A team of researchers has managed to introduce a number of human brain cells into mouse brains, making them partly human. The new mice have grown to be more intelligent than their ordinary mice counterparts and have a demonstrated an improved performance in cognitive ability tests.
This may seem too bizarre for some people, but scientists aspire to draw a lot of useful insights from these trials. For example, through examining brain diseases in whole organisms, they will be able to expand their knowledge on the ways health problems develop.
In another experiment, human glial cells were injected into mouse pups that were poor at making myelin, a protein that insulates nerves. Many of the human cells matured into oligodendrocytes, brain cells that specialize in making myelin. This suggests that the cells somehow detected and compensated for the defect.
This could be useful in treating diseases in which the myelin sheath is damaged, such as multiple sclerosis.
Human glial cells
For the purposes of the study that appears in the Journal of Neuroscience, the scientists from the University of Rochester Medical Center have begun by distinguishing glial cells from given human embryos. Glial cells are among the key cell forms that act as building blocks of the nervous system together with neurons. Glial cells serve a lot of duties within the nervous system such as supporting and protecting neurons from damage. However, unlike nerve cells, they are not involved in the electrical signaling of brain cells, which is a way to communicate information between cells.
The scientists grafted glial cells from donated human fetuses into the brains of young mice, where they were observed to have transformed into a new form of glial cells with a star-like shape, called “astrocytes”. These large-sized cell types are one of the most common forms of glial cells, they envelop their thick tendrils around synapses, which are active junctions between neurons through which chemical and electrical messages flow. This function allows astrocytes to strengthen the synaptic connections to nearby neurons.
During the course of the year, the astrocytes spread so much that they displaced existing cells, which lead to regions of the brains largely, and sometimes entirely, of human brain cells. The cell count reached 12 million, from the original 300,000 cells each mouse received through injection. However, they eventually stopped due to the physical limits of the space available.
“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.”
The human brain is relatively large and very wrinkled. Wrinkles increase the surface are for neurons.
Due to the fact that human astrocytes are 10-20 times bigger than mice astrocytes and have 100 times more projections, the injected cells could orchestrate the communication between neural pathways more effectively than existing mice cells. “It’s like ramping up the power of your computer,” says Goldman.
However, Goldman rejects the idea that this procedure of human cell injections could make mice more human. “This does not provide the animals with additional capabilities that could in any way be ascribed or perceived as specifically human,” he says. “Rather, the human cells are simply improving the efficiency of the mouse’s own neural networks. It’s still a mouse.”
To evaluate the cognitive ability improvements of the mice the scientists had carried out multiple cognitive tests and compared them with the performance metrics of existing mice.
The results have shown that the mice scored better at these tests than typical mice. One test involved the mice remembering that a sound was associated with a mild electric shock. When the mice heard the sound they froze 4 times as long as the control group, this has been interpreted as suggesting that their memory was 4 times better than the control mice group
“These were whopping effects,” says Goldman. “We can say they were statistically and significantly smarter than control mice.”
At this point, researchers can’t tell for sure whether the cells in mice are working in a similar manner to human cells and admit that there is still a lot of work to be done. In particular, they are hoping to find out how the cells affect memory and learning.
They have already started to graft human glial cells into rat brains, but say they will avoid using primates because of ethical considerations. “We briefly considered it but decided not to because of all the potential ethical issues,” Goldman says.