By injecting human cells into mouse pups, scientists have created mice whose brains are part human. These hybrid mice grew up to be smarter than their peers, performing much better in tests for memory and cognition. Although this may sound like the plot of a terrible Sci-Fi film, researchers hope to glean a lot of valuable information from these experiments. For example, by studying brain diseases in whole organisms rather than in cells in a dish, researchers should gain a better understanding of how the conditions develop and progress.
For the study, which has been published in The Journal of Neuroscience, researchers from the University of Rochester Medical Center started off by isolating immature glial cells from donated human fetuses. Glial cells are one of the two main cell types that build the nervous system, the other being neurons. Glia perform a variety of roles in the nervous system, such as providing support and protection for neurons, but unlike nerve cells they do not participate directly in electrical signaling, which is a form of communication used to transmit information between cells.
The scientists then injected these cells into the brains of baby mice, where they were found to differentiate into star-shaped glial cells called astrocytes. These large cells, which are the most common type of glial cell, wrap their bushy tendrils around synapses, the active junctions between neurons through which chemical or electrical signals flow. In doing so, astrocytes provide support to neighboring neurons and also strengthen their synaptic connections, meaning they play a pivotal role in conscious thought.
Within just one year, the astrocytes had proliferated so much that they displaced the native cells, resulting in populations of cells in some brain areas that were largely, and sometimes entirely, of human origin. The cells eventually reached 12 million, but they only stopped replicating because they reached the physical limits of the space.
“We could see the human cells taking over the whole space,” lead researcher Steven Goldman told New Scientist. “It seemed like the mouse counterparts were fleeing to the margins.”
Because human astrocytes are dramatically larger than mouse astrocytes and possess 100 times as many projections, the transplanted cells could coordinate the signaling in neural networks much more efficiently than native cells. This essentially gave their brains an upgrade, but it didn’t make the animals more human.
The researchers then performed various memory and cognitive tests on the mice and compared them with control mice, which revealed that they were significantly smarter than their peers. One test even suggested that their memory was four times better than the controls. “These were whopping effects,” Goldman remarked.
At this stage, the researchers aren’t sure whether the cells are functioning in the same way in the mice as they do in humans, but they have more work planned to find out how the cells are affecting memory and learning. They’ve already started grafting human cells into rat brains, but they won’t be using primates because of ethical considerations.