Here’s how the process generally works:

We now know that the brain, like most organs in the body, already does replenish itself to some extent (this is actually a pretty new idea). It has a supply of largely undifferentiated cells (a kind of stem cell) which divide, move throughout the brain, and grow into other neural cells.

This works fine for most of us, but for some people neural degeneration seems to be occurring faster than this replenishment can repair. Unlike other organs, we can’t just graft a bunch of brain tissue to take the place of lost stuff. So the idea is instead to bolster the existing system of replacement.

And that’s where artificially produced stem cells come into the picture. We know that if the stem cells are taken from early enough in development (like an embryo), they can differentiate into any tissue in the body (unlike the ones that remain in adulthood and can only generally become a few types). We also know that if even foreign cells have the same DNA as their recipient, they won’t be rejected by the immune system, thereby avoiding very nasty transplant problems.

To produce those embryonic stem cells, a genetic engineer will take a cell (usually from an aborted embryo), remove all the DNA that it originally had, and insert the DNA of the patient who will recieve it. All the cells that made up the original embryo will be separated, modified, and inserted into people. This (ideally) doesn’t directly kill any of those cells, but they certainly are no longer an embryo any more, and arguably not the original creature they were before either.

And here’s where the religious part comes in. If you believe that an embryo has a soul, or is otherwise as much of a person as anyone else, then what is being done to them is arguably not much different from taking a healthy, living person, cutting them into pieces, and distributing all their organs into other people.

Of course, most of the embryos that are used would have been destroyed anyway, so the analogy isn’t entirely apt. But it is still an ethically tricky area. Some will try and invoke a slippery slope and say that if it’s okay to do it to embryos, shouldn’t we fear that it’s okay to do it to anyone? And some religions are just against such kinds of extreme medical procedures in any case… some even go so far as to view diseases as tests and judgements of the Divine, and therefore attempts to circumvent such things as evil.

But for most, I think, it’s the destruction of an embryo that’s the difficult-to-accept part. Hope that answers all your questions. Peace.