Serious doubts over "Eggs forever?".

A recent commentary in this journal by Byskov et al. (2005) claims that, despite published results from numerous independent lines of investigation from our laboratory and others, there does not "exist any evidence for neo-folliculogenesis in the adult mammalian ovary." While we agree with Byskov et al. that our work represents a radical departure from the age-old dogma that mammalian females permanently lose the capacity for oocyte and follicle production during the perinatal period, careful examination of all of the available data leaves no doubt that adult female mammals retain the capacity for oogenesis and folliculogenesis. These findings do not change the fact that exhaustion of the oocyte pool occurs with advancing chronological age--a process responsible for driving the menopause in women--but rather question the basic mechanism underlying age-related ovarian failure. In this regard, studies of aging male mice have demonstrated that testicular atrophy is associated with a dramatic decline in the number, activity and quality of germline stem cells that maintain spermatogenesis during adulthood (Zhang et al., 2006). Therefore, to the contrary of the opinion of Byskov et al. that such a process would be "considered exceptional among stem cells," it is certainly reasonable to hypothesize that a similar deterioration of female germline stem cell function underlies the decline in oocyte quality and the onset of ovarian failure in aging females. Further, while we accept that a departure from conventional thinking can take years to gain widespread acceptance, we feel this resistance to change should not be construed as the sole means to voice opinions about the validity of our data or the maturity of our principal conclusion.