Is it possible to transplant human brains inside an immune ablated animal host, such as dogs?

Question

Is it possible to transplant human brains inside an immune ablated animal host, such as dogs?

Answer

The brain, like any other organ of the mammalian organism, can be transplanted, in theory. The first attempts of brain tissue xenotransplants were attempted by Gilman Thompson in 1890, between cats and dogs. Although the neocortical pieces exchanged were viable, they were also neuron-free, and possibly composed of scar tissue. A century later, in 1979, experiments in rats showed that rat foetal dopamine neurons were able to reduce experimentally-induced Parkinsonism in rats. Whole autologous brain structures such as the adrenal cortex was transplanted in rats, with or without administration of 200 units/100 gram weight of human chorionic gonadotrophic hormone. Catecholamine and norepinephrine assays showed that the transplants were more active in the presence of the human hormone and possibly functional seven weeks post-transplantation. In 2002, thirty patients with advanced Parkinson disease received human foetal brain tissue. Results showed that there was an alleviation of the symptoms of the disease and importantly there was no graft versus host related inflammation, either systemically or locally. There are still many obstacles that need to be overcome. Firstly, once the connection of the brain to the spinal cord has been severed, its successful reconnection to the host’s (or the donor’s) is yet to be reported. Secondly, although the brain is an immunoprivileged organ, i.e. the blood-brain-barrier somewhat hinders immune cell invasion, local inflammation due to the innate immune response could trigger a massive neuronal and glial cell loss. Thirdly, the difference in brain size between dogs and humans makes it extremely difficult to achieve a successful transplantation using the former as a host. Lastly, the endocrine system within humans and certainly within species is finely-tuned to meet the needs of each brain, thus any disruption in that balance will most likely have unfavourable effects. After nearly 150 years of research, brain transplantation remains fictional.

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