11.2 Female Reproductive Cycle and Hormonal Regulation

The menstrual cycle is a recurring monthly cycle of hormonal and physiological changes in the female reproductive system that prepares the uterus for potential pregnancy. It averages 28 days and is regulated by a coordinated interplay of hormones from the hypothalamus, anterior pituitary, and ovaries.

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Phases of the Menstrual Cycle

The menstrual cycle is divided into ovarian and uterine phases that occur simultaneously.

Ovarian Cycle

Uterine Cycle

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Hormones and Their Roles

1. Gonadotropin-Releasing Hormone (GnRH)

• Secreted by the hypothalamus
• Stimulates the anterior pituitary to release FSH and LH

2. Follicle Stimulating Hormone (FSH)

• Secreted by the anterior pituitary
• Stimulates development of primary follicles into Graafian follicles
• Induces follicular cells to secrete estrogen

3. Estrogen

• Secreted by growing follicles
• Causes proliferation (repair and thickening) of the uterine endometrium
• At high levels, triggers a positive feedback surge of LH
• Exerts negative feedback on FSH/LH at moderate levels

4. Luteinizing Hormone (LH)

• Secreted by the anterior pituitary
• The LH surge (~day 14) triggers ovulation — rupture of the Graafian follicle
• Stimulates the ruptured follicle to form the corpus luteum

5. Progesterone

• Secreted by the corpus luteum
• Maintains the secretory endometrium for potential implantation
• Exerts negative feedback on the anterior pituitary, inhibiting FSH and LH
• If fertilization does not occur, the corpus luteum degenerates → progesterone drops → menstruation begins

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Key Structures

Graafian Follicle

A mature ovarian follicle containing the secondary oocyte, surrounded by follicular cells that produce estrogen. It ruptures at ovulation.

Corpus Luteum

A yellowish glandular structure formed from the ruptured Graafian follicle after ovulation. It is the primary source of progesterone during the luteal phase. If fertilization does not occur, it degenerates into the corpus albicans by day 28.

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Hormonal Feedback Mechanisms

Negative Feedback

• Moderate estrogen + progesterone → inhibit GnRH, FSH, and LH → prevent new follicle development during the luteal phase

Positive Feedback (LH Surge)

• High estrogen (just before ovulation) → stimulates a massive LH surge → triggers ovulation

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Summary: Hormonal Timeline

$\text{GnRH}\to \text{FSH}\to \text{Follicle growth}\to \text{Estrogen}\uparrow \to \text{LH surge}\to \text{Ovulation}$

$\text{Corpus Luteum}\to \text{Progesterone}\uparrow \to \text{Endometrium maintained}$

$\text{No fertilization}\to \text{Corpus Luteum degenerates}\to \text{Progesterone}\downarrow \to \text{Menstruation}$

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