Hormonal Regulation of the Ovarian Cycle and Contraception

Hormonal Changes During the Ovarian Cycle

The levels of LH, FSH, estrogen, and progesterone fluctuate significantly throughout the ovarian cycle:

• FSH stimulates follicular maturation, peaking near ovulation on the 14th day, after which it remains at low concentrations.
• The LH surge, beginning around the 9th day, promotes endometrial proliferation and the release of testosterone, which is converted into estradiol via aromatase.
• Peaks in FSH and LH trigger follicular rupture and the formation of the corpus luteum.
• The corpus luteum produces progesterone and estrogen.
• Progesterone transforms the proliferative endometrium into a secretory state, preparing it for nidation.
• If fertilization does not occur, the corpus luteum degrades, leading to a drop in progesterone and estrogen, which triggers endometrial shedding.
• If fertilization occurs, the corpus luteum persists, and the placenta eventually takes over hormone production by the third month of pregnancy.
• High levels of estrogen exert negative feedback on FSH and LH, preventing the maturation of additional follicles during the second half of the cycle.

Ovarian Regulation Mechanisms

The regulation of the ovarian cycle involves complex feedback loops:

• LHRH stimulates the release of FSH and LH.
• FSH acts on granulosa cells to stimulate follicular maturation and aromatase activity.
• LH acts on thecal cells to produce androstenedione and testosterone, which are converted to estradiol.
• Estradiol provides negative feedback on FSH and influences secondary sexual characteristics, including breast growth, uterine development, vaginal epithelium maintenance, and cartilage ossification.
• Progesterone from the corpus luteum prepares the uterine lining for implantation. Following fertilization, the embryo releases hCG, which mimics LH to maintain the corpus luteum.

Mechanism of Contraceptive Action

Contraceptives utilize hormonal regulation to prevent pregnancy through several pathways:

1. Suppression of Follicular Growth: High levels of exogenous estrogen and progesterone inhibit FSH, preventing follicular development.
2. Inhibition of Ovulation: The combined action of estrogen and progesterone prevents the mid-cycle LH and FSH surge, effectively blocking ovulation.
3. Endometrial Thinning: Hormonal contraceptives limit endometrial growth, which often results in reduced menstrual pain due to lower prostaglandin release.
4. Cervical Mucus Alteration: While estrogen may increase mucus volume, progesterone makes the cervical mucus thicker and more viscous, creating a physical barrier that prevents sperm penetration.