Human Brain Cells Flourish in Mice: A New Frontier in Neuroscience Research

In a groundbreaking experiment conducted at Stanford University in California, mice have been implanted with millions of functioning human neurons, creating what researchers are calling “alien-cortical mice.” These mice were genetically engineered to lack key parts of their brains, including the cerebral cortex, the region responsible for complex cognitive functions like perception, decision-making, and attention. The findings, published in the journal Nature, suggest these modified mice could provide a novel avenue for studying neurological disorders such as schizophrenia, cerebral palsy, and dementia.

The experiment involved genetically modifying mice to develop without most of their cerebral cortex and hippocampus. At two days old, these mice received injections of human “organoids”—miniature cell structures grown in labs from stem cells, mimicking human organs. Each organoid contained approximately 100,000 human cells. In total, researchers replaced about 14 million mouse neurons with roughly 4 million human neurons. The Stanford team emphasized that this procedure does not result in mice that think like humans.

The transplantation was attempted on 29 mice, with successful integration in 25. Over three months, the human brain tissue grew significantly, expanding about five times in size, integrating with the mice’s blood supply, and occupying over 90 percent of the vacated brain space. Despite human tissue comprising roughly half of their brain volume, the “alien cortex” mice exhibited largely normal locomotion and vocalization. In cognitive tests, specifically a maze challenge, mice with human brain cells performed better than those without, indicating a contribution to cognition.

Remarkably, some of the human tissue developed into von Economo neurons, specialized cells typically found in highly social species like humans and apes, but absent in mice and difficult to cultivate in labs. This research may also shed light on cerebral palsy. While human infants are susceptible to oxygen deprivation during childbirth, a rare occurrence in mice, the modified mice in this study showed brain damage, altered gait, and poor limb coordination when subjected to low oxygen conditions, mirroring aspects of the human condition.

Studying human brain diseases is exceptionally challenging due to the brain’s complexity and ethical limitations in accessing living human brain tissue. While lab-grown “organoids” have been a valuable tool, they lack the intricate connections and environmental context of a living brain. Sergiu Pasca, a professor at Stanford University and senior author of the study, highlighted the potential of this research to offer unprecedented insights into the human brain, stating, “For the past two decades, there’s been a quest to try to build models of the human brain outside of the human body. This is not going to replace all the models we had before, but it’s going to provide us access to other aspects of human brain function that would be very difficult to study otherwise.”

As brain organoid research advances, so does the discussion around ethical regulations. Concerns have been raised about the potential for these cell clusters and grown brains to develop sentience or consciousness. The researchers deliberately concluded their experiments when the human nerve cells were six months old, before significant neural connections with the animal’s brain could form and before any signs of consciousness might emerge.

John Evans, a sociology professor at the University of California, San Diego, noted that while this specific experiment doesn't present major ethical issues, it paves the way for future research that could. He suggested that current research should remain limited to rodents to avoid creating neurohybrid organisms that could spark significant ethical debates. Establishing clear boundaries for this field of research remains a critical challenge.