Mice with millions of human neurons in their brains

Researchers implanted millimeter-scale cortical organoids made from reprogrammed human skin cells into mice engineered not to develop their own cortex. Placed two days after birth, the organoids vascularized, grew and connected with host tissue during a narrow plasticity window, producing animals whose cortexes are over 90% human neurons and contain roughly four million human cortical cells - comparable in developmental stage to a six‑month human fetus though far smaller than an adult cortex. Imaging and diffusion MRI show directed connectivity from human grafts into the mouse brain, and the chimeric brains even produced Von Economo neurons, a large specialized cell type previously seen only in large-brained mammals. Despite extensive human integration the mice show only minor behavioral differences, reflecting imperfect cross‑species wiring and missing cell types.

The humanized mice reproduced clinically relevant effects: induced oxygen deprivation created lesions resembling cerebral palsy, and earlier work with organoids carrying Timothy syndrome informed a genetic therapy that reversed pathology in rodents and is headed toward clinical testing. Researchers emphasize the model’s value for studying developmental psychiatric and neurological disorders and for testing interventions, while acknowledging limits - slower human neuronal maturation, incomplete cortical composition, and ethical constraints against extending such experiments to primates - leading to independent oversight and calls for caution.