I was watching Robert George, of the President’s Council on Bioethics, talk about this on C-SPAN this morning, and they had a nice graphic. I will try to find it, and post it as an update:

Now scientists are exploring methods for resetting the genetic switches inside various cells to the positions that will make them embryonic again. Both of the two major approaches now under study use existing embryonic stem cells (widely available from previously destroyed embryos and eligible for study using federal funds) to help ordinary cells become stem cells.

In one approach pioneered by Robert Lanza and colleagues at Advanced Cell Technology in Worcester, Mass., researchers pluck single cells from eight-cell embryos — embryos so young they do not have stem cells yet.

Fertility doctors have known for years that early embryos seem unfazed by the removal of any one of their eight virtually identical cells, called blastomeres. In fact, it is common today to remove a single, representative blastomere from a laboratory-conceived embryo and test that cell for disease genes before deciding whether to transfer that embryo into a woman’s womb.

Working with early mouse embryos, the team has found that single blastomeres, when cultivated in dishes with embryonic stem cells, can become what appear to be embryonic stem cells themselves. Chemicals secreted by the embryonic cells apparently flip the right genetic switches in the blastomeres to make them act “stemmy.”

About a quarter to one-third of blastomeres treated this way can be coaxed to become embryonic stem cells or closely related embryo cells, said Lanza, who declined to release specific data pending publication in a peer-reviewed journal.

I would recommend catching this segment, when it’s up on C-SPAN’s website. Professor George strikes me as one of the more reasonable and pragmatic members of the bioethics panel.