To generate normal stem cell lines, scientists start with a fertilized embryo, usually discarded from an in vitro fertilization clinic. They collect a specialized ball of cells, called the inner cell mass, from the embryo when it is just five to six days old. Cultured in a dish, the cells develop into a line of embryonic stem cells that can, depending on the conditions, either regenerate itself or differentiate into specialized cell types, such as heart cells, liver cells, or brain cells. Scientists must continually make new stem cell lines, because existing lines may accumulate mutations, making them unfit for therapies and many types of research.
Cloned stem cells, however, are even more difficult to make than regular embryonic stem cells. Scientists take the DNA from a differentiated cell, such as a skin cell, and insert it into an egg that has been stripped of its own DNA. The egg then starts dividing, much as a regular embryo would. If it survives long enough, its inner cell mass can be harvested and used to grow stem cells. Scientists have generated stem cells from cloned mouse embryos but have not replicated that feat in humans. Unlike naturally fertilized embryos, cloned embryos are hard to keep alive long enough – almost a week – that their inner cell masses can be gathered.
Hwang had claimed to do this with remarkable efficiency, using a small number of eggs. Human eggs are a precious resource that is very difficult to obtain, so the frugal use of eggs is critical to making nuclear transfer practical. But subsequent investigations revealed that Hwang and colleagues lied not only about their results but also about the number of eggs they used in their experiments. According to a report from South Korea’s National Bioethics Committee, Hwang used 2,221 eggs in his failed experiments, rather than the 427 eggs reported in his two Science papers. Scientists now have no idea how many eggs are required to successfully clone a line of human stem cells.
When the nucleus of an adult cell is put into an egg, some unknown factors in the egg turn back the clock on it, reverting it to its embryonic state. “It’s like pushing the reformat key on a computer. You reformat it to become some other kind of cell,” says Snyder. “We don’t understand the molecular pathways that do this….As far as we know, the only thing that can do this is the egg.”