Scientists at Stanford University have successfully created mice containing partially human brains by transplanting lab-grown human brain cells into animals that were genetically engineered to lack a cerebral cortex and hippocampus. The procedure involved injecting newborn mice with human brain organoids derived from reprogrammed skin cells, with each animal receiving multiple injections totaling approximately 100,000 human brain cells. Within three months, the human tissue integrated with the mouse’s blood supply and filled the cavity where the mouse brain tissue was missing, ultimately comprising about half the brain volume alongside approximately 4 million human cells.
The research aims to advance understanding and treatment of brain disorders including schizophrenia, epilepsy, cerebral palsy, intellectual disability, and rare forms of dementia. By using patient cells to create brain tissue that can be studied in living animals, researchers can observe how neurological conditions develop in human brain tissue and evaluate potential drug treatments. According to Sergiu Pașca, the Stanford professor leading the research, this approach addresses a significant gap in neuroscience, as the complexity and inaccessibility of the human brain has hindered development of therapeutics compared to other medical fields.
The work builds on previous efforts in the field of neural organoids, where human brain cells grown in laboratories self-assemble into complex structures mimicking aspects of real brains. The xenocortical mice—those containing the human tissue transplants—did not show cognitive enhancement from the transplants, though their balance and memory issues improved slightly. Researchers demonstrated the model’s potential by exposing some animals to low-oxygen conditions, revealing vulnerability of human nerve cells to oxygen deprivation relevant to cerebral palsy.
The research has generated ethical considerations regarding animal welfare and the consciousness of organoid tissue. Pașca noted the work received extensive ethical oversight from its inception. Emily Jackson, chair of a recent report on neural organoids for the Nuffield Council on Bioethics, emphasized the necessity of closely monitoring transplanted animals to assess impacts on their wellbeing. Some experts noted that while organoid work may reduce overall animal research, alternative in vitro approaches remain the field’s longer-term goal.
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