Development of a Novel Vertebrate Model of Postnatal Oogenesis.

Oogenesis in mammals was historically believed to cease in the perinatal period, thus endowing females with a finite supply of oocytes. Ovarian failure, whether by natural or pathological means, was consequently attributed to irreversible depletion of this finite oocyte reserve. These traditional concepts have been challenged recently, with accumulating evidence supporting the existence of female germline stem cells (fGSCs) in the postnatal mammalian ovary. While unexpected and highly controversial, these findings raise the distinct possibility that ovarian failure could be a target of therapeutic modification, with significant clinical implications for women's health. Due to the prevailing dogma regarding mammalian oogenesis, progress in exploring the function and regulation of vertebrate fGSCs and their role in postnatal oogenesis has been slow. Among the many limitations has been a lack of genetically relevant, experimentally tractable models of vertebrate oogenesis. Herein, we describe development of a novel vertebrate model of fGSC biology, designed specifically to provide insights into postnatal oogenesis and ovarian regeneration. We have combined chemical and genetic tools to generate a unique transgenic zebrafish model of targeted oocyte ablation that permits direct observation of fGSC-mediated regeneration. The approach employs a reversible, gene-directed enzyme prodrug therapy system, using the zona pellucida C (zpc) gene promoter to drive oocyte-specific expression of a bacterial nitroreductase enzyme. Oocyte-specific expression of nitroreductase catalyzes the reduction of a prodrug (metronidazole) to a cytotoxic alkylating agent, designed to induce oocyte-specific cell death with no deleterious effects on fGSCs. Prodrug removal is predicted to result in fGSC-mediated oogenesis to replace ablated oocytes. Preliminary studies have confirmed that a seven day exposure to metronidazole results in rapid oocyte ablation. We now propose to use this approach to investigate postnatal oogenesis, with the goal of identifying regulatory factors underlying fGSC function. This unique genetic resource will provide valuable insights into the molecular and cellular control of postnatal oogenesis, while providing a novel model of ovarian regeneration. We are hopeful that this work will assist in resolving the debate surrounding the existence of fGSCs and postnatal oogenesis in mammals, and contribute to the development of novel therapeutic strategies urgently required to treat or postpone premature ovarian failure. (poster)