Meet a mouse whose brain cortex is made up of human cells

A team of neuroscientists at Stanford University, led by Sergiu Pașca, has successfully introduced human brain cells into the brains of genetically modified mice. The researchers used human brain organoids—small, lab-grown clusters of neural tissue that mimic parts of the human brain. These organoids were injected into mice whose brains were genetically altered to lack most of their cortex and hippocampus, two critical brain regions responsible for functions like memory and movement. The human cells filled the empty space, growing to occupy nearly half of the mice’s brain volume within weeks. This experiment demonstrates how human neural tissue can integrate and function within another species, raising new possibilities for studying brain development and diseases.

The mice without human brain cells showed noticeable memory deficits. In a maze test, they struggled to remember which areas they had already explored, suggesting their missing brain regions impaired their ability to navigate. In contrast, mice with human brain cells performed better in the same maze test, indicating that the human tissue contributed to improved cognitive function. The mice with human cells appeared physically normal, walking and squeaking like typical mice, but their enhanced maze performance suggests the human tissue played a role in their brain activity. This outcome highlights the potential of human brain organoids to compensate for missing brain functions in animal models.

Sergiu Pașca and his team believe these genetically modified mice, which they call xenocortical mice, could become valuable tools for studying brain injuries and neurological disorders. The human cells in the mice’s brains may provide insights into how human brain tissue behaves in a living organism, offering a new way to test treatments for conditions like Alzheimer’s or Parkinson’s disease. Pașca also noted that brain organoids are already being explored for connecting to computers to play video games and as potential replacement parts for treating stroke victims. However, the research also raises ethical questions about the limits of mixing human and animal tissues in brain studies.

Pașca and his colleagues have consulted ethics experts to address concerns about the implications of this research. One major worry is the possibility that animals with human brain tissue could develop human-like consciousness, though Pașca argues this is unlikely in mice due to their small brain size and the significant evolutionary distance between humans and mice. He strongly cautions against conducting similar experiments in higher species, such as monkeys, where larger brain volumes and closer evolutionary ties could blur the cognitive boundaries between humans and animals. Pașca emphasizes that such experiments in primates are not justified at this time and should be avoided.