Oocytes Spontaneously Generated Gretchen Vogel

Science - May 2, 2003

In a development that could alter the ethical landscape surrounding human embryonic stem (ES) cells, scientists have found that mouse ES cells can develop into oocytes in culture dishes. It is not yet clear whether the cells can be fertilized and develop into embryos. But if so, and if human ES cells turn out to have similar powers, such cells might allow researchers to get around some of the expense and ethical questions that arise from using donated eggs for therapeutic cloning experiments. The procedure is deceptively simple, report Karin Hübner, Hans Schöler, and their colleagues in a paper published online this week by Science (www.sciencemag.org/cgi/content/abstract/1083452). The cells developed spontaneously in dense cultures of ES cells. The trick was identifying the cells that took on characteristics of oocytes, says Schöler, a developmental biologist at the University of Pennsylvania in Philadelphia. He and his colleagues developed a fluorescent marker for early-stage germ cells, cells that may turn into sperm or eggs. After 8 days, up to 40% of the cells glowed green. "I'm sure a lot of people will realize now that they had these early germ cells," Schöler says. But without the marker, "you wouldn't have any idea what is happening."

The researchers began to spot other similarities to germ cell development: Groupings of ES cells seemed to act like follicles, surrounding some of the apparent germ cells and even producing estradiol, a precursor of estrogen. And after 16 days in culture, some of the oocyte-like cells expressed proteins typical of meiosis, the specialized cell division that sperm and egg cells undergo. After 26 days, some of the cells broke away from their companion cells, much as normal eggs depart from follicles.

Perhaps most surprising, after about 40 days in culture, structures that looked like early embryos appeared. Most common were rounded groups of cells that expressed proteins similar to those produced by 16-cell embryos. In a few cases, the scientists found structures that resembled blastocyst-stage embryos, the stage at which ES cells are derived. Schöler says the clusters are likely parthenotes, embryos that develop from unfertilized eggs. Normal mouse oocytes are known to form parthenotes in culture in response to chemical signals or temperature changes, but despite multiple attempts to implant them in a womb, none has ever survived to birth.

Developmental biologists say the observations are a tantalizing glimpse of what might be possible in cell culture. The markers the team developed provide good evidence that some ES cells are taking on characteristics of oocytes, says Azim Surani of the Wellcome Trust Institute of Cancer and Developmental Biology in Cambridge, U.K. But he and Alan Spradling of the Carnegie Institution of Washington in Baltimore urge caution. Spradling suspects that the cells "are not exactly equivalent to normal oocytes," which require complex and delicate interactions with surrounding cells to develop properly. To really prove that they are oocytes, Surani says, the team needs to show that the cells can be fertilized and produce viable offspring.

Such experiments will have significant ethical implications, says bioethicist Arthur Caplan of the University of Pennsylvania, who helped advise Schöler. "This paper is an ethical earthquake," Caplan says. If ES cells could be a source of human oocytes, scientists might be able to use them for nuclear transfer experiments rather than eggs from human donors, which are in short supply. And if the cells prove to be functional enough for nuclear transfer but not for producing offspring, they might answer one of the main arguments against therapeutic cloning: that it creates embryos only to destroy them. But if the cells are capable of being fertilized and developing normally, Caplan predicts, they will only increase the concerns of people opposed to ES cell research.