Human Ovulatory Cycle Phases and Hormonal Triggers

Duration: Approximately $28$ days.
Context: Includes spontaneous ovulation and occurs throughout the year, producing one egg per month.
Three Phases: * Menstrual phase (destructive). * Follicular phase (proliferative or estrogenic). * Luteal phase (secretory or progestational).

Timeline: Approximately $4-5$ days in length.
Uterine Changes: The uterine lining undergoes degeneration.
Follicular Development: Pulses of $LH$ and $FSH$ stimulate several antral follicles.
Hormonal Shift: Estrogen levels in the blood begin to rise.

Timeline: Starts on the last day of menses and lasts until ovulation (approximately Day $14$ ).
Hormonal Feedback: Estrogen generally exerts negative feedback, but shifts to positive feedback by Day $12-13$ .
Follicle Maturation: Progresses from a few ripe tertiary follicles (Days $10-12$ ) to one Graafian follicle by Day $13$ .
Uterine Preparation: Endometrium thickens, uterine glands enlarge, and the blood supply increases.

Timing: Occurs at the end of the follicular phase (Day $14$ ).
Primary Trigger: A peak in estrogen (Days $12-13$ ) causes a $GnRH$ surge and a subsequent $LH$ surge.
Role of $LH$ : * Initiates meiosis in the oocyte of the Graafian follicle. * Triggers ovulation approximately $9-12$ hours after the peak of the surge.
Fate of Other Follicles: All other antral follicles are doomed to atresia.

Timeline: Occupies the last half of the cycle (approximately $14$ days).
Corpus Luteum ( $CL$ ): The follicle is transformed into the $CL$ by the $LH$ surge; it secretes progesterone and estradiol.
Hormonal Feedback: High progesterone and estradiol levels exert negative feedback on the hypothalamus, keeping $LH$ and $FSH$ levels negligible.
Outcome Scenarios: * Fertilization/Implantation: Placental $hCG$ rescues the corpus luteum. * No $hCG$ : The corpus luteum regresses, leading to the menstrual phase.