The Effects of Exogenous Hormones on the Progression of Oocytes through Prophase I of Meiosis

The process of germ cell cyst breakdown and primordial follicle formation that begins during embryonic development in mammalian ovaries is vital for future reproductive success because it establishes the total pool of viable egg cells the female will have for the rest of her life. Germ cells arrive at the gonad in the mouse at approximately 10.5 days post coitum and begin to form germline cysts through incomplete mitosis. At 13.5 days post coitum, the germ cells enter meiosis and begin to progress through prophase I of meiosis I. Germ cells progress through a series of sub-phases of prophase I starting at pre-meiotic interphase and then moving through leptotene, zygotene, pachytene, and eventually arresting at the diplotene stage. The germ cells enter diplotene arrest beginning at 17.5 days post coitum, which corresponds with the start of cyst breakdown. As more germ cells arrive at diplotene and cysts begin to break down to form primordial follicles, some oocytes experience programmed cell death. The protein MSY2 is believed to be upregulated at the diplotene stage and therefore could be used as a marker for diplotene. Through analysis of the MSY2 protein from 15.5 days post coitum through postnatal day 5 using immunohistochemistry, we were able to see an increase in expression over time. The increase of expression was expected as our current model suggests that more cells should be arriving at diplotene as development progresses. Synaptonemal complex protein 3 (SYCP3) has been shown to be expressed during the pachytene stage of meiotic prophase I and therefore can be used as a marker for this developmental stage. Analysis of SYCP3 starting at 13.5 days post coitum through postnatal day 5 using immunohistochemistry showed the greatest percentage of oocytes in the pachytene stage at postnatal day 1. Exogenous hormone exposure on developing ovaries can disrupt cyst breakdown and follicle formation, but the process by which the hormone exposure disrupts the cyst breakdown is not well understood. It is also unclear how hormones affect progression through prophase I of meiosis. Recreating environments that mimic exogenous hormone exposure that a female might experience can lead to a better understanding of how early such exposure can lead to further issues with fertility. To examine the effect or effects of hormones on meiosis, ovaries were dissected at 17.5 days post coitum and grown in culture for four days, reaching postnatal day 3, in the presence of 10 -6 M estradiol, 10 -6 M progesterone, both 10 -6 M estradiol and 10 -6 M progesterone, or neither hormone. Ovaries were fixed and double labeled with MSY2 and oocyte marker TRA98 or SYCP3 and oocyte marker VASA. We found a decrease in MSY2 expression and an increase in the number of oocytes in the pachytene stage following hormone treatment, suggesting that estradiol and progesterone can block progression to the diplotene stage of meiosis. This work will contribute to our understanding of the effects of hormones on oocyte development by determining if there are any disruptions in development following the exposure. Information regarding the timing of hormone exposure with regards to the extent of developmental disruptions will allow for a clearer timeline of when an individual could experience fertility issues through such exposure. The Effects of Exogenous Hormones on the Progression of Oocytes through Prophase I of Meiosis