For several days in mid-September, neuroscientist and stem-cell biologist Sergiu Pașca became inseparable from his phone. As one of the principal authors on a study by researchers at Stanford University in California reporting the successful transplant of laboratory-grown human brain ‘organoids’ into mice, he was doing many interviews every day.
The mice had been bred so that almost all of their cerebral cortex was missing from their brains. The transplanted lab-grown human tissue survived and developed working connections to the mice’s brains and spinal cords (K. Kaganovsky et al. Nature 658, 430–441; 2026).
It is without doubt a landmark achievement. Animal studies can reveal only so much about the human brain, and options to study live human brain tissue inside a person are limited. Around a decade ago, the creation of organoids — tiny 3D cellular structures grown in the lab that mimic some functions of real organs — changed this. The latest finding, which represents the most extensive integration of human brain cells into an animal so far, opens up further possibilities to understand conditions such as cerebral palsy and autism spectrum disorders in a nervous system, and to test possible interventions.
At the same time, lab-grown organoids are prompting ever deeper ethical questions. Researchers are asking whether the animals receiving implanted brain organoids might gain any human-like properties of consciousness or cognition. How should these characteristics be measured, and what extra oversight should there be for such studies?
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