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Can brain tissue from one species be implanted into the brain of another species? Because the brain is tolerant to the introduction of antigens (it is said to be immuno-privileged), I was wondering whether neurons would be subject to graft rejection?

If neurons from another species survive in their new host, would they create functional connections?

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    $\begingroup$ To add on AliceD's comment. You should define a specific trait on which you are interested in. For example, you may ask if mice neurons tend to be shorter than human neurons (yes for many motor neurons) or whether there is more myelin on the mice neurons than on human neurons. $\endgroup$ – Remi.b Jul 13 '15 at 0:12
  • $\begingroup$ Added information to the question, explaining what i mean by "differences" $\endgroup$ – Mystery Jul 13 '15 at 0:30
  • $\begingroup$ @AliceD: it now excludes for example differences in size $\endgroup$ – Mystery Jul 13 '15 at 0:33
  • $\begingroup$ Ok, I see now, there isnt an "easy" answer to my question. I dont think, continuing this question will lead to any useful information, due to the topics complexity... the question can be closed $\endgroup$ – Mystery Jul 13 '15 at 22:30
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    $\begingroup$ I know of some papers where neural stem cells (of rats/humans) have been transplanted in mice and there is a change (enhancement in vague terms) of function. AFAIK there is some structural difference too (the size of the processes etc). I can answer your question in a while (a week or two) because I need to dig up these papers. Lots of deadlines at this moment. $\endgroup$ – WYSIWYG Jul 14 '15 at 10:30
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Short answer
Neural xenografts can survive and form functional connections in the host.

Background
Across-species (xenogeneic grafting) grafting of cerebral cortex from a fetal rat to an adult mouse brain and vice versa, have shown that immunocompetent hosts can reject a graft in less than 30 days. Rejection results in destruction of the graft tissue presumably by macrophages. Immunosuppression can be used to prevent graft rejection (Mason et al., 1986).

A review article on this topic shows that in the absence of immunosuppression, neural xenografts (donors were fetal to adult-aged) in a rat host brain survive in 37% of the cases in case of a mouse donor, 16% in case of a hamster donor, 9% in case of a rabbit rabbit, 7% from a pig donor, and 0% when human donor tissue was used. Hence, species-separation seems to be a determining factor in xenograft rejection. Induction of MHC-I and MHC-II expression in the xenograft mediates graft rejection in the brain. Hence, despite the fact that the brain is immunopriviliged because of the relative low expression levels of MHC class proteins, it is still sufficient to reject grafts (Pakzaban & Isacson, 1994).

Xenografts have been shown to generate functional afferent and efferent axonal connections with the host brain when grafts of the striatum, substantia nigra, hippocampus and retina were homotopically transplanted.

Functional (behavioral) studies have confirmed these physiological studies. Rats with Parkinson's disease could be successfully treated with substantia nigra grafting from mouse, hamster, rabbit, pig and even humans. Similarly, retinal xenografts have resulted in functional pupil responses, hippocampal xenografts have restored learning deficits, and suprachiasmatic nucleus xenografts have been shown to restore circadian rhythms (Pakzaban & Isacson, 1994).

References
- Mason et al., Neurosci (1986); 19(3): 685-94.
- Pakzaban & Isacson, Neurosci (1994); 62(4): 989-1001.

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