Date of Thesis

Spring 2019


The hypothalamic-pituitary-gonadal (HPG) axis is responsible for the production of the hormones testosterone and estradiol, and testosterone is thought to contribute to regulation of the axis through a negative feedback mechanism. Regulation by negative feedback involves the product of a pathway turning off that pathway when enough product is made. However, because the enzyme P450 aromatase converts testosterone to estradiol, estradiol may also contribute to regulation of the HPG axis and other phenomena that have been attributed to testosterone, like the inhibition of immune function. Previous studies have injected birds with an aromatase inhibitor (presumably reducing estradiol production) and shown a subsequent increase in the immunity of the injected birds as compared to their controls, implying that estradiol was decreasing immune function. In order to support the hypothesis that it is estradiol, not testosterone, which is inhibiting immune function, it is important to show that a decrease in estradiol and not a subsequent increase in testosterone is what is leading to the decreased immunity. In order to test this hypothesis that estradiol is inhibiting immune function, it is important to determine whether estradiol is a contributor to negative feedback of the HPG axis. In this thesis, I tested the hypothesis that estradiol affects negative feedback in the HPG axis. Chicken embryos were injected with either Fadrozole, an aromatase inhibitor, or a vehicle solution on day 13 of incubation. Two days later, the embryos were bled for hormone quantification and genetic sexing. Genetic sexing was obtained through DNA extraction, PCR, and gel electrophoresis. Hormone levels were compared across treatment and sex through ELISA assays. By using embryos, the immediate effects of Fadrozole on negative feedback of the HPG axis were assessed. In contrast, recent studies of looking at androgen exposure have mainly focused on the post-hatch stages of development.

Estradiol levels in female Fadrozole treated embryos were significantly lower than in female control embryos, but this pattern was not seen in males. This could be due to the greater amount of estradiol and aromatase in females as compared to males. Additionally, females had a significantly higher level of estradiol than males, which is consistent with published literature and is most likely due to the major role that estrogen plays in female development. There was no significant difference in testosterone levels between treatment groups or sexes, implying that it is the decrease in estradiol, and not a subsequent increase in testosterone, that lead to the increase in immunity in Fadrozole treated embryos in previous studies. This work also implies that estradiol is not affecting negative feedback in the HPG axis of chicken embryos but that it may in fact be affected by testosterone.


estrogen, testosterone, HPG axis, Fadrozole, development

Access Type

Honors Thesis

Degree Type

Bachelor of Science



First Advisor

Z. Morgan Benowitz-Fredericks