In a study published on September 16 in the journal Nature, scientists from Stanford Medicine successfully transplanted fragments of lab-grown human brain tissue into genetically modified mice. These mice were specially bred to have a near-complete absence of the cerebral cortex, the part of the brain responsible for higher functions like thought and sensory processing. The human tissues survived in the mice's brains, developed, and established functional connections with the mice's brains and spinal cords. According to Sergiu Pasca, a professor of psychiatry, these animal models offer a unique opportunity to study how disease-related changes in human brain circuits manifest in a complete nervous system. The study could help better identify and understand the biological causes of conditions like schizophrenia, epilepsy, profound autism, and cerebral palsy. Pasca noted that neuroscientists could learn more about the causes and mechanisms of neurodevelopmental disorders and disorders related to pregnancy, and test potential interventions to correct or prevent them. He added that while animal models have been extremely useful, certain biological characteristics appear to be unique to humans. The mice had a deficit of about 14 million mouse brain cells and ended up with about 4 million human cells, half the volume of their brain. The procedure allows the harvesting of cells from patients with brain disorders and transforming them into brain tissue in the laboratory. These animals can then be studied to observe how the disease develops in human brain tissue and how medications could treat these conditions. Three months after the operation, the human tissues had connected to the mouse's circulatory system. Some human neurons had established connections with the mouse's brain cells and spinal cord. As part of the study, the researchers exposed some animals to five hours of low oxygen concentration. This highlighted the vulnerability of human nerve cells to oxygen deprivation, which during pregnancy and childbirth can cause cerebral palsy. While brain organoids represent hope for brain medicine, this approach has raised a wave of ethical concerns, particularly regarding animal welfare. Emily Jackson, a professor of law at the London School of Economics and president of a recent report on neuronal organoids for the Nuffield Council on Bioethics, explained that animal welfare is a major concern and it will be necessary to closely monitor these animals to assess the impact of these research on them. The study marks a significant step forward in understanding human brain development and disease, but it also underscores the need for careful ethical oversight in such research.