Human ES cells grow in mice brains

US scientists have created mice with human brain cells, by injecting human embryonic stem cells (ES cells) into the brains of fetal mice growing in the womb. The research, published in the Proceedings of the National Academy of Sciences (PNAS), offers proof that human ES cells can become working brain cells, able to connect with surrounding cells. The scientists, based at the Salk Institute for Biological Sciences in California, say the mice could also be used to test new drugs for treating human brain diseases.

The team injected about 100,000 cells into each mouse, but only a fraction survived, so over 99 per cent of the animals' brains were still made up of mouse cells. However, the finding that the cells that did survive managed to connect up and work properly provides hope for scientists trying to develop cell-based therapies for conditions such as Parkinson's disease. Previous studies had shown that human fetal brain cells could survive when injected into adult rodents, but it was unclear if they were working or not. Commenting on the latest research, US stem cell researcher Irving Weissman said that 'it's the best evidence yet that they are integrating and functioning', adding 'it's a nice advance'.

The injected cells formed biologically active nerve cells (neurons), as well as other types of brain cell, such as protective glial cells. The researchers were able to track their progress as the cells had been altered so that they fluoresced under the microscope, when the scientists analysed samples of the mouse brains. The human cells had migrated to the forebrain, where they grew to the size of mouse neurons - and where they fired electrical signals, showing they had formed connections with neighbouring mouse cells.

Lead researcher Fred Gage says the work is geared towards understanding human brain diseases, rather than finding a cure. One approach could be to inject human ES cells into the brains of mice with versions of diseases such as Alzheimer's or Parkinsson's disease. This could reveal whether these conditions affect neurons after emerging elsewhere, or whether they emerge in diseased neurons themselves.