Xenobiotics; effects on female fertility

Over the course of the 20th century there has been an increasing trend in western women opting to delay childbirth in the pursuit of social and economic stability. This development has highlighted the need to identify and characterise ovotoxic xenobiotics (foreign chemical compounds) which threaten their fertility. Arguably the most insidious ovotoxic xenobiotics are those which target the irreplaceable primordial follicle pool for destruction, resulting in pre-mature ovarian senescence. Although many of these xenobiotics have been identified, the molecular mechanisms behind their ovotoxicity remain largely unknown. Employing a neonatal mouse model rich in primordial follicles, the studies presented in this thesis were aimed at characterising the mechanisms of ovotoxicity for six xenobiotics known to target immature follicles (4-vinylcyclohexene diepoxide; Methoxychlor; Menadione; Benzo-a-pyrene, 7,12-Dimethylbenz-[a]anthracene; 3-Methylcholanthrene). The effects of short term xenobiotic exposure on long term oocyte viability were also examined to determine whether follicles which survive ovotoxic destruction were still functionally viable. Microarray analysis and quantitative PCR revealed a unique ovarian response to each xenobiotic involving a number of genes linked to follicular growth/development, cell death and tumorigenesis in vitro. Immunohistological and histomorphological analysis confirmed the microarray data, and revealed a consistent mechanism of ovotoxicity involving primordial follicle activation alongside developing follicle atresia in vitro and in vivo. Immunohistological and pharmacological inhibition studies also revealed an essential role for the PI3K/Akt/mTOR signalling pathways in 3-Methylcholanthrene and 7,12-Dimethylbenz-[a]anthracene induced primordial follicle activation and survival. Studies into the effects of short term neonatal exposure on long term female fertility revealed no difference in the number of healthy oocytes ovulated in neonatally treated adults compared to controls. However, comprehensive oocyte viability analysis revealed a decreased capacity for fertilisation caused by oxidative damage to the oolemma membrane due to mitochondrial electron transport chain leakage. The studies conducted in this thesis have identified a common mechanism of xenobiotic induced primordial follicle depletion via a homeostatic mechanism of developing follicle recruitment, with some ovotoxic xenobiotics inducing primordial follicle survival as opposed to atresia. In addition, the contents of this thesis also provide the first documented evidence of short term neonatal exposure causing long term oocyte dysfunction through xenobiotic induced oxidative stress.