FRED DE SAM LAZARO, guest anchor: A series of significant developments have emerged in recent weeks in the field of stem cell science. Each adds exciting prospects for treating disease; each adds vexing complexity. With us to help understand the science and the issues: Rick Weiss, science writer for THE WASHINGTON POST. Rick, why don't we begin with the basics: What are stem cells, and why are they so controversial?

RICK WEISS (Science Writer, THE WASHINGTON POST): These are the cells that are at the core of a very early developing human embryo. What scientists are very excited about them is that they can develop into all the different kinds of the tissues, so you can imagine using these cells to grow replacement tissues to help treat a variety of diseases. The controversy, of course, is in order to get at the cells you need to destroy developing human embryos, and for those who believe that even the earliest stages of human life have moral standing, this is of course tantamount to murder.

DE SAM LAZARO: We had a story earlier this month from a team in Massachusetts that claimed to have surmounted this hurdle.

Mr. WEISS: Right. This team figured out how to take an even earlier stage of the human embryo, when there are just eight cells, pluck one of those cells out of the embryo, and grow it into a colony of human embryonic stem cells, leaving the other seven cells alone to develop normally, as it turns out they will, into an embryo. They felt that this would allow them to get federal funding because it doesn't harm the embryo. That's the rule for federal funding. But as it turns out, the government is not allowing funding for this work because they say the only way to prove that those remaining seven cells can develop into a full human being is to actually transfer them to a woman and have them born as a baby. And, of course, these embryos are for research purposes and are not meant to be born, so that's not going to happen.

DE SAM LAZARO: We did have a story sometime in November, as I recall, from teams which had found a way to create stem cells without embryos?

Mr. WEISS: Right. This is another promising approach. This is a team of scientists in Madison and Japan who converted ordinary skin cells into embryonic stem cells directly in the lab, without having to make any embryos or destroy embryos. But this is a method that still is experimental, requires viruses to make the changes. These viruses can cause cancers. So this method is really not ready for experimentation in people yet.

DE SAM LAZARO: And yet again was a story from California this month about a team that had essentially cloned new embryos?

Mr. WEISS: Right. So going back to the embryo method, these people took skin cells -- actually the CEO of the company took his own skin cells, cloned those cells to make living human embryos of a stage from which you could take stem cells. This is another promising method for getting stem cells, but another controversial one, because it does involve the creating of embryos, cloned human embryos, which among other things if transferred to a woman by perhaps a rogue scientist would lead to the birth of the world's first cloned baby.

DE SAM LAZARO: This has become demonstrably much more feasible than we had thought?

Mr. WEISS: Cloning has come a long way since Dolly the sheep was born 10 years ago.

DE SAM LAZARO: Okay, and finally in the very short time that we have, give us a sense of time frame. If I'm suffering from Parkinson's or a disease that could be helped, how long might I expect?

Mr. WEISS: It's not going to be quick. I think it's safe to say five to 10 years before some real therapies start to come out of stem cells. There are still animal studies to be done and early human clinical trials. The field looks promising, but it does take time to really get somewhere with this.

DE SAM LAZARO: Rick Weiss with THE WASHINGTON POST, thanks so much for coming by today.

Mr. WEISS: Thank you.