Ovarian and Menstrual Cycles

• Ovarian Cycle vs. Menstrual Cycle

  • Ovarian cycle and menstrual cycle are synchronized but distinct.

  • The menstrual cycle is simpler as the uterus responds to hormonal cues without producing hormones itself.

  • The ovarian cycle is more complex due to the involvement of multiple hormones.

  Hormone Graph

  • A graph showing hormone peaks during different phases of the ovarian cycle is crucial.

  Review Questions

  • The questions covered:
    * Synapses and hormone release
    * ADH (Vasopressin) and the Posterior Pituitary
    * GnRH and the Hypothalamus (Arcuate Nucleus)
    * Location of GnRH entry into the bloodstream
    * Release of FSH (Follicle Stimulating Hormone) from the anterior pituitary, affected by estrogen and testosterone
    * Hormone half-lives (ABC hormones)

  Follicle Development

  • Females are born with all the eggs they will ever have, paused at a specific stage of development.

  • Each cycle (approximately 28 days), FSH stimulates some follicles to begin growing.

  • Multiple follicles start developing, but typically only one matures fully.
    * Twins are an exception; nature favors single pregnancies due to resource demands.
    * Fraternal twins: two follicles are released.
    * Identical twins: one follicle splits during development.

• Primary follicles develop in response to FSH, leading to a mature or dominant follicle.

• A dip in FSH helps to eliminate developing follicles that haven't fully matured, ensuring only the most developed follicle ovulates.

Ovulation

• Ovulation leads to the release of an egg, ideally into the fallopian tube.

Ectopic Pregnancy

• Ectopic pregnancy: pregnancy occurs outside the uterus.
  * Most commonly in the fallopian tube.
  * Rarely, a fertilized egg escapes and implants in the abdominal pelvic cavity.

• Full-term pregnancy is impossible in ectopic scenarios; surgical removal is necessary.

• Symptoms include severe, one-sided lower abdominal pain in reproductive-age females.

• Rupture of the fallopian tube can cause life-threatening blood loss.

• Loss of the affected fallopian tube impacts future fertility.

Hormone Production

• Ovaries produce estrogen and progesterone.

• During the follicular phase (primary follicle to ovulation), the tissue surrounding the follicle (zona pellucidum or corona radiata) produces estrogen.

• After ovulation, the structure transforms into the corpus luteum, producing progesterone.
  * Latin for yellow body

• The corpus luteum eventually shrivels up and becomes the corpus albicans, which doesn't produce hormones.

Ovarian Reserve and Cycles

• Females have tens of thousands of follicles at birth.

• Average of 450 ovarian cycles between menarche (first period) and menopause (last period).

• Older ovaries show remaining undeveloped follicles and corpus albicans as residual tissue.

• Irregular cycles vary in length; some individuals may experience longer or less frequent cycles.

Uterine Cycle

• The uterine cycle involves the endometrium, the inner layer of the uterus.
  * Myometrium: muscular layer responsible for cramps.
  * Endometrium: grows and sheds each cycle.
  * Has 3 phases including the menstruation phase.

Menstruation Phase

• Menstruation phase: endometrial lining is shed (typically lasts around 7 days).

• Evolutionary safeguard to eliminate less than thriving pregnancies.

• Menstruation is relatively rare in the animal kingdom (primates, elephant shrews).

• The menstrual cycle is linked to the high level of resources humans invest in offspring, ensuring quality over quantity.

Ovarian Events and Hormones

• Follicular Phase (Day 1-14):
  * Maturation of primary follicles.
  * Also known as the preovulatory phase.
  * Follicles produce estrogen.

• Ovulation: occurs around day 14.

• Luteal Phase (Post-Ovulatory Phase):
  * Corpus luteum develops and produces progesterone.

Hormonal Regulation

• GnRH (Gonadotropin-Releasing Hormone) is where the whole process begins.
  * The hypothalamus releases GnRH in a pulsatile fashion.

• GnRH stimulates the production of FSH (Follicle Stimulating Hormone) and LH (Luteinizing Hormone).
  * FSH responds more strongly to GnRH.

• FSH stimulates follicle growth, leading to increased estrogen production.

Estrogen Feedback

• Estrogen provides feedback to the brain about follicle development.

• High levels of estrogen inhibit FSH production (negative feedback).

• However, at high concentrations, estrogen switches to positive feedback, stimulating GnRH and LH production.

• This surge of LH drives ovulation.

Positive Feedback Loop for Ovulation

• High estrogen $\rightarrow$ more GnRH $\rightarrow$ more FSH/LH $\rightarrow$ more estrogen.

• Runaway train effect leading to ovulation.

• Follicle ceases estrogen production following ovulation, and the corpus luteum forms.

Luteal Phase and Progesterone

• The luteal phase follows ovulation, during which the corpus luteum produces progesterone.

• Progesterone maintains the uterine lining and suppresses GnRH and FSH production, preventing new follicle development.

• Progesterone always exerts negative feedback.

• Important for if pregnancy happens

• Birth control pills utilize progesterone to suppress the HPG axis.

Corpus Luteum and Menstruation

• If pregnancy doesn't occur, the corpus luteum shrinks into the corpus albicans, ceasing progesterone production.

• The drop in progesterone signals the endometrial lining to shed, triggering menstruation.

• The last pills in birth control packs contains no progesterone to initiate menstruation.

Narrow Window for Pregnancy

• Pregnancy can only occur within a narrow window, approximately from day 9 to 16 of a 28-day cycle.

• In vitro fertilization involves FSH and LH to harvest follicles.

• Sperm can survive for several days inside the female reproductive tract.

• The ovum survives only about 24 hours after ovulation.

• Having sex before ovulation can increase the chance of pregnancy.

Additional Notes

• Plan B interferes with progesterone production or action, preventing implantation.

• Dysmenorrhea is painful menstruation. Meneralgia is heavy menstruation.