The results demonstrate that even outside the body embryonic stem cells remain totipotent, or capable of generating any of the body's tissues, said lead researcher Hans R. Schöler of Penn's School of Veterinary Medicine.
"Most scientists have thought it impossible to grow gametes from stem cells outside the body, since earlier efforts have yielded only somatic cells," said Schöler, professor of reproduction medicine and director of Penn's Center for Animal Transgenesis and Germ Cell Research. "We found that not only can mouse embryonic stem cells produce oocytes, but that these oocytes can then enter meiosis, recruit adjacent cells to form structures similar to the follicles that surround and nurture natural mouse eggs, and develop into embryos."
Schöler said oocyte development in vitro may offer a new way for embryonic stem cells to be produced artificially, sidestepping the ethical concerns articulated by President Bush and others. Implanting a regular nucleus from any of the body's cells into such an oocyte would yield a totipotent stem cell.
The findings may force legal revisions in nations such as Germany whose lawmakers, assuming that stem cells' potency outside the body was limited, have passed legislation banning research with totipotent stem cells.
The Penn scientists pulled off this feat using a gene called Oct4 as a genetic marker. After the stem cells were plated in a regular Petri dish -- densely but without special feeder cells or growth factors -- the scientists used fluorescent markers linked to Oct4 and other telltale genes to assay oocyte development. After 12 days in culture, the cells organized into colonies of variable size. Shortly thereafter, individual cells detached from these colonies.
"These germ cells then accumulated a coating of cells similar to the follicles surrounding mammalian eggs," Schöler said. "Starting on day 26, oocyte-like cells were released into the culture -- similar to ovulation -- and by day 43, embryo-like structures arose through parthenogenesis, or spontaneous reproduction without sperm."
In the experiment described this week in Science, both male- and female-derived stem cells developed into female gametes. Schöler and colleagues now plan to test whether oocytes developed in vitro can be fertilized.
"We would like to use these oocytes as a basis for therapeutic cloning, and hope that our results can be replicated with human embryonic stem cells," Schöler said.
Schöler was joined in the research by Karin Hübner, James Kehler, Rolland Reinbold, Rabindranath de la Fuente and Michele Boiani of Penn's School of Veterinary Medicine; Lane K. Christenson, Jennifer Wood and Jerome Strauss III from Penn's School of Medicine; and Guy Fuhrmann of the Centre de Neurochimie in France. The work was funded by the National Institutes of Health, the Marion Dilley and David George Jones Funds, the Commonwealth and General Assembly of Pennsylvania and the Association pour la Recherche sur la Cancer.