Module 3 Lesson 5 - 3
Lesson 5 — Female Hormones: Menstrual Cycle
Oogenesis
A female begins to produce eggs while still a fetus. At birth, a female baby has about 2,000,000 immature egg cells, called primary oocytes, inside her ovaries. At puberty, about 400,000 of these oocytes remain. Only about 400 primary oocytes will mature in a woman's lifetime, usually one for each menstrual cycle during the woman's reproductive years. At menopause, the number of primary oocytes is reduced to zero.
The process of forming female gametes is called oogenesis. Until puberty, the primary oocytes stay in a state of suspended animation as immature egg cells in the ovaries. While in the ovary, each primary oocyte is surrounded by several follicle cells, which are the structural supporting cells that surround and develop the egg. The primary oocyte and the follicle cells are referred to as an immature follicle.
At the onset of puberty, some immature follicles develop further into secondary oocytes because of a rise in follicle stimulating hormone (FSH). Normally, only one oocyte matures each month.
Similar to Sertoli cells in the male reproductive system, the follicle cells perform a secretory and structural support function for the developing gamete. The follicle cells give structural support by fully surrounding the egg, thereby helping to nourish and protect the primary oocyte and helping it develop into a secondary oocyte. The follicle cells give secretory support by releasing estrogen and some progesterone.
Ovulation occurs when the secondary oocyte is expelled from the ovary. Once the sperm burrows through the outer barrier of the secondary oocyte, the oocyte completes its cell division into a female gamete (ovum), ready to be fertilized by a male gamete (sperm). You will study the genetic details of oogenesis (meiosis in females) in Module 5.
Menstrual Cycle
Read pages 495 - 499
Follicular Stage (Day 1 to 14)
Flow Phase (Day 1 to Day 5) On day 1 of the menstrual cycle, the endometrium is shedding due to low levels of progesterone and estrogen. This is called menstruation or period and can last 2 to 7 days.
In response to the low levels of estrogen and progesterone, the hypothalamus increases the production of gonadotropin releasing hormone (GnRH) and it triggers the anterior pituitary to release follicle stimulating hormone (FSH) and luteinizing hormone (LH).
FSH and LH travel by blood to the ovaries. Follicle stimulating hormone (FSH) stimulates the immature follicles in the ovaries to develop. The primary oocyte in a follicle develops into a secondary ooctye and the follicle cells release estrogen. At the end of menstruation, this increase in estrogen levels triggers thickening of the endometrium at about day 6, and it continues to day 14.
Growing follicles continue to release more estrogen. Through negative feedback, increases in estrogen levels suppress the production of GnRH in the hypothalamus and FSH and LH in the anterior pituitary.
Ovulation (Day 14)
As the level of FSH starts to decline through the negative feedback system, typically one follicle will have grown large enough not to require FSH to continue its development. The large follicle continues to release more estrogen and this spike in estrogen levels around day 12 triggers the large surge of LH through positive feedback on day 14, resulting in ovulation of the secondary oocyte. As a result, estrogen initially affects LH levels through negative feedback, but around day 12, a spike in estrogen causes LH levels to surge through positive feedback.
Fertilization and Pregnancy
If fertilization occurs in the Fallopian tube, the fertilized egg or zygote divides several times as it travels down the tube and the resulting developing embryo implants on the endometrium. The build-up of blood vessels and tissue in the endometrium support the development of the embryo. The elevated levels of progesterone and estrogen are maintained by the embryo through the production of human chorionic gonadotropin hormone (hCG) by the embryo. hCG maintains the corpus luteum. This ensures the maintenance of the endometrium and prevents any new development of follicles. You will learn much more detail about how the zygote develops into an embryo and then a fetus in Module 4.
Luteal Stage (Day 15 to Day 28)
The large surge of LH through positive feedback on day 14 also transforms the follicle into a yellow body called the corpus luteum. The corpus luteum releases a large amount of progesterone and some estrogen.
Progesterone makes the uterus more hospitable for the potential fertilization of the egg and developing embryo and fetus. It is an important hormone in maintaining pregnancy as it maintains the endometrium by preventing menstruation and uterine contraction.
Through a negative feedback system, increased progesterone and estrogen levels suppress the production of GnRH, FSH, and LH. Suppression of GnRH, FSH, and LH means that no new immature follicles can be developed during the luteal stage.
If fertilization does not occur about 10 to 12 days after ovulation, the corpus luteum degenerates into a white body called corpus albican and stops producing progesterone and estrogen.
Reduced levels of progesterone and estrogen result in increased production of FSH and LH, and a new group of immature follicles starts developing. Low levels of progesterone results in the break down of the endometrium lining. The discarded tissues travels down the vagina and exits the body, marking the beginning of flow phase and a new menstrual cycle again.