In research published this week, scientists report that they've successfully transplanted human stem cell-derived neurons into the brains of living mice. That's right, we're talking about a functioning trans-species transplant of brain matter. The researchers took human embryonic stem cells, and grew them in a culture with mouse neurons that had a specific trait — they're activated by light. The stem-cell derived neurons don't normally have this ability, but progressively gained it when grown with the mouse neurons.
The stem-cell neurons were then implanted into a living mouse's hippocampus, where the transplants were able to reciprocally interact with the mouse's neuronal network, and integrate into it. They became part of the network, and functioned normally.
While the whole "transplanting into a mouse" thing is very cool, it's not really why this work is important. The best part is that you can train neurons, and then successfully transplant them into a brain, giving us another avenue to help treat those effected with Parkinsons and Alzheimers diseases, stroke, and epilepsy.