US: Scientists in California have created living mice with working human brain cells inside their skulls by injecting human embryonic stem cells into the brains of foetal mice.
The research offers the first proof that human embryonic stem cells - vaunted for their potential to turn into every kind of human cell, at least in laboratory dishes - can become functional human brain cells inside a living animal, reaching out to make connections with surrounding brain cells.
The human cells had no apparent impact on the animals' behaviour. About 100,000 cells were injected into each animal; just a fraction survived. The animals' brains were still more than 99 per cent mouse - a precaution that helped to avoid ethical objections to creating animals that were "too human".
The finding that the human cells are working in their new environment provides encouragement for those who hope to develop stem-cell-based therapies for neurodegenerative diseases such as Parkinson's.
More immediately, mice with humanised brains could provide a living laboratory where scientists can study human brain diseases and drug companies can test the safety of experimental medicines.
The work, published in yesterday's issue of the US Proceedings of the National Academy of Sciences, is the latest in the ethically challenging field of human-animal "chimera" research - a reference to the chimera of Greek mythology, which had a lion's head, a goat's body and a serpent's tail.
In previous studies, scientists had injected brain cells from aborted human foetuses into the brains of rodents and shown that the human cells could survive and migrate to various brain regions, but because those human brain cells were relatively mature, they were larger than their rodent counterparts and it often was unclear if they were working.
The new work showed that those human cells developed into all the major kinds of cells normally found in mammalian brains, namely neurons and nerve-nurturing glial cells.
It also showed that the neurons were biologically active and made what appear to be good connections, or synapses, with adjacent mouse cells.