Stanford Scientists Grow Human Brain Tissue Inside Mice, Filling Over 90% of Missing Cortex Space

Stanford University scientists have developed a new model for studying human brain development and neurological disorders by transplanting lab-grown human brain tissue into mice engineered to lack most of their cerebral cortex. According to findings published in Nature on September 16, the human tissue survived and expanded within months to occupy more than 90 percent of the cortical space, forming functional connections with the mouse brain and spinal cord.

The research, led by Stanford psychiatrist Sergiu Pasca, aims to offer a clearer window into how human neurons mature, interact, and respond to injury inside a living animal. The cerebral cortex is the outer layer of the brain responsible for complex thought, language, attention, and decision-making. Studying this region has historically been challenging because living human brain tissue is rarely accessible for research, and animal models often fail to capture human biological features accurately.

To overcome these hurdles, Stanford researchers spent over a decade refining techniques to grow three-dimensional clusters of human brain cells, known as cortical organoids, from skin cells reprogrammed into stem cells. While these organoids can self-organize into structures resembling the developing human cortex, they traditionally lack a blood supply, immune input, and full sensory or motor connections when kept solely in a culture dish.

In earlier research, the team transplanted human cortical organoids into newborn rats, observing advanced neuronal development compared to cell culture. However, the rat's native cortex developed rapidly and competed for space, limiting how much human tissue could integrate. To minimize this competition for the new study, the researchers engineered a strain of mice that lack the starter cells for most of the neocortex and related structures.

As adults, these specialized mice possess only about two percent of the cortical tissue found in ordinary mice, leaving a substantial cavity. The research team grew cortical organoids from healthy human donors and surgically placed them into two-day-old mouse pups. Over the subsequent three months, the human tissue thrived and expanded significantly.

By the three-month mark, the transplanted human tissue accounted for more than 90 percent of the cortical volume in the mice. The human neurons successfully extended projections, formed synapses, and integrated with remaining brain regions and the spinal cord. Behavioral testing conducted between three and six months post-surgery indicated that the xenocortical mice performed similarly to normal mice, though subtle differences in gait and memory were observed.

A key advantage of this xenocortical model is the ability to utilize cells derived from specific individuals. Because the organoids carry the donor's genetic material, researchers can create models reflecting the biology of specific patients, whether healthy or affected by neurodevelopmental disorders like autism, schizophrenia, epilepsy, and cerebral palsy.

In a proof-of-concept experiment, the modified mice were exposed to five hours of low oxygen. The human-origin cortical tissue sustained substantial damage, and the mice exhibited balance and movement difficulties mirroring aspects of cerebral palsy. Normal mice and mice without human tissue remained largely unaffected by the same oxygen deprivation, offering a potential method to study why human cortical neurons are vulnerable to oxygen stress during pregnancy or birth.

Another notable finding was the spontaneous appearance of von Economo neurons, or VENs, within the human tissue inside the mice. These large, cigar-shaped neurons are linked to social awareness and decision-making and are impacted in conditions such as frontotemporal dementia. Previously, VENs had only been observed in post-mortem human brains and certain large-brained social animals, rather than in lab cultures or earlier rodent models.

Work combining human neural tissue with animal hosts raises ethical considerations. Pasca and his colleagues have engaged ethicists, neurobiologists, patient advocates, and legal scholars over years of experimentation, including hosting a conference in Asilomar, California, in late 2025 to discuss the implications of stem cell models.

Proponents of the research emphasize its potential to alleviate suffering for hundreds of millions of people affected by untreatable neurological conditions. Stanford holds patents on aspects of the technology, and the research team anticipates that other laboratories will utilize the approach to investigate the underlying mechanisms of profound neurological and psychiatric disorders.