Stem Cells from Fertilized Eggs
New results by Eggan and his HSCI co-workers may have made that research easier by showing how to extract stem cells from single-celled fertilized embryos, or zygotes, which researchers have long considered impossible.
Collecting unfertilized eggs has proved challenging, Eggan says, because donors are not compensated beyond the cost of the extraction procedure. A year after receiving approval to study cloning in human cells, "we have yet to find a woman who's willing to participate as an egg donor," he says.
Eggan's group hit on a way around this barrier by injecting the chromosomes from embryonic and adult mouse cells into zygotes that were chemically frozen midway in the process of dividing into two cells. Twenty percent of the cells cloned in this way grew into early embryos, called blastocysts, and 5 percent of them yielded embryonic stem cells, which is comparable with results obtained from unfertilized eggs.
When implanted into female mice, nine of 174 cloned blastocysts survived until birth, although they died soon after, possibly because the reprogramming was incomplete, the team notes in a third Nature report.
The researchers believe the experiment worked because the zygotes were paused at a stage in which the nucleus has dissolved and the molecules responsible for reprogramming are mixed throughout the cytoplasm where they can affect the injected DNA. They believe prior experiments failed because researchers removed the reprogramming factors from the zygotes along with their nuclei.
It may sound potentially more ethically troublesome to clone stem cells from a fertilized egg than an unfertilized one. "This certainly does not lessen the ethical issues at stake," Cameron says, because extracting stem cells from the reprogrammed embryo would destroy it.
It benefits researchers, however, because Eggan's group found that the technique works on zygotes carrying an extra set of chromosomes, which can occur if two sperm fertilize an egg. Up to 5 percent of all zygotes have extra chromosomes and they may exist in the tens of thousands in the banks of in vitro fertilization (IVF) clinics, whic routinely discard such zygotes because they develop abnormally, Eggan says. His team, he says, has already begun studies on donated fertilized eggs.