The optic nerve is a large cable composed of ganglion cell axons, which acts as a viaduct conducting visual information from the retina to the brain. Several diseases, such as glaucoma, can damage the optic nerve and cause permanent blindness. Optic nerve regeneration restores damaged axons in the optic nerve so that vision returns. Regeneration of the optic nerve is currently impossible in humans, but research in this area has advanced significantly in the last few years and several promising ideas exist. Building nerve grafts and manipulating enzymes and other molecular components involved in cell growth are some of the techniques which are being investigated to attempt to engineer optic nerve regeneration.
There are no nerves in the central nervous system, which includes the brain, spinal cord, and retina, that regenerate. Nerves in other parts of the body are however capable of regeneration. In rats, grafting or transplanting peripheral nerve cells onto an injured optic nerve promoted nerve regeneration, a technique which may work in humans as well. Researchers are working on a man-made gel or polymer, based on the peripheral nerve grafts, which could be placed on an injured optic nerve to act as scaffolding or a guide for the axons to harness for regeneration.
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The puzzle of optic nerve regeneration will probably be solved, at least in part, by manipulating molecular pathways in the body, and several promising ideas in this area involve the phosphatase and tensin homolog (PTEN) gene, the brain-derived neurotrophic factor (BDNF) gene, and a growth factor called oncomodulin. In humans, the PTEN gene is a piece of genetic material containing instructions on how to make the PTEN enzyme, which is a key factor in stopping cell growth. The hope is that if the PTEN enzyme can be blocked, axon regeneration will be possible. BDNF is a growth factor expressed by the BDNF gene, and is important in neuronal development in the optic nerve. Over-production of this growth factor through gene therapy has resulted in regeneration of nerve connections in mice.
Oncomodulin is a cell growth factor which, when added to a dish of retinal nerve cells, practically doubled the growth of the axons. When oncomodulin was placed on damaged optic nerves in rats, along with a chemical which amplifies the effects of oncomodulin, the result was a significant increase in optic nerve regeneration. Once researchers are able to get the axons to regenerate, they will need to then make them grow to the right spots and make the right connection in order for the visual information to be transferred to the brain, so getting the axons to grow is just one part of the very complex puzzle of optic nerve regeneration.