assessed for safety, vision preservation, and visual improvement in patients with severe glaucoma.1 This trial is very exciting in its potential to validate a completely new category of treatments for glaucoma patients. Currently, treatment is limited to medications or surgery that lower intraocular pressure, which is very important, but not sufficient as there are some patients whose glaucoma continues to get worse even with low eye pressure.
For the remaining steps, many scientists are studying ways in which to instruct injured neurons to sprout axons (fibers) and regenerate by supplying factors that stimulate and instruct the process. Scientists are also exploring “axonal guidance,” which provide cues for the axons to grow in the correct direction and to the proper location. There is even some recent evidence in animal models of glaucoma that manipulating specific genes and stimulating activity of the retina may lead to axon regeneration and reconnections with the correct targets in the brain. Researchers now believe that in order to achieve meaningful optic nerve regeneration, multiple treatments that stimulate growth and suppress the tissue’s growth inhibition signals need to be combined. It is a tall order but one that is being actively investigated around the world.
This content was last updated on: March 18, 2017
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