In parthenogenesis, an unfertilised egg keeps two sets of chromosomes and begins developing as if it had been fertilised. Some insects and reptiles can reproduce this way but even though an electric or chemical stimulus can induce parthenogenesis in mammals, the resulting embryos die after a few days. And that, according to its proponents, is the beauty of the technique as far as stem cells are concerned: it produces embryos that could never become human beings. So destroying these embryos to obtain stem cells would avoid the ethical concerns that have led to restrictions or bans on embryonic stem cell research in many countries.
However, while the technique works in mice and monkeys (New Scientist, 26 October 2001, p 14), attempts with human eggs have not got far. Until now, that is. A team led by fertility specialist David Wininger at biotech firm Stemron of Maryland has grown parthenogenetic human embryos to the blastocyst stage, at which stem cells can be obtained. Cells taken from one of the embryos survived for a few days (Stem Cells, vol 21, p 152).
"It's the first time I know of parthenogenetic cells in humans," says Kent Vrana of Wake Forest University School of Medicine in North Carolina, whose team pioneered the work in monkeys.
The next step is to get the cells to grow in culture indefinitely: that is, to obtain a stem cell line. In monkeys, such a cell line has been growing for over two years, and it makes the human experiments all the more relevant.
According to Vrana, extensive analysis of the monkey cells suggests that they are indistinguishable from normal embryonic stem cells. "They are identical to ESCs by every known criterion we have tested," he says, adding that details will soon be published in a peer-reviewed journal.
A lot of work still has to be done to ensure any tissues made from parthenogenetic stem cells are absolutely normal, says Jerry Hall of the Institute for Reproductive Medicine and Genetics in Los Angeles. But he is optimistic. "Patients are so interested in this procedure, and we are confident enough in its feasibility, that we have been willing to store eggs for use as soon as safety and effectiveness is shown," he says.
Since eggs are needed to make parthenogenetic stem cells, one potential problem is that the technique could not be used to make matching stem cells for men or for women after menopause.
Therapeutic cloning, by contrast, could provide matching stem cells for any individual.
However, because cells made by parthenogenesis have two identical sets of chromosomes, rather than one set each from the father and the mother, they have less variation in the surface proteins on cells that can trigger immune reactions.
Wininger thinks it will possible to establish a bank of parthenogenetic stem cells that could provide cells to suit most individuals. And such banks would be much cheaper than creating stem cells from scratch for each individual.
Sylvia Pagan Westphal, Boston. New Scientist issue: 26 April 2003.
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