"If these findings hold up in humans, all theories about the aging of the female reproductive system will have to be revisited," says Jonathan Tilly, PhD, of the Vincent Center for Reproductive Biology at MGH, the paper's lead author. "We also may need to revisit the mechanisms underlying such environmental effects on fertility as smoking, chemotherapy and radiation. Eventually this could lead to totally new approaches to combating infertility in cancer patients and others." Tilly is an associate professor of Obstetrics, Gynecology and Reproductive Biology at Harvard Medical School.
For several years Tilly's group has been studying the mechanisms behind the death of oocytes and follicles, the tiny sacs in which the eggs grow. In both mice and humans, the vast majority of oocytes are destined to die through a process called programmed cell death or apoptosis, the body's natural way of eliminating unneeded or damaged cells. The team's earlier research confirmed that oocytes destroyed by chemotherapy drugs or radiation also die through apoptosis, opening the possibility of designing ways to stop ovarian damage in female cancer patients and perhaps to postpone normal ovarian failure. However, to provide an essential context to their efforts to inhibit oocyte apoptosis, the researchers decided to measure the numbers of healthy and dying follicles in mouse ovaries through the animals' lifespan.
What they found was remarkable. Measurements taken during the early stages of life found a steady, low level of dying follicles, but as the mice reached adulthood the number of dying follicles increased markedly. In young adult animals, the researchers measured 1,200 dying follicles per ovary, compared with about 3,000 healthy follicles remaining. Similarly elevated levels of dying follicles were measured well into maturity. Although such a surprisingly high rate of follicle loss would be expected to completely deplete a fixed population of oocytes within a matter of days or weeks, female mice retain healthy egg cells well past one year of age.
To make sure they were accurately measuring the rate at which follicles were dying, the researchers evaluated whether dead and dying follicles were being cleared from the ovaries. Their results confirmed that the dying follicles were being cleared within three days of death and thus represented a continuing level of cell death, not an accumulation of "cellular corpses."
"Finding such a high level of follicle degeneration without a corresponding reduction in the number of healthy follicles brought us up against the dogma of a fixed supply of oocytes," Tilly says. "The only probable interpretation was that the postnatal ovary had to be retaining the ability to make new oocytes and follicles." To investigate such a potentially revolutionary possibility, the researchers ran several additional experiments.
Tilly says that the concept of a fixed pool of oocytes, first asserted almost a century ago, has been so widely accepted that he is not aware of any studies over the past 50 years that have questioned its accuracy. "The ovaries visibly lose healthy follicles throughout life, while the testes continue to look the same. It was assumed that the decline in total follicles represented the gradual disappearance of a limited supply of oocytes. No one ever attempted to measure the actual rate of oocyte or follicle death before because the dogma was so persuasive," he explains.
Among many potential implications of the study is a different mechanism underlying ovarian aging. It is known that eggs released by older women are more likely to be abnormal, which has been attributed to the eggs themselves being older. But the problems could instead be the result of aging of the germline stem cells that produce the oocytes. If these stem cells could be identified and isolated, a whole new set of options for treating or preventing infertility might open up.
"These are basic biological findings that may change everything in our field," Tilly says. "Although there's no way to say how long it may take for these findings to actually affect the care of patients, we're very excited about the new set of scientific questions we have to investigate."
Additional authors of the Nature study are co-first authors Joshua Johnson, PhD, and Jacqueline Canning, along with Tomoko Kaneko, MD, PhD, and James Pru, PhD, all of the Vincent Center for Reproductive Biology. The study was supported by Vincent Memorial Research Funds and the National Institute on Aging.
Massachusetts General Hospital, established in 1811, is the original and largest teaching hospital of Harvard Medical School. The MGH conducts the largest hospital-based research program in the United States, with an annual research budget of more than $400 million and major research centers in AIDS, cardiovascular research, cancer, cutaneous biology, medical imaging, neurodegenerative disorders, transplantation biology and photomedicine. In 1994, MGH and Brigham and Women's Hospital joined to form Partners HealthCare System, an integrated health care delivery system comprising the two academic medical centers, specialty and community hospitals, a network of physician groups, and nonacute and home health services.