A mouse grew a human cortex, and it wired itself in

I read this one twice to make sure I had it right. A team at Stanford, led by Sergiu Pașca, published a study in Nature on September 16 about mice that were engineered so that almost no cortex of their own would form. Single-cell profiling put it at about 98% of the cortex and hippocampus missing. The mice still walked around and squeaked.

Then the researchers took lab-grown human brain tissue, called cortical organoids, and transplanted it into newborn mice like that. It worked in 25 out of 29 animals. The human tissue didn't just sit there. It divided and spread into the empty space, growing about 4.7-fold between the second and third month, until human cells made up most of the cortical volume.

The part that got me was the wiring. Human-derived projections reached all the way down to the cervical spinal cord. That didn't happen in earlier experiments, where organoids went into mice that still had their own cortex. Give the tissue room and it builds long-range connections. The grafts also produced von Economo neurons, which is a cell type you don't usually see in a dish.

The practical point is disease modeling. The team used these mice to study hypoxia, the lack of oxygen that damages developing brains, and the human tissue was hit harder than the mouse tissue, with effects on how the animals walked. That gives you a model for cerebral palsy that nobody could build before.

I think the ethics conversation around this is going to get complicated fast, and I think it should.

Source: https://medicalxpress.com/news/2026-09-mice-human-brain-organoids-space.html