The reproductive system of women produces oocytes (eggs) and hormones, and these functions are controlled by hypothalamic and anterior pituitary hormones. The follicle is the functional unit of the ovary. Usually one follicle matures and ovulates each month as controlled by a cyclical feedback interplay between ovarian hormones and the hypothalamic–
learning outcomes
You should be able to:
Link the events in the ovarian and menstrual cycles.
Describe the interaction between the granulosa cells and the thecal cells of the follicle in limiting the number of eggs that are ovulated in each cycle.
Explain how the blastocyst prevents menstruation.
Explain how mechanical and hormonal stimuli interact during labor.
What events in the ovary are responsible for the onset of menses, and what prevents menses if pregnancy occurs?
The events in the ovary that are responsible for the onset of menses are the degradation of the corpus luteum and the consequent decrease in production and release of estrogen and progesterone. However, if a blastocyst implants in the uterus, it produces human chorionic gonadotropin (hCG), which maintains the corpus luteum until the extraembryonic membranes take over the production of estrogen and progesterone.
What are the roles of thecal cells and granulosa cells in the ovarian cycle, and how do they limit the number of eggs that are ovulated?
The granulosa cells nurture the developing egg, providing it with nutrients, growth factors, and mRNAs. The granulosa cells also produce estrogen and progesterone in response to FSH. Estrogen is produced by the granulosa cells from testosterone that is produced in the outer cells of the follicle, the thecal cells. Estrogen stimulates the granulosa cells to produce FSH receptors, but at the same time estrogen exerts negative feedback on the production and release of FSH from the anterior pituitary. Since multiple follicles begin to mature in each menstrual cycle, only the one with the most FSH receptors fully matures as the estrogen levels rise and FSH levels fall.
How does positive feedback between hormonal and mechanical processes play a role in childbirth?
Prior to the onset of labor, the contractility of the uterine muscle increases, and at the same time the growing fetus is causing more stretch of the uterine muscle. Stretch of the uterine muscle stimulates contractions, pressing the head of the fetus onto the cervix. That stimulates neural signals to the hypothalamus, causing it to release oxytocin from the posterior pituitary. Oxytocin increases the uterine contractions, which in turn puts more pressure on the cervix, resulting in even more oxytocin being released from the posterior pituitary—
Understanding the physiology of human reproduction has led to numerous methods and technologies for controlling it, either to prevent unwanted pregnancies or to overcome infertility.