Artificial limbs let receivers lead more productive and fulfilling lives. Organ transplantations give new life to people. Science has advanced so far that it can even tinker with the workings of the brain to explore ways in which lost brain functions can be revived. Experiments are already underway to determine if neuronal transplantation can replace and restore the functionality of lost or damaged neurons.

Experiments conducted on laboratory mice provide a glimmer of hope. For instance, in a recent experiment, embryonic neurons were transplanted into the visual cortex of vision-impaired mice. These animals began to see a few weeks after the transplantation!

The findings of experiments like this one are exciting and have already led scientists to wonder if neuronal transplantation holds the key to curing brain disorders and cognitive, motor, and sensory impairment.

Neuronal transplantation and plasticity of the human brain

The term “plasticity” refers to the ability of the neuronal pathways and synaptic connections to change in response to novel experiences. It was once believed that the neural pathways and connections in the brain became fixed after an individual reaches a certain age. Not only laymen but also scientists in some quarters believed that only a child’s brain can shape and reshape itself in response to events and experiences. But the above-mentioned experiment on mice turns this idea right on its head.

Scientists have been carrying out experiments on neuronal transplantation over the past decade or so. One study points to the immense significance of stem cells in aiding neural regeneration after transplantation. Stem cells are primitive cells that can not only regenerate but also develop and differentiate into other types of cells with various functionalities.

In humans, it is believed that embryonic stem cells (ESCs) can be transplanted to reverse the effects of diseases, aging, developmental defects, and other types of tissue damage.

Neuronal transplantation: how did the mice get their sight back?

In a landmark experiment, embryonic neurons with GABA were injected into the visual cortex regions of adult laboratory mice with visual impairment. The neurotransmitter GABA is instrumental in controlling vision and several motor and cortical functions in organisms.

Several weeks after the transplantation, the mice were tested for their visual capabilities. It was found that those who were injected with the neurons not only displayed normal visual clarity but also younger and more flexible brains. It is evident from this experiment that the implanted neurons integrated seamlessly into the GABA-deficient region of the mice brain. What is interesting to note is that after transplantation, the neurons migrated to the appropriate cortical regions of the tissue associated with visual acuity, metamorphosed, and took over the characteristics and functionalities of the lost or damaged cells that were associated with vision.

Scientists are excited at another finding from this experiment. They have discovered that the transplantation of the neurons set into motion a critical period of neural development in the mice. “Critical period” refers to a time period when there is maximum plasticity of the brain. Usually this period occurs in childhood. But this experiment shows that the critical period can also be induced in adulthood. In this experiment, the implantation of the neurons created a new “critical period” that corresponded to the time after the transplantation that the neurons took to integrate into the visual cortical system of the mice and acquire the characteristics of the relevant cells.

The results indicate that neural plasticity in adult human brains may also not be fixed and that they can change under the influence of chemical and physical factors. This revolutionary experiment has got scientists excited about the self-renewal and self-generation possibilities of the human “plastic” brain.

Neuronal transplantation as a cure for brain disorders

Researchers have long been wondering if neuronal transplantation can stem the advance of and/or reverse the effects of progressive neurodegenerative diseases like Parkinson’s disease (PD), Huntington’s disease (HD), and schizophrenia. The scientists feel hopeful because diseases like PD are triggered by dysfunctional neuronal pathways or when there is loss of or damage to the neurons that hamper their ability to function normally.

PD is caused by a progressive loss of dopamine neurons in a specific part of the brain. Dopamine therapy is a standard treatment procedure for PD. According to the experimental findings reported in one study carried out on laboratory mice, fetal cells transplanted into the dopamine-deficient region of the brain can develop as fully-functional dopamine neurons to replace the lost or damaged cells and take on their functionalities. This can restore lost cerebral function and reduce the symptoms of PD in an animal case study.

Scientists have also carried out experiments to test the feasibility of this therapeutic approach on individuals afflicted with HD. In one experiment, two people with moderate HD were transplanted with fetal cells from the pre-basal ganglia region. These embryonic cells survived in the new environment and differentiated into the intended type of cell. But six years after the implantation, it was found that though the symptoms of the disease did not progress in the individuals, they were not cured either. Incidentally, the two patients who took part in the experiment survived 74 months and 79 months respectively after the transplantation.

Scientists have achieved some degree of success with neuronal transplantation in case of PD. On the other hand, the partial setback in the experiment on people suffering from HD indicates that they should continue to explore more sophisticated techniques of neuronal implantation and find out about the other factors (internal or external) that contribute to the success of the transplantation or the various developmental factors that trigger the creation of a critical period.

The limited amount of laboratory success of the neuronal transplantation procedure should not discourage scientists from searching for answers to the above problems. People are already hinging their hopes on this flicker of hope that the experiment to bring back vision in laboratory mice has brought them.

References

Davis, M., Figueroa Velez, D., Guevarra, R., Yang, M., Habeeb, M., Carathedathu, M., & Gandhi, S. (2015). Inhibitory Neuron Transplantation into Adult Visual Cortex Creates a New Critical Period that Rescues Impaired Vision Neuron, 86 (4), 1055-1066 DOI: 10.1016/j.neuron.2015.03.062

Frank, S., & Biglan, K. (2007). Long-term fetal cell transplant in Huntington disease: Stayin’ alive Neurology, 68 (24), 2055-2056 DOI: 10.1212/01.wnl.0000267703.35634.e1

Keene, C., Sonnen, J., Swanson, P., Kopyov, O., Leverenz, J., Bird, T., & Montine, T. (2007). Neural transplantation in Huntington disease: Long-term grafts in two patients Neurology, 68 (24), 2093-2098 DOI: 10.1212/01.wnl.0000264504.14301.f5

Nguyen, J. et al. (2009). Neuronal Transplantation: A Review. Practical Handbook of Neurosurgery p.1574-1584.

Sowden, J. (2014). Chapter 4 – Restoring Vision to the Blind: Stem Cells and Transplantation Translational Vision Science & Technology, 3 (7) DOI: 10.1167/tvst.3.7.6

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