Study finds that bone marrow and blood cannot replenish eggs, questioning earlier findings
By Melissa Lee Phillips | June 15, 2006
Mature ovulated mammalian eggs can't arise from circulating stem cells, according to a report in this week's Nature. The study questions the physiological relevance of putative regenerated oocytes another group had reported in a 2005 Cellpaper, but does not directly refute those findings, the authors say.
"I don't think the results are contradictory, because the endpoints are different," said Nature study co-author Amy Wagers, of Harvard University. The previous work examined immature oocytes in the ovary, while the current paper analyzes ovulated eggs ready for fertilization. But the cells seen in the earlier studies "are not likely to be available for fertilization and they're not likely to contribute to female fertility," she added. "There's no definitive proof of what those cells actually do."
Cells circulating in the blood may express markers found in germ cells, said lead author Kevin Eggan, also of Harvard, "but I think I can say with a high degree of certainty from our studies that those cells don't become eggs that could be fertilized," Eggan told The Scientist.
Reproductive biologists have long believed that female mammals are born with a limited number of oocytes that they can't replenish. In 2004, however, a Naturepaper by Joshua Johnson and colleagues from Jonathan Tilly's lab at Massachusetts General Hospital showed that ovaries of adult mice may be able to regenerate oocytes. In 2005, the group published results suggesting that regenerated oocytes come from stem cells that circulate in the blood and are derived from bone marrow.
"The hypothesis that the Johnson papers were putting forth is just revolutionary," said Patricia Hoyer of the University of Arizona, who was not involved in any of the work. If this hypothesis were validated, "it would just totally turn our thinking around," she said.
To see if circulating cells could turn into mature eggs, Eggan's group surgically joined wild-type mice and mice ubiquitously expressing green fluorescent protein (GFP). "These are remarkable mice, which basically, over time, develop a common circulatory system," Eggan said.
These parabiotic pairs were joined together at 4-8 weeks of age and remained together for 6-8 months. After this time, each pair member had, as expected, a mixture of GFP-positive and wild-type blood cells. However, when the researchers induced ovulation in these mice and then collected ovulated oocytes, they found no GFP-positive oocytes in the wild-type mice or vice versa. They found some cells associated with oocytes that were from the parabiotic partner, but all of these cells stained positive for CD45, which is a marker common to blood cells, Eggan said.
The researchers then repeated their experiments in mice pre-treated with chemotherapy to see if injury to bone marrow, ovary, or germ cells might induce oocyte regeneration. Again, they found that no GFP-positive cells crossed into the wild-type partner to become mature oocytes.
Finally, the researchers performed bone-marrow transplants with GFP-transgenic cells in mice pre-treated with chemotherapy agents or with low-dose irradiation to sterilize them completely. They found GFP-positive blood cells in these treated wild-type mice but no GFP-positive oocytes.
The studies "demonstrate the bottom-line physiological potential of the results that Johnson had been reporting," Hoyer said. "I think they've done a good job of demonstrating what we've believed all along."
The implication of the 2005 Johnson paper was that transplanted cells in women receiving bone-marrow transplants could give rise to germ cells that might be fertilized and produce offspring, said Wagers. "Our data argue that that's very unlikely."
According to Antonin Bukovsky of the University of Tennessee, Knoxville, who was not involved in the study, "the data presented are convincing regarding the ovulated oocytes," but unless the authors can show that ovaries do not contain pre-ovulated oocytes from the parabiotic partner, Tilly's group's data remain valid, Bukovsky told The Scientist in an Email.
It's possible that immature oocytes derived from bone marrow could contribute to the ovarian endocrine environment necessary for ovulation, Tilly speculated. But Eggan said that their studies argue against this effect in chemotherapy-treated mice, since neither parabiosis nor bone-marrow transplants increased endogenous ovulation.
It's also possible that scientists may be able to figure out how to manipulate these cells to turn them into eggs that could mature and become fertilized, Tilly told The Scientist. "I'm not saying that, long-term, these cells will be meaningful, but I think at this stage we have to be careful not to simply rule them out."
Melissa Lee Phillips
Links within this article
K. Eggan et al., "Ovulated oocytes in adult mice derive
from non-circulating germ cells," Nature, published online June 14, 2006.
J. Johnson et al., "Oocyte generation in adult mammalian ovaries by putative germ cells in bone marrow and peripheral blood," Cell, July 29, 2005.
J. Johnson et al., "Germline stem cells and follicular renewal in the postnatal mammalian ovary," Nature, March 11, 2004.
C.Q. Choi, "New oocytes from bone marrow?" The Scientist, July 28, 2005.
C. Holding, "Oocytes direct from embryonic stem cells," The Scientist, May 2, 2003.