10.3 Menstrual cycle and hormonal regulation

The menstrual cycle is a complex dance of hormones orchestrating changes in the female reproductive system. It involves four phases: menstrual, follicular, ovulatory, and luteal, each with distinct physiological changes and hormone fluctuations.

Key players in this hormonal ballet include gonadotropins like FSH and LH, and ovarian hormones estrogen and progesterone. These work together through intricate feedback mechanisms, regulated by the hypothalamus-pituitary-ovary axis, to control the cycle's progression and prepare for potential pregnancy.

Phases of menstrual cycle

Physiological changes and duration of each phase

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• The menstrual cycle is divided into four main phases: menstrual, follicular, ovulatory, and luteal
  1. Menstrual phase (3-7 days): Shedding of the uterine lining (endometrium) if pregnancy has not occurred, marked by menstrual bleeding
  2. Follicular phase (from first day of menstruation to ovulation): Growth and development of ovarian follicles stimulated by follicle-stimulating hormone (FSH), leading to a rise in estrogen levels
  3. Ovulatory phase (around day 14 of a 28-day cycle): Surge in luteinizing hormone (LH) triggers the release of a mature egg from the ovary (ovulation), estrogen levels peak
  4. Luteal phase (after ovulation until the start of the next menstrual period): Remnants of the ovarian follicle transform into the corpus luteum, which secretes progesterone to prepare the uterine lining for potential implantation of a fertilized egg

Hormonal changes during the menstrual cycle

• Estrogen levels rise during the follicular phase, peak at ovulation, and decline during the luteal phase
  • Estrogen stimulates the proliferation of the uterine lining (endometrium) in preparation for potential implantation of a fertilized egg
• Progesterone levels increase during the luteal phase, secreted by the corpus luteum
  • Progesterone maintains the uterine lining and supports early pregnancy
• Absence of pregnancy leads to a decline in progesterone and estrogen levels, triggering menstruation and the start of a new cycle

Key hormones in menstrual regulation

Gonadotropins and their functions

• Gonadotropin-releasing hormone (GnRH): Secreted by the hypothalamus, stimulates the anterior pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
• Follicle-stimulating hormone (FSH): Stimulates the growth and development of ovarian follicles, which contain immature eggs, and promotes the production of estrogen by the follicles
• Luteinizing hormone (LH): Triggers ovulation, the release of a mature egg from the ovary, and stimulates the formation of the corpus luteum and the secretion of progesterone

Ovarian hormones and their roles

• Estrogen: Primarily produced by the growing ovarian follicles
  • Stimulates the proliferation of the uterine lining (endometrium) in preparation for potential implantation of a fertilized egg
  • Exerts negative feedback on FSH secretion
  • Rising levels provide positive feedback on LH secretion, leading to the LH surge and ovulation
• Progesterone: Secreted by the corpus luteum after ovulation
  • Maintains the uterine lining and supports early pregnancy
  • Exerts negative feedback on LH secretion

Feedback mechanisms of reproductive hormones

Negative feedback

• Rising levels of hormones inhibit further secretion of the same or other hormones
• Examples:
  1. High levels of estrogen and progesterone inhibit the secretion of GnRH, FSH, and LH
  2. Estrogen exerts negative feedback on FSH secretion

Positive feedback

• Rising levels of hormones stimulate further secretion of the same or other hormones
• Example: The LH surge that triggers ovulation is an example of positive feedback, as rising estrogen levels stimulate a sudden increase in LH secretion

Absence of pregnancy and cycle restart

• Decline in progesterone and estrogen levels in the absence of pregnancy removes the negative feedback on GnRH, FSH, and LH secretion, allowing the cycle to begin anew

Hypothalamus-pituitary-ovary axis

Role of the hypothalamus

• Secretes gonadotropin-releasing hormone (GnRH) in a pulsatile manner, which stimulates the anterior pituitary gland to release FSH and LH

Role of the anterior pituitary gland

• Secretes FSH and LH in response to GnRH stimulation
• FSH and LH act on the ovaries to regulate follicular development, ovulation, and hormone production

Role of the ovaries

• Contain follicles that house immature eggs (oocytes)
  • In response to FSH stimulation, the follicles grow and secrete estrogen
  • Rising estrogen levels provide negative feedback on FSH secretion and positive feedback on LH secretion
• The LH surge triggered by high estrogen levels causes ovulation, the release of a mature egg from the dominant follicle
• The remaining follicular cells transform into the corpus luteum, which secretes progesterone to maintain the uterine lining

Cycle restart

• The decline in progesterone and estrogen levels at the end of the luteal phase, in the absence of pregnancy, allows the cycle to restart with the onset of menstruation and the removal of negative feedback on GnRH, FSH, and LH secretion