- The aim of the female menstrual cycle is to allow for oocyte release and prepare the uterus for a possible pregnancy.
- It begins at puberty, ranging from the ages of 10 to 16, and ends at menopause at an average age of 51.
- It may last anywhere from 21 days to 35 days with an average duration of 28 days
- The average woman has 450 menstrual cycles in her lifetime
- Hormones are secreted in a negative and positive feedback manner to control the menstrual cycle. It is controlled by the hypothalamus which releases gonadotropin-releasing hormone (GnRH) in an increased, pulsatile fashion once puberty starts.
- GnRH is then transported to the anterior pituitary where it activates its 7-transmembrane G-protein receptor. This provides a signal to the anterior pituitary to secrete stimulating follicle hormone (FSH) and luteinizing hormone (LH).
- FSH and LH provide input to the ovaries. Within the ovarian follicle, there are 2 cell types responsible for hormone production, theca cells, and granulosa cells.
- LH stimulates theca cells to produce progesterone and androstenedione by activating the enzyme, cholesterol desmolase. Once androstenedione is secreted, the hormone diffuses to the nearby granulosa cells.
- FSH stimulates the granulosa cells to convert androstenedione to testosterone then 17-beta-estradiol by activating the enzyme, aromatase. As levels of 17-beta-estradiol or progesterone increase based on the phases of the menstrual cycle, there is negative feedback back to the anterior pituitary to lower the levels of FSH and LH being produced and subsequently, the levels of 17-beta-estradiol and progesterone produced.
- An exception to this is during ovulation, in this case, once a critical amount of 17-beta-estradiol is produced it provides positive feedback to the anterior pituitary to produce increased amounts of FSH and LH. This feedback system is represented in figure 1. Additionally, within the feedback system, the granulosa cells produce inhibin and activin, which inhibit and stimulate FSH release from the anterior pituitary, respectively.
- This feedback mechanism is controlled by upregulating, to increase hormone production, or downregulating to decrease hormone production, the GnRH receptors on the anterior pituitary.
Phase 1: The Follicular, or Proliferative Phase
- Day 0 to day 14 of the menstrual cycle (assuming 28 day cycle)
- Cycle variation usually due to the variable follicular phase.
- Main hormone oestrogen, specifically 17-beta-estradiol.
- Due to upregulation of the FSH receptors within the follicle
- As the cycle progresses 17-beta-estradiol will provide negative feedback to the anterior pituitary.
- The action is to grow the endometrial layer of the uterus with increased amounts of stroma and glands and increasing the depth of the arteries that supply the endometrium, the spiral arteries.
- This phase creates an environment that is friendly to sperm.
- 17-beta-estradiol achieves this by creating channels within the cervix allowing for sperm entry.
- The channels are made within the abundant, watery and elasticity changes of the cervical mucous.
- A primordial follicle begins to mature to a Graafian follicle. The surrounding follicles begin to degenerate which is when the Graafian follicle becomes the mature follicle. This sets up the follicle for ovulation, the next step.
- ALWAYS Occurs 14 days before menses.
- At the end of the proliferative phase, 17-beta-estradiol levels are at a high due to the follicle maturation and increased production of the hormone.
During this time only, 17-beta-estradiol provides positive feedback for FSH and LH production.
- This occurs when a critical level of 17-beta-estradiol is reached, at least 200 picograms per millilitre of plasma.
- There is a high FSH and LH "the LH surge". As a result, the mature follicle breaks, and an oocyte is released.
- The changes to the cervix as initiated during the follicular phase further increases allowing for increased, waterier cervical mucous to better accommodate the possible sperm. The levels of 17-beta-estradiol fall at the end of ovulation.
Phase 2: The Luteal or Secretory Phase
- Day 14 to day 28 of the cycle.
- Progesterone stimulated by LH is the dominant hormone
- This prepares the corpus luteum and the endometrium for possible fertilized ovum implantation. As the luteal phase ends, progesterone will provide negative feedback to the anterior pituitary to decrease FSH and LH levels and subsequently, the 17-beta-estradiol and progesterone levels.
- The corpus luteum is a structure formed in the ovary at the site of the mature follicle rupture to produce 17-beta-estradiol and progesterone, which is predominate at the end of the phase due to the negative feedback system. The endometrium prepares by increasing its vascular supply and stimulating more mucous secretions.
- This is achieved by the progesterone stimulating the endometrium to slow down endometrial proliferation, decrease lining thickness, develop more complex glands, accumulate energy sources in the form of glycogen, and provide more surface area within the spiral arteries.
- Cervical mucous as the progesterone decreases and thickens the cervical mucous making it non-elastic, since the fertilization time period passed, and sperm entry is no longer a priority.
- Progesterone increases the hypothalamic temperature, so body temperature increases during the luteal phase. Near the end of the secretory phase, plasma levels of 17-beta-estradiol and progesterone are produced by the corpus luteum.
- If pregnancy occurs, a fertilized ovum is implanted within the endometrium, and the corpus luteum will persist and maintain the hormone levels. However, if no fertilized ovum is implanted, then the corpus luteum regresses, and the serum levels of 17-beta-estradiol and progesterone decrease rapidly.
- When the hormone levels decrease, the endometrium layer as it has been changed throughout the menstrual cycle is not able to be maintained. This is called menses, considered day 0 to day 5 of the next menstrual cycle.
- The duration of menses is variable. Menses or menstrual bleeding is when there is sloughing of the endometrial lining and its blood. To continue the process of the menstrual cycle, primordial follicles begin to develop and start the follicular phase again in hopes of a pregnancy.