A team led by Stanford University neuroscientist Sergiu Pașca introduced mice whose brains are filled with human cells by roughly half in a study published in Nature. The researchers used mice genetically modified to lack most of the cells in the cortex and hippocampus. This allowed human brain organoids implanted after birth to grow and form connections with the mouse nervous system.
Previous work by Pașca’s group had shown that human organoids can survive and function in the brains of juvenile rodents. In the new experiment, mice lacking brain tissue could walk and make sounds, but struggled to remember the sections of the maze they had explored. Mice given human cells performed better in the test; the finding showed that human tissue plays a role in cognitive functions.
Pașca believes these models, which he calls “xenocortical mice,” could be useful for studying brain injuries. Carsten Charlesworth, who was not involved in the study, drew attention to the power of genetic engineering and stem cell technology to reshape biology. Pașca stated that the mice do not possess human-like cognitive capacity because of their small brains and the evolutionary distance between the species; however, he opposed conducting similar experiments in primates.
Why it matters
The significance of the study lies in opening up for experimental investigation the question of whether human brain tissue can not only survive within another species’ nervous system, but also participate in functions associated with behavior. While this approach provides a framework for how models that could be used to study brain injuries might be established, it also concerns the fields of neuroscience, stem cells and genetic engineering. However, although the results show the contribution of human cells to the cognitive test, this does not mean that these mice possess human-like cognitive capacity. The open ethical question is whether applying the same method to primates would be acceptable and at what point the role of human cells in the animal brain would create different boundaries.