Scientists at Stanford University have successfully transplanted human brain tissue into mice, creating animals with approximately half their brain volume composed of human cells. The procedure involved taking mice that were genetically engineered to lack a cerebral cortex and hippocampus, then injecting them with human brain organoids derived from reprogrammed skin cells. Each mouse received multiple injections totaling approximately 100,000 human brain cells into the cavity where their own brain tissue was absent.
Within three months of the procedure, the human tissue integrated with the mouse’s blood supply and nearly filled the empty space, occupying roughly half the rodent’s brain by volume. Some human neurons formed connections with the mouse’s existing brain cells and spinal cord. Although the human tissue remained structurally immature and not organized as it would be in a human brain, the xenocortical mice showed modest improvements in cognitive function and motor control compared to baseline measurements.
Researchers led by Sergiu Pașca noted that this approach enables them to study how brain disorders develop in human tissue within living organisms and to test potential pharmaceutical treatments. The technique involves taking cells from patients with conditions such as schizophrenia, epilepsy, and dementia, converting them into brain tissue in laboratory settings, and then cultivating that tissue within the animals for observation and experimentation. The team demonstrated the model’s potential by exposing some mice to oxygen deprivation, revealing the vulnerability of human nerve cells to such conditions.
The work has generated significant ethical concerns within the scientific community, particularly regarding animal welfare and questions about whether organoid tissue might achieve consciousness or experience pain. Pașca indicated that the research underwent comprehensive ethical review from inception. Experts emphasized the need for continued monitoring to assess effects on animal subjects. Some researchers noted that while this approach may be necessary for studying certain disorders, the scientific community continues pursuing fully in vitro solutions to reduce animal research requirements.
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