First evidence embryonic stem cells can become human brain cells reported.

The first evidence that human embryonic stem cells can become functioning human brain cells inside a living animal and make connections with surrounding brain cells has been created by scientists at the Salk Institute for Biological Sciences in La Jolla, CA.

The scientific team, led

by Fred Gage, announced they successfully created mice with tiny amounts of human brain cells by injecting 100,000 human cells into the brain of each 14-day-old mouse embryo. The human cells had no apparent impact on the animals' behavior - the brain was still 99% mouse - which the team said protected the research from ethical charges that they were creating a humanized mouse.

"This illustrates that injecting human stem cells into mouse brains doesn't restructure the brain," Gage said, according to the Associated Press.

The researchers also reported that the injection of the cells had no apparent impact on the animals' behavior. That finding, they said, provides hope that realistic models of neurological disorders like Parkinson's disease can be created.

"Let's say you're in the last stages of research before testing a new drug in humans. This could help tell you what effect it will have on human neurons inside a brain," Gage said, according to the Washington Post.

Previous studies had shown that brain cells from aborted fetuses injected into the brains of rodents could survive and migrate to different regions of the brain. But researchers were never sure if the larger human cells were actually working.

The new work clearly showed, the scientists said, that the human cells developed into all the major kinds of cells normally found in mammalian brains. It also showed the neurons are biologically active and appear to make good synapses with adjacent mouse cells.

The research was published in the December 2005 issue of the Proceedings of the National Academy of Sciences.