Human brain organoids integrate extensively in mice lacking cortical cells

Lab-grown human brain organoids transplanted into newborn mice that lack a cerebral cortex expanded to occupy over 90% of the vacant brain cavity and formed connections with the host nervous system, researchers reported. The study, published in Nature, describes the most extensive integration of human brain cells into an animal model to date and aims to provide a more realistic platform for testing drugs and investigating developmental brain disorders.

The mice were engineered so that precursor cells destined to become the cerebral cortex failed to survive, leaving a large empty space. Without competition from mouse neurons, the implanted human tissue grew nearly fivefold within two to three months, sending projections deep into the spinal cord and differentiating into specialized neuron types, including large spindle-shaped cells resembling von Economo neurons.

"We now have a very powerful new system for understanding what makes the human brain unique and uniquely susceptible to disease," said co-author Sergiu Pașca, a neuroscientist at Stanford University. The team emphasized that extensive ethical oversight, including review by independent bioethics panels, guided the work.

Developmental neurobiologist Madeline Lancaster, who was not involved in the study, noted that the procedure was performed days after birth, after the core wiring of the mouse brain was established, reducing concerns that human cells could affect higher-order cognition. Behavioral tests showed no enhancement of the rodents' intellect.

The approach builds on earlier work in which human organoids were grafted into newborn rats, but those models faced competition for space from rapidly growing rodent brain tissue. By removing that competition, the current study achieved far greater integration, offering a promising avenue for evaluating therapies such as antisense oligonucleotides for conditions like Timothy syndrome, autism and epilepsy.