Here’s how Stanford scientists got human brain tissue to thrive in mice to advance neuroscience research

Researchers genetically modified the brains of mice using human tissue after finding them to be 'a hospitable environment'. Scientists at Stanford Medicine have successfully transplanted self-organising bits of laboratory-grown human brain tissue, known as cortical organoids, into mice that have been specially bioengineered and bred to have most of their cerebral cortex missing. The cerebral cortex is the outer layer, or “rind,” of the brain that controls higher-level functions like awareness, reasoning, understanding, language, attention and decision-making. Lab-bred mice with no cortex According to the researchers Sergiu Pasca and Alison Singer, the missing cerebral cortex in the mice’s brains proved to be a “hospitable environment”. The scientists observed that the human tissue survived, thrived, grew, and developed working connections with the mice’s brain and beyond, reaching the spinal cord. Sergiu Pasca said that these animal models offer a unique opportunity to study how disease-associated alterations in human brain circuitry manifest in an intact nervous system. ‘Never seen before results’ The researchers were surprised to find an important nerve cell type in these mice that hasn’t been seen in laboratory cultures; it has only been seen in autopsied human brains. According to Pasca, the new methodology should speed research into the underlying biological causes of schizophrenia, epilepsy, profound autism and cerebral palsy. “Neuroscientists will be able to learn much more about the causes and mechanisms of neurodevelopmental and pregnancy-incurred disorders, and to test possible interventions to correct or prevent them,” Pasca highlighted. Complex brain studies Pasca further explained that such conditions are extremely difficult to study at the molecular and cellular level because the brain is more complex than any other organ and living human brain tissue is almost always inaccessible. Autism Science Foundation president Alison Singer said one in every 218 American children meets the criteria for profound autism. “This is an unarguably disabled state that requires round-the-clock supervision to ensure safety, as well as for assistance with activities of daily living such as getting dressed, using the bathroom and preparing food,” she noted. Xenocortical mice Singer said that profoundly autistic patients are vulnerable to sleep disorders, epilepsy and self-injury, and they are disproportionately likely to exhibit aggressive behaviour. The researchers developed “xenocortical” mice in the laboratory and also found evidence of human cortical nerve cells’ vulnerability to oxygen deprivation, which can cause brain injury during pregnancy or around birth. “While animal models have been extremely helpful, some biological features seem to be uniquely human,” Pasca said. Limitations “We can now generate a broad range of brain cell types, present throughout the developing human nervous system,” Pasca said. They transformed human skin cells into stem cells that can differentiate into most of the body’s cell types, then gently modified the cells to form self-organising clusters of brain tissue that closely resemble circuits in the cerebral cortex. Pasca acknowledged the study’s limitations. “We couldn’t study complex human behaviour in a dish,” he said. To address these limitations, Pasca noted the idea of integrating human organoids into a living rodent’s brain, particularly when the brain was still in an early stage of circuit formation. Back in 2022 Dating back to 2022, when the study was still in its early days, Pasca and his colleagues transplanted human cortical organoids into newborn mice’ brains and saw much better neuronal development than could be achieved in a petri dish. The implanted human organoids occupied a minuscule part of the brain when transplanted, and just six months later they occupied one third of the mouse brain hemisphere. These researchers have unlocked an advanced avenue in neuroscience because, after eight months of testing, they observed that individual neurons from the human organoids in the rodent’s brains were substantially larger and more electrically active than those in the organoids. Despite being generated the same way, at the same time, these neurons remained in a dish, instead of being transplanted. New and impressive discovery unlocked Researchers noted that the organoids displayed more sophisticated branching patterns and integrated with their rodent-neuron neighbours to form functional brain circuits. “The cells we implant carry the genetic material of the person they’re derived from -whether that person is a patient or a healthy individual – allowing us to study downstream disease effects in brain cells and circuits,” Pasca said. In the study conducted in 2022, Pasca’s team implanted brain tissue from young patients with a rare but devastating genetic condition called Timothy syndrome, characterised by severe autism and heightened vulnerability to epilepsy and schizophrenia, into neonatal mice’s cortical regions. Proof is in the pudding Pasca and his team pinpointed the disorder’s molecular defect and later found a candidate drug to treat it. That drug is now entering its second phase of safety trials, but even then, Pasca said, there were limits. “The rat neurons developed more quickly than the human neurons did.” “The two parallel developing systems are in competition for turf,” Pasca explained. Apallial mice In the latest study, Pasca’s team came up with a way to reduce the competition by producing a laboratory mouse strain that was genetically engineered so that the “starter cells” prevent the evolutionary cerebral cortex- the neocortex- and other brain structures from forming. After a series of technically challenging steps, the team successfully generated mice that survived in healthy condition despite the absence of most of their cortex and hippocampus, a brain structure important for memory formation. These mice are called “apallial mice” because the missing cortical tissue develops from a prenatal brain structure called the pallium. Observation While largely much indistinguishable from normal mice at first glance, Pasca said that the apallial mice exhibited minor but measurable behavioural quirks: a slightly more cautious gait, probably reflecting somewhat compromised fine-motor coordination, and a tendency to be more forgetful about recently encountered novel environments. Their behavioural performance also varied more from one individual to the next compared to the normal control mice, suggesting that the cortex has a stabilising role in brain function. Putting their theory to test In a proof-of-concept experiment showing how these mice could be used to study disease, researchers exposed xenocortical mice to five hours of low oxygen. Pasca said the exposure caused substantial damage to their human-originated cortical tissue. They observed that the oxygen-deprived xenocortical mice appeared to have trouble moving steadily and maintaining their balance. This is also seen in children with cerebral palsy. Testing results and ethics Pasca noted a contrast between normal and apallial mice, which were virtually unaffected by reduced oxygen levels over a long period of time. “Finding out what accounts for this difference could yield clues about human neural susceptibility to oxygen deprivation, shed light on mechanisms underlying cerebral palsy and provide a platform for testing potential therapeutic strategies,” Pasca said. Pasca stressed the importance of remaining within ethical boundaries while reflecting on his research and emphasised that they have received input from ethicists, neurobiologists with expertise in primate and human cortical biology, patient advocates, philosophers and legal scholars. “An overriding argument questioned the ethics of not conducting this research in the face of the suffering of hundreds of millions of people afflicted with neurological disorders that today are incurable but tomorrow could yield treatments we discover by using this model,” Pasca concluded.