Mammalian females ovulate periodically over their reproductive lifetimes, placing significant demands on their ovaries for egg production. Whether mammals generate new eggs in adulthood using stem cells has been a source of scientific controversy. If true, these “germ-line stem cells” might allow novel treatments for infertility and other diseases. However, new research from Carnegie’s Lei Lei and Allan Spradling demonstrates that adult mice do not use stem cells to produce new eggs. Their work is published by the Proceedings of the National Academy of Sciences the week of April 29.
Before birth, mouse and human ovaries contain an abundant supply of germ cells, some of which will develop into the eggs that will ultimately be released from follicles during ovulation. Around the time of birth these germ cells have formed a large reserve of primordial follicles—each containing a single immature egg. Evidence of new follicle production is absent after birth, so it has long been believed that the supply of follicles is fixed at birth and eventually runs out, leading to menopause.
During the last decade, some researchers have claimed that primordial follicles in adult mouse ovaries turn over and that females use adult germ-line stem cells to constantly resupply the follicle pool and sustain ovulation. These claims were based on subjective observations of ovarian tissue and on the behavior of extremely rare ovarian cells following extensive growth in tissue culture, a procedure that is capable of “reprogramming” cells.
Caption: Developing cells in the ovary of a 13.5-day-old mouse embryo. Photo courtesy of Carnegie Institution for Science.