Chapter 28H: Female Reproductive Cycle: Hormonal Regulation and Phases

Introduction to the Female Reproductive Cycle

• The female reproductive cycle is divided into two main phases of the ovarian cycle: the Follicular Phase (preovulatory phase) and the Luteal Phase (postovulatory phase).

• The cycle's timeline is typically measured from $Day ext{ } 0$ (marking the beginning of a new cycle) to $Day ext{ } 28$ (the end of the preceding cycle), emphasizing its cyclical nature.

• The diagram of the cycle integrates information across several panels:

  • Gonadotropic Hormone Levels: Displays the fluctuating levels of Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH).

  • Follicle Development: Illustrates the growth and changes within the ovarian follicle.

  • Ovarian Hormone Levels: Shows the concentrations of estrogen, progesterone, and inhibin.

  • Endometrial Lining: Depicts the transformations of the uterine endometrium throughout the cycle.

• Body temperature changes can also be tracked to indicate ovulation, though it's not the primary focus of this discussion.

The Follicular Phase (Preovulatory Phase)

• Hormonal Initiation: The cycle begins with the release of FSH and LH from the anterior pituitary, stimulated by Gonadotropin-Releasing Hormone (GnRH) from the hypothalamus (as discussed in previous lectures).

• Follicle Growth and Estrogen Production:

  • FSH and LH stimulate the growth and development of ovarian follicles.

  • As the follicles mature, the number of granulosa cells within them increases significantly.

  • This increase in granulosa cells directly correlates with a substantial rise in estrogen production, as estrogen is synthesized from precursors within these cells.

• Androstenedione and Estrogen Synthesis:

  • LH specifically stimulates thecal cells (interstitial cells) within the follicle to produce androstenedione, a steroid hormone.

  • Androstenedione serves as a key precursor that granulosa cells then convert into estrogen. This conversion pathway can also involve testosterone as an intermediate.

• Uterine Proliferative Phase: The rising estrogen levels signal the uterus to enter the proliferative phase.

  • During this phase, the endometrium, which was shed during menses, undergoes repair and begins to thicken, laying down new tissue in preparation for a potential pregnancy.

• Hormonal Feedback and the LH Surge (Around $Day ext{ } 14$):

  • As estrogen levels rise, they initially exert a negative feedback effect on FSH and LH, causing their levels to dip before $Day ext{ } 14$. This feedback loop indicates that the follicle is maturing sufficiently.

  • Concurrently, progesterone and inhibin levels modest rise. Inhibin provides specific negative feedback to suppress FSH production, contributing to the dip.

  • Critically, once estrogen levels reach their peak, they switch from negative to positive feedback, triggering a massive surge in LH known as the luteal surge.

  • This LH surge is the primary event that directly leads to ovulation, the release of the secondary oocyte from the mature follicle.

The Luteal Phase (Postovulatory Phase)

• Corpus Luteum Formation: After ovulation, the ruptured follicle transforms into a temporary endocrine structure called the corpus luteum (or corpus luteum).

• Hormonal Shift and Dominance:

  • Estrogen production decreases immediately after ovulation.

  • Progesterone becomes the dominant hormone of this phase. Its name literally means