In a cautiously worded "ADDENDUM" published in this week's issue of Nature, the authors of the original paper now provide some additional details of their technique:
At the request of the Editors at Nature, we wish to clarify some questions that have arisen since the advance online publication (AOP) of our Letter on 23 August 2006. In our Letter, we showed that human embryonic stem-cell lines can be generated from a single cell after its removal from an 8–10-cell embryo. To minimize the number of embryos used, we removed multiple cells from each embryo, and none of the biopsied embryos were allowed to develop in culture.Given the significance of the additional details, and the "request of the Editors of Nature", one wonders how how close the clarification came to characterization as "CORRIGENDUM" or "ERRATUM" instead of the far more forgiving "ADDENDUM".
In our experiments, the isolated blastomeres from each embryo were cultured together in the same medium that was used to culture the parent embryo, and were arranged to avoid contact with each other. Diffusible factors from the other blastomeres present in the media may assist recovery and growth of the blastomere. We have not excluded the possibility that only a subset of blastomeres of an 8–10-cell embryo are capable of forming human embryonic stem cells. These caveats are worth considering for future studies, but do not negate our central finding that blastomeres extracted from an 8–10-cell embryo by mechanical micromanipulation can form human embryonic stem-cell cultures.
Bioethicists who viewed the new technique as vitiating the claim that embryos must be destroyed to generate ESCs now face renewed disappointment. As the authors clarify, "we removed multiple cells from each embryo, and none of the biopsied embryos were allowed to develop in culture". In other words, ACT has not yet, in fact, demonstrated that ESCs can be generated from embryos that survive the loss of a blastomere. Though ACT's original claims may still be proven at some point, this reversal of scientific fortune should embolden opponents of ESC research to renew their criticisms.