Xenocortical mice did not appear out of nowhere. They are the latest step in a decade-long arc of experiments, each one moving human neural tissue a little further into another animal's brain. Here is the timeline, with the papers.
2017 — Human neurons in the mouse cortex
A team led by researchers including Raquel Real transplanted human neural progenitor cells into the mouse cortex and showed that the human cells matured and took on the characteristic identities of cortical neurons. The mouse brain was still overwhelmingly mouse — the human contribution was a sprinkle of cells — but the proof of principle was in: human neurons could grow up inside a mouse.
2018 — Vascularized human organoids inside a mouse brain
Ali Mansour, working in the lab of Fred Gage at the Salk Institute, transplanted human brain organoids into the mouse brain and achieved the critical milestone: the organoids became vascularized by the host's blood supply and showed maturation and functional integration that dish-grown organoids could not achieve. Nature Biotechnology called it an in vivo model of functional and vascularized human brain organoids. For the first time, a human "mini-brain" had a blood supply.
2022 — Human cortical organoids wired into rats
Sergiu Pașca's Stanford lab published in Nature the transplantation of human cortical organoids into the brains of young rats — animals with a full, intact cortex, unlike the engineered mice of 2026. The human neurons matured dramatically, sent connections throughout the rat brain, and — strikingly — could influence the animals' behavior: stimulating the human graft produced reward-seeking responses. The trade-off, noted in later coverage, was space: the rat brain was crowded, and the graft had little room to grow.
2026 — The cavity opened: xenocortical mice
The September 2026 Nature paper removed the constraint. Instead of adding human tissue to an already full brain, the Pașca lab engineered mice whose cortex and hippocampus barely form — leaving an empty cavity — and injected human organoids into the vacancy. Three months later the graft was vascularized, expanded to about four million cells, and made up roughly half the brain by volume, with connections reaching toward the spinal cord. The word "xenocortical" entered the scientific vocabulary, and The Guardian put the result on its front page.
2017: a sprinkle of human cells. 2018: a blood supply. 2022: behavior. 2026: half the brain.
What comes next
Two directions pull at once. One continues the in vivo arc: larger grafts, more integration, better models of human disease. The other pushes toward the opposite pole — fully in vitro organoids that never enter an animal at all, which researchers like Madeline Lancaster see as the field's ethical destination. The history so far suggests both will keep moving. The line they trace is the same one: more human brain, further from a human body.
Sources
- Real, R., et al. (2017). Human neural progenitors differentiate into distinct neuronal lineages after transplantation into mouse cortex. Cell Reports, 19, 978.
- Mansour, A. A., et al. (2018). An in vivo model of functional and vascularized human brain organoids. Nature Biotechnology, 36, 432–441.
- Revah, O., et al. (2022). Maturation and circuit integration of transplanted human cortical organoids. Nature, 611, 518–526.
- Pașca, S. P., et al. (2026). Xenocortical transplantation of human cortical organoids in mice. Nature. doi:10.1038/s41586-026-11032-2
- Sample, I. (2026, September 16). "Scientists create mice with part-human brains." The Guardian.
