ICYMI: Human Brain Organoids Grow in Mice Missing Much of Their Cortex

This side view of a mouse brain shows nerve fibers from a transplanted human organoid (colored green and red) extending into the original tissue composing the animal's brain (colored blue).

The Pașca Lab

In a Frankensteinian feat, researchers at Stanford University transplanted human brain organoids into mice missing much of their cortex. The organoids grew into networks of active neural tissue and formed viable connections with the animals’ nervous systems. The work offers a model for studying neurodevelopment and injury processes. The team published the results of their experiment September 16 in the journal Nature.

This image illustrates how human organoids containing cells found in the human cortex integrate into a mouse brain that has failed to develop a cortex. The first image shows a normal mouse brain without any human intervention in its developmental course. The “apallial” mouse brain refers to mice genetically engineered to survive with little of their cortex: in this case, stunting half their brain volume. Researchers introduced human cortical organoids into the space left by the mice’s missing brain tissue. The resulting grafts developed and integrated into portions of the mouse's brain and spinal cord. The team referred to these mice as “xenocortical” — the prefix “xeno” means “stranger” or “foreign.”

The Pașca Lab

Scientists genetically engineered mice to lack much of their neocortex and hippocampus, reducing their brain volume by nearly half. They then grafted human cortical organoids, or structures derived from stem cells resembling features of the cortex, into cavities left by the missing tissue in the living newborn mice. The grafts filled the missing cortical tissue volume in the mice’s brains. This achievement, along with other feats using brain organoids, provides a novel model for studying neurodevelopment and injury.

Big Picture: Scientists use brain organoids to study human brain development, function, and disease. Previously, members of the same research team transplanted cortical organoids into smaller sections of developing rat brains to form connections with their nervous systems. But the recent study showed organoids can be grafted onto larger volumes of an organism’s brain, providing more opportunities for human tissues to connect across mice’s nervous systems. The researchers involved acknowledge the unique ethical considerations posed by the approach. In November 2025, Sergiu Pașca, senior author on the 2026 Nature paper, co-organized an event for scientists, ethicists, patient advocates, journalists, and policymakers to discuss developing oversight systems around neural organoids and assembloids.

Read More: These Mice Have Partly Human Brains. The New York Times

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About the Author

Bella Isaacs-Thomas

Bella is the Staff Writer and Editor for BrainFacts. She previously worked as a digital science reporter at PBS NewsHour, where she was named a fellow in the 2022-2023 class of the National Science-Health-Environment Reporting Fellowships (SHERF) and earned a 2021 D.C. Science Writers Association Newsbrief Award for writing. She graduated from the University of Michigan with a bachelor's degree in 2018.