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Toxic Responses of the Female Reproductive System
Published in Stephen K. Hall, Joana Chakraborty, Randall J. Ruch, Chemical Exposure and Toxic Responses, 2020
Joana Chakraborty, Maureen McCorquodale
The central nervous system (CNS) plays an integrative role in the reproductive process. The neurons in the hypothalamus of the brain secrete the gonadotropin releasing hormone (GnRH). The anterior pituitary cells have receptors for GnRH. In response to GnRH, the anterior pituitary cells secrete follicle stimulating hormone (FSH) and luteinizing hormone (LH). FSH secretion is necessary for the development of the follicles, while both FSH and LH are needed for their final maturation and a burst of LH is required for ovulation and the initiation of corpus luteum formation. Estrogens, the female sex hormones, are produced by the theca cells and the corpus luteum. Granulosa cells also produce estrogens, which remain in the follicular fluid. Mature corpus luteum secretes progesterone. Progesterone is responsible for changes in the endometrium of the uterus. If the implantation takes place, then the early embryo, is capable of prolonging the life of the corpus luteum. The implanting embryo, which is now called the blastocyst is composed of different types of cells. The syncytiotrophoblast cells of the blastocyst produce a hormone which is called human chorionic gonadotropin (hCG). This hormone is critical for maintaining the progestagenic activity of the corpus luteum. After the formation of the placenta, various hormones are secreted by this organ which become central to the maintenance of pregnancy. Exogenous chemicals altering the placental activity may be damaging or lethal to the fetus.
How the quest to improve sheep reproduction provided insight into oocyte control of follicular development
Published in Journal of the Royal Society of New Zealand, 2018
Ovarian follicular growth and ovulation is controlled by complex communication among the hypothalamus, pituitary, ovary and uterus (Figure 2). The hypothalamus secretes gonadotrophin releasing hormone (GnRH), which acts on the pituitary to cause synthesis and release of the gonadotrophins, follicle stimulating hormone (FSH) and luteinising hormone (LH) (Clarke & Arbabi 2016). In turn, FSH and LH act on cells in the ovarian follicle to support its growth and maturation (Webb & Campbell 2007). Initially, FSH acts on granulosa cells to stimulate proliferation and differentiation. LH stimulates production of androstenedione from the theca that then is used as a substrate by granulosa cells to produce oestradiol. As the follicle grows, increased amounts of inhibin are also produced by the granulosa cells and, together with oestradiol, these feedback to the pituitary to inhibit FSH synthesis and release (Webb & Campbell 2007). Oestradiol, as well as progesterone produced by the corpus luteum, also inhibit GnRH release from the hypothalamus, suppressing the release of the gonadotrophins (Goodman et al. 2002). However, as the follicle continues to grow, the granulosa cells mature and they begin to express LH receptors (LHR). FSH concentrations fall during selection of the follicles that will go on to ovulation. Follicles that are mature enough to express LHR on the granulosa cells are able to survive through the additional support of LH, as FSH concentrations are suppressed. Less mature gonadotrophin dependent follicles, that have yet to express LHR on the granulosa cells, die in this environment. The mature follicles continue to grow and synthesise increasing levels of oestradiol (Webb & Campbell 2007). During the follicular phase, following the regression of the corpus luteum triggered by release of prostaglandin F2α from the uterus (Niswender et al. 2000), oestradiol reaches a critical threshold. At this stage, oestradiol actually switches from negative to positive feedback, causing the release of the preovulatory gonadotrophin surge (Caraty et al. 1995), and ovulation of the mature follicle(s) (usually one or two in sheep).