What happens when human brain tissue is implanted into rats

. Henry David Thoreau’s maxim is one which scientists generally take to heart when investigating complex natural phenomena. And there is no known natural phenomenon more complex than the human brain. Since 2011, therefore, Sergiu Pasca of Stanford University, who studies it, has been doing just that. His simplified models are structures called cortical organoids. They are spheres a few millimetres across, composed of specially grown human nerve cells, which act as simulacra of brain tissue. Now, however, he is ready for a bit more complexity. As he and his colleagues report this week in , they are wiring their organoids into rats’ brains (see picture above). And not only that. They have also found that their implanted organoids can perceive the outside world in a limited way, by tapping into a rat’s senses, and can affect that world by controlling the animal’s behaviour. On a more practical note, when grown from cells taken from patients with a particular genetic illness, implanted cortical organoids can yield information on the cellular abnormalities underlying that illness in a way which conventional approaches have not been able to.