Wired on Friday: Dr Arnold Kriegstein left his neural stem cell research position at Columbia University 10 months ago to head up the developmental and stem cell biology department at the University of California, San Francisco.

At the time he was attracted by the university's impressive track record - the term "stem cell" was coined at UCSF - and by its associated work in organogenesis and developmental biology, cancer, immunology and tissue transplantation.

Since Kriegstein took the job, though, UCSF has moved even closer to the centre of the stem cell maelstrom.

In November 2004, Californian voters approved $3 billion (€2.5 billion) in funding for embryonic stem cell and other biomedical research. This funding was used to create the California Institute for Regenerative Medicine (CIRM), which has since found its home in San Francisco, across the street from the new biotech campus for UCSF.

Kriegstein reports that, somewhat to his chagrin, he is spending less and less time in the lab and more and more time hiring new investigators and educating the public. "The role is a lot more ambitious than I had thought," he says ruefully. That's hardly surprising. Stem cell research seems almost magical in its promise of therapies for appalling, degenerative disorders. At the same time, it's a bitterly divisive issue. Why?

Stem cells come in three kinds. Totipotent cells are essentially fertilised eggs. They have the potential to make every kind of cell found in the human body. As the egg divides and cells multiply, they separate into trophoblasts, which become the placenta, and an inner cell mass, which becomes the embryo.

Human embryonic stem cells - and these are the controversial ones - are derived from the embryo. Unlike the fertilised egg, these cells are pluripotent - they can make practically all, but not all, cells found in the human body. They can't make totipotent fertilised eggs, but they can make skin and bone cells, blood cells and neurons.

Stem cells can also be derived from umbilical cord blood and from adult cells. These are called somatic stem cells (as opposed to embryonic stem cells) and they are multipotent (as opposed to pluripotent). They are one step closer to being specialised cells. A cord blood cell, for example, can make many different kinds of blood cell, but it can't make a skin cell or a neuron.

Pluripotent embryonic stem cells offer some of the most exciting possibilities for treating intractable, devastating conditions like Alzheimer's and diabetes. But this power comes at a terrible price. Developing a stem cell line means destroying the blastocyst (or embryo) from which that stem cell line is derived.

It's very difficult to derive a stem cell line at all: UCSF is one of only a handful of places in the world where this has been done. The embryos are typically obtained from IVF clinics. Stem cell lines are derived from the leftover embryos, which might otherwise be frozen, donated to other couples or destroyed. Supporters of stem cell research argue that stem lines are derived from embryos that would not otherwise have found any useful purpose. Opponents believe it is a slaughter of innocents.

"I don't want you to think that we as scientists gloss over the implications of what we are doing," says Dr Kriegstein. He points to UCSF's ethical oversight committee, which carefully scrutinises every proposal around oocyte (egg) and embryo donation and stem cell research.

As he describes it, the oversight committee enforces at least two fundamental values. First, the proposed use of biological materials must be appropriate. Second, the investigators must ensure that the donors of biological materials give their informed consent for whatever applications are proposed. This is where we start getting into murky ethical waters. If you believe embryo donors own their embryos, informed consent makes a lot of sense. If you believe, on the other hand, that embryos are fully human beings, and that their mother and father stand in a guardianship, then it's hard to see how anyone could meaningfully consent to the embryos' destruction in the pursuit of research.

Polls consistently show that the majority of people in the US believe that the potential benefits from stem cell research outweigh concerns about the use of embryos for this purpose, and voter support for funding in California clearly underlined this position. There are, however, many outspoken critics.

There is a glimmer of hope for a win-win resolution in reports that investigators have developed pluripotent stem cells - the powerful, embryo-like cells - from umbilical cord blood, without having to destroy any embryos.

Whether this happens, Dr Kriegstein believes that objections to stem cell research will fade as ordinary people's loved ones are offered real hope for serious illnesses. "There was a lot of objection to IVF at first," he says, "but when people were presented with a live, healthy baby, so much of the resistance just melted away."