New Research Challenges the Long-Held Belief That Women Have a Fixed Egg Supply

For more than seven decades biologists taught that female mammals are born with a finite ovarian reserve that dwindles to menopause. The view was cemented by mid-20th-century experiments of British scientist Solly Zuckerman, who found no new oocytes in animals at different ages. In 2004, Jonathan Tilly and colleagues argued that the rate of follicular atresia did not match the remaining egg count, suggesting ongoing oocyte production. Subsequent studies have isolated rare oogonial stem cells (OSCs) in adult mice, cows, pigs, non-human primates and humans. Genetic labeling in mice showed newly formed oocytes that could generate offspring, and recent work indicates OSCs persist into old age but lose key developmental gene activity, a pattern also observed in post-menopausal human tissue.

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Supporters such as Tilly and reproductive biologist Evelyn Telfer contend that OSCs can develop into follicle-like structures in adult ovarian tissue, opening the possibility of post-natal oocyte formation. Critics, including obstetrics professor Aaron Hsueh, question the reliability of Tilly’s proprietary antibodies and argue that Zuckerman’s findings remain the definitive evidence against adult oogenesis. Telfer notes that entrenched consensus can hinder funding for unconventional research, while acknowledging that the functional capacity of OSC-derived eggs in humans remains unproven.

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Researchers are exploring engineered ovarian organoids that combine a patient’s OSCs with hormone-producing somatic cells. The goal is not immediate baby production but restoration of natural hormone function for individuals who have lost it, offering an alternative to conventional hormone-replacement therapy. Preliminary work has isolated “dormant” OSCs from tissue donated decades after menopause, but safety, efficacy and long-term reliability of such implants have yet to be demonstrated.

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• “Previous to this work, every response to questions about infertility or menopause was rooted in the concept that women only have a fixed number of eggs to work with,” — Jonathan Tilly, Biologist, Northeastern University

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• “You’re part of a community, and that community holds these views, and if you’re going against those views, it’s difficult to get funding,” — Evelyn Telfer, Chair of Reproductive Biology, University of Edinburgh

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• “These cells underwent a transformation within the tissue,” — Evelyn Telfer

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• “I’m not completely against this idea [of ovarian stem cells], but I’m against Tilly’s approach,” — Aaron Hsueh, Professor of Obstetrics and Gynecology, Stanford University

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• “It really changes [our] perspective on why ovaries fail,” — Jonathan Tilly

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• “We need to do a lot more of the basic work,” — Evelyn Telfer

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