Clumps of human neurons implanted into the brains of newborn rats have successfully integrated into the animals' brain circuitry, according to a study published yesterday. The procedure is expected to shed light on psychiatric disorders like autism by illuminating how genetic mutations influence neural circuits. Stanford researchers, led by Dr. Sergiu Pașca, reported the clumps of human cells—known as organoids—replicated millions of new neurons over several months, wiring themselves into the rat's brain. The grafted tissues were shown to be integrated when they responded to a puff of air on the animals' whiskers, among other tests. The team will use the procedure to observe how diseased organoids grow within neural circuits, which isn't possible in an isolated petri dish. The novel technique raised ethical questions, with the team consulting ethicists prior to experimentation on questions of animal welfare and how to classify organisms with hybrid brains. Learn the basics of the humane use of animals for research here.