Over the last decade there has been a growing interest in the ability of olfactory glia to promote neural regrowth. Olfactory glia are a type of cell that wraps around the axons of nerves that relay the sense of smell from the nose to the brain. In humans and other mammals, olfactory neurons have unique properties – unlike neurons in other parts of the brain and spinal cord, olfactory neurons are replaced continually. They also regrow naturally after injury, such as after attack by the cold virus, and the regenerating neurons grow towards and make appropriate connections with their target neurons in the brain. The key question is whether olfactory glia have similar regenerative effects in injured spinal cords.
Scientists supported by Spinal Research were among the first to show that olfactory glia promote axon regeneration when they are transplanted into spinal cord injuries. These cells also form a new myelin coating around spinal cord neurons.
Investigating how best to use these cells to repair spinal cord injury is an important area of research. Scientists now know that olfactory glia are not a single type of cell – at least two have been identified. The proportion of each cell type varies depending on factors, such as how the cells are collected and the exact conditions in which they are grown in the laboratory. It is likely that each type of olfactory glia has a specific function and it is vital to establish which is responsible for axon regrowth and remyelination. One suggestion is that the different types of cells work in combination, so that all are needed: one type of olfactory glia surrounds the damaged neurons and shields them from inhibitory scar tissue. Additional olfactory glia grow to create a ‘bridge’ that links the two ends of the damaged spinal cord and forms a path for the regenerating nerves across the injury site.
In some cases injury does not sever all the nerve fibres at the wound site, some remain but lose the myelin coating that helps them transmit signals quickly and effectively. Because myelin is essential for rapid transmission of these signals, demyelinated neurons are unable to function efficiently. Using olfactory glia to restore the myelin coating to neurons in the spinal cord could boost the performance of surviving neurons.
Research is continuing to identify which cell types are most effective in promoting spinal cord repair. The regenerative properties also need to be confirmed by other research groups in different models of injury. Other questions to be answered include how best to collect olfactory glia from human patients and grow them in the laboratory to obtain sufficient numbers to be used in transplant studies.
Spinal Research funds a range of studies investigating how best to use these cells as well as methods to collect them from human tissue and grow them in laboratory dishes. One day it might be possible to harvest olfactory glia from individual patients to provide a tailor-made treatment for each injured person.
The regenerative properties of another type of glial cell, called Schwann cells, are also being studied. Schwann cells normally only occur in the in the peripheral nervous system, where they surround nerve axons and make the myelin covering. Like olfactory glia, transplanting Schwann cells into injured spinal cords stimulates growth of damaged axons and they also restore myelin to artificially demyelinated neurons. However, although Schwann cell grafts promote axon growth across the injury site, these newly regrown nerve fibres become trapped in the Schwann cell transplant – they are unable to ‘escape’ from the graft and so cannot form connections with surviving neurons. Because of this, olfactory ensheathing glia are likely to be more effective than Schwann cells in promoting neural regeneration.
