Female Reproductive Endocrinology: Menstrual Cycle

Female Reproductive Endocrinology: Menstrual Cycle

• The menstrual cycle, a roughly 28-day cycle, is divided into follicular and luteal phases.
• Follicular phase: The first half, preceding ovulation; variable in length (10-14+ days).
• Luteal phase: The second half, following ovulation; more consistent length.

Hormonal Regulation of the Menstrual Cycle

• LH (Luteinizing Hormone) and FSH (Follicle-Stimulating Hormone): Peak at ovulation.
• Estrogen: Increases before ovulation.
• Progesterone: Low in the follicular phase, rises sharply in the luteal phase.

Ovarian Events During the Cycle

Follicular Phase

• Multiple follicles with ova develop.
• One follicle matures into a dominant follicle.
• Dominant follicle secretes Estrogen.
• Estradiol (a form of estrogen), triggers positive feedback loop leading to ovulation.

Luteal Phase

• Corpus luteum forms after ovulation, secretes both progesterone and estrogen.
• High progesterone levels trigger negative feedback loop.

Follicular Phase: Detailed Look

Early Follicular Phase

• LH stimulates androgen production in thecal cells.
• FSH stimulates aromatase activation in granulosa cells, converting androgens to estrogen.

Mature Follicle

• LH and FSH stimulate estrogen production.
• Estrogen increases FSH receptors in granulosa cells (positive feedback loop).
• Increased FSH receptors lead to more estrogen production.

Dominant Follicle

• High estrogen production and low androgen production make the dominant follicle.
• Estrogen increases FSH receptors for continued follicle development.
• FSH supports follicle growth & nutrient production for the ovum.
• Granulosa cells also produce inhibin, regulating FSH.

Key Takeaways: Follicular Phase

• LH triggers androgen production; FSH promotes estrogen synthesis.
• Estrogen enhances FSH sensitivity, leading to dominant follicle.
• Hormonal regulation selects the dominant follicle.

Luteal Phase

• Corpus luteum formation after ovulation.
• LH is the key for production of both estrogen and progesterone in the corpus luteum
• Both estrogen and progesterone are produced by LH stimulation.
• Progesterone and Estrogen create Negative Feedback

Hormonal Changes and Feedback Loops

Early Follicular Phase

• High FSH stimulates multiple follicles.
• Estrogen production increases from these follicles.
• High estrogen leads to negative feedback, suppressing FSH.
• Dominant follicle becomes highly sensitive to FSH.

Mid-Cycle (Ovulation)

• High estrogen from the dominant follicle triggers a positive feedback loop.
• Resultant surge in LH and FSH initiates ovulation.

Luteal Phase

• Corpus luteum produces estrogen and progesterone.
• Progesterone and estradiol, create negative feedback, reducing LH and FSH.

Pulsatile Hormone Secretion

• Frequent, low-amplitude LH pulses during the follicular phase (estradiol only).
• Slow, progesterone-infuenced LH and FSH pulses during the luteal phase.

Summary of the Menstrual Cycle

• Early Follicular Phase: FSH stimulates initial follicle development, LH stimulates androgen production; FSH converts to estrogen.
• Dominant Follicle: Highest estrogen output makes dominant follicle ; Estrogen promotes growth and heightened sensitivity to FSH.
• Ovulation: High estrogen triggers positive feedback loop, resulting in LH and FSH surge.
• Luteal Phase: Corpus luteum produces progesterone and estrogen, initiating negative feedback on hormone.
• End of Cycle: Corpus luteum regress; reduction of progesterone and estrogen results in next cycle initiation.

Key Points

• The menstrual cycle is a complex hormonal interplay.
• Understanding the hormonal feedback loops (positive and negative) is crucial to comprehension.
• Review the hormonal changes alongside the corresponding phases.