Endocrine Signalling & Reproductive Biology 5

Introduction to Reproductive Endocrinology

  • Focus on women's reproductive system, particularly the ovary, menstrual cycle, and ovarian cycle.
  • Distinction between menstrual and ovarian cycles, though they occur concurrently.

Overview of the Ovaries

  • Ovaries function as a single organ; there are two, but they coordinate like one.
  • Monoovulatory species; typically one oocyte is ovulated per cycle.
  • Feedback regulation is crucial for single oocyte ovulation.

Ovarian and Menstrual Cycles

  • Begin counting days from onset of menses:
    • Day 0 is marked by the LH surge.
    • LH surge leads to ovulation approximately 36 hours later.
  • Two main phases: follicular phase (before ovulation) and luteal phase (after ovulation).
    • Follicular phase lasts about 14 days; ovarian follicle develops during this time.
    • Luteal phase also lasts about 14 days; following ovulation, the corpus luteum forms.
  • Uterine cycle synchronized with ovarian cycle:
    • Menstruation lasts up to 7 days.
    • Proliferative phase follows, where the endometrium regenerates.

Hormonal Regulation of the Ovarian Cycle

  • Gonadotropin release from the hypothalamus regulates the cycle:
    • FSH (Follicle-stimulating hormone) increases at the beginning of the follicular phase.
    • LH (Luteinizing hormone) has a mid-cycle surge to induce ovulation.
  • Antral follicles produce inhibin B that negatively regulates FSH production.
  • Dominant follicle grows through increased estrogen production, promoting rapid growth and ovulation.

Hormonal Functions

  • Hormones involved in the cycle:
    • FSH: Stimulates follicular development.
    • LH: Triggers ovulation and maintains the corpus luteum.
    • Oestradiol: Supports endometrial growth, has feedback effects on FSH and LH.
    • Inhibin B: Produced by granulosa cells; negatively regulates FSH.
    • Inhibin A: Produced by corpus luteum; also influences FSH regulation.
    • Progesterone: Maintains the secretory state of the endometrium.

Feedback Mechanisms

  • Hormonal feedback loops:
    • Positive feedback from estrogen leads to LH surge; negative feedback stabilizes hormone levels post-ovulation.
    • The corpus luteum eventually dies if pregnancy does not occur, leading to a decrease in progesterone and estrogen, allowing FSH to rise again, restarting the cycle.

Follicle Development

  • Progression from primordial to Graafian follicle takes approximately 3-6 months.
  • Different stages of follicle development are characterized by:
    • Presence of granulosa and theca cells.
    • Development of antrum and hormone production.
    • FSH-dependent growth in the latter stages of development.

Key Stages of the Ovarian Cycle

  1. Early-Mid Follicular Phase:
    • Steady pulses of FSH induce growth of several follicles.
  2. Late Follicular Phase:
    • Increased estrogen from the dominant follicle; inhibin B suppresses FSH.
  3. Ovulation:
    • LH surge triggers ovulation; positive feedback maintains heightened estrogen levels.
  4. Luteal Phase:
    • Corpus luteum produces high levels of estrogen and progesterone; prepares endometrium for possible implantation.

Role of Estrogen in the Female Body

  • Multiple effects including:
    • Regulating metabolism (proteins, carbohydrates, lipids).
    • Maintaining reproductive structures and supporting mammary gland function.
    • Influencing bone health and distribution of body fat.
  • Antimüllerian hormone: Indicates ovarian reserve, reflects the health and number of ovarian follicles.

Conclusion

  • Understanding the complex interplay of hormones in the ovarian and menstrual cycles is crucial for comprehending female reproductive health and the mechanisms leading to conditions like menopause.
  • These processes exemplify how endocrine regulation influences physiological changes throughout a woman's reproductive life.