Routes and mechanisms of embryonic hormone exposure and endocrine disruption in a viviparous lizard

The mechanisms of embryonic exposure to hormones in viviparous reptiles are speculative, despite detailed understanding of exposure in oviparous species and eutherian mammals. The yolk of oviparous species and the placenta of eutherian mammals are key sources of steroid to developing embryos. Importantly, most viviparous reptiles utilise a yolk and a placenta to support embryogenesis. Maternally-derived testosterone and oestradiol have major impacts on development. The yolk and the placenta are therefore both potential sources of testosterone and oestradiol to embryonic viviparous lizards, but this remains to be tested. Many endocrine disrupting chemicals (EDCs) impair gonadal development by affecting testosterone and oestradiol concentrations. Some EDCs disrupt the activity of aromatase which catalyses the conversion of testosterone to oestradiol. Embryonic development can be affected either by alterations in maternal hormone concentrations or by direct exposure to EDCs. Embryonic viviparous reptiles are at particular risk due to the multiple routes of potential exposure. However, no study has examined the routes of embryonic exposure to EDCs or examined EDC effects in viviparous reptiles. The major aims of this study on the viviparous lizard, Niveoscincus metallicus, were to confirm that the yolk is a source of testosterone and oestradiol to developing embryos; to demonstrate the placenta and embryonic tissues as sites of aromatase; and to examine the effects of gestational exposure to two EDCs (diethylstilbestrol and the herbicide atrazine) on gonadal development. Yolks of N. metallicus were sampled from vitellogenesis through to the final stages of gestation. As in in oviparous reptiles, the yolk of N. metallicus contains testosterone and oestradiol, however, the stages at which the steroids decline are not comparable to oviparous species, as the placenta also provides steroid. Aromatase activity was measured in the placenta and embryonic tissues during three progressive stages of development. Placental aromatase activity was highest in the early stages of development, suggesting oestrogen synthesis in N. metallicus supersedes yolk reserves. The effects of oestrogenic EDCs on gonadal development were characterised by dosing gestating females with diethlystilbestrol at 100 or 10 ˜í¬¿g/kg and examining the gonads of neonates. The effects of gestational exposure to a single dose of atrazine at 10 ˜í¬¿g/kg on gonadal development were also characterised by examining the gonads of neonates. Diethlyslstilbestrol and atrazine disrupt gonadal development: by comparing the developmental effects observed from exposure to diethylstilbestrol and atrazine in males and females, I conclude that atrazine disrupts gonadal development in N. metallicus via increased aromatase activity. This thesis has expanded our understanding of the endocrine environment in which embryos of viviparous lizards develop, and has provided new evidence of the potential impacts of EDCs on viviparous vertebrates.