Scientists, by experimenting, have transplanted human brain tissue into the heads of mice and rats. Surprisingly, the pinhead-sized ''plugs'' of human brain cells not only grew extensive and lasting links with rodent neurons but also appeared to start firing as though they had become part of the animals' brains.
The research could now be a powerful tool for studying how the brain works and how its disorders could be treated.
However, this also raises ethical questions on the extent to which a human brain should be hybridised with another species.
There were two separate studies. Both teams began with human stem cells which were converted into neurons and grown into tiny 3D structures known as cerebral organoids.
One of the groups, led by Isaac Chen at the University of Pennsylvania, grafted the organoids into the visual cortices of 11 adult rats. Two months later, the scientists discovered that the cells were still present in seven of the animals' brains, where they had formed connections stretching as far as 1.55mm. The human tissue did not produce any electrical activity on its own, but it seemed to light up when the rats' eyes were stimulated. This suggests that it might have, in a way, integrated with its new environment.
The second group led by Fred Cage at the Salk Institute in La Jolla, California, involved a similar procedure in mice.
The problem with all these is that once the door is open, it could lead to all sorts of scenarios including the 100 per cent humanisation of an animal's brain, with all the attendant ethical concerns.
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