Chapter 26 · The reproductive system · Topic 153

Hormonal control of the ovarian and uterine cycles

A&P IIphysiologyRead the notes

1Why this matters

Priya, 19, runs 100 km a week for her college team and eats barely enough to keep up. Her periods stopped eight months ago, and now she has a stress fracture in her shin. Nothing is wrong with her ovaries or uterus. Her brain has slowed the hormone pulses that drive them, her estrogen has fallen, and her bones are thinning. To see how one signal from the hypothalamus runs the whole monthly cycle, you need the feedback loops on this page.

2What this builds on

3Quick check before you start

1. In a positive feedback loop, the response:

  1. Opposes the original change
  2. Strengthens the original change until something outside the loop ends it
  3. Keeps the variable at its set point
Show the answer

Positive feedback pushes the variable further in the same direction; an outside event, such as delivery of the baby in childbirth, has to stop it.

  • Opposes the original change:
  • Correct: Strengthens the original change until something outside the loop ends it:
  • Keeps the variable at its set point:

2. How does GnRH reach the anterior pituitary?

  1. Along axons into the posterior pituitary
  2. Through the general circulation after passing the heart
  3. Through the hypophyseal portal system
Show the answer

Hypothalamic releasing hormones enter capillaries in the hypothalamus and are carried by portal veins straight to capillaries in the anterior pituitary.

  • Along axons into the posterior pituitary:
  • Through the general circulation after passing the heart:
  • Correct: Through the hypophyseal portal system:

3. Which hormone does the corpus luteum secrete in the largest amount?

  1. Progesterone
  2. FSH
  3. Testosterone
Show the answer

The corpus luteum is a temporary gland that secretes mainly progesterone, with estradiol and inhibin.

  • Correct: Progesterone:
  • FSH:
  • Testosterone:

4How it works, step by step

  1. The old corpus luteum dies, so progesterone, estradiol and inhibin fall.Negative feedback lifts and FSH rises; without progesterone, the endometrium's functional layer breaks down and is shed (the menstrual phase).
  2. The rise in FSH rescues a group of follicles, and their granulosa cells release estradiol and inhibin.Estradiol rebuilds the functional layer (the proliferative phase), and estradiol and inhibin lower FSH, so only the dominant follicle keeps growing.
  3. Estradiol from the dominant follicle stays high for about two days.The hypothalamus and pituitary switch to positive feedback, and LH surges.
  4. The LH surge acts on the mature follicle.Ovulation follows about 34 to 36 hours later, and the follicle's cells become the corpus luteum.
  5. The corpus luteum releases progesterone, estradiol and inhibin.The endometrium becomes secretory, and negative feedback keeps FSH and LH low until the corpus luteum dies 12 to 14 days later and the cycle restarts.

5Core concepts

Cell-to-cell communicationHomeostasis

6A common mistake

The wrong idea: Ovulation happens on day 14 of every cycle.

What actually happens: Day 14 is ovulation in the 28-day model cycle only. Cycle length varies, and most of the variation falls before ovulation, in the follicular phase. The luteal phase is steadier, usually 12 to 14 days, so ovulation comes about two weeks before the next period starts: around day 18 in a 32-day cycle, and around day 10 in a 24-day one.

7Check yourself

Anything you miss goes into your review queue.

1. In the middle of the cycle, estradiol from the dominant follicle stays high for about two days, and instead of holding LH down it now drives a surge of LH. What kind of control is this?

  1. Negative feedback
  2. Positive feedback
  3. A neural reflex with no hormone involved
  4. Antagonism between two hormones
Show the answer

Sustained high estradiol now pushes GnRH and LH up instead of down, and the rising gonadotropins had been driving the follicle to make still more estradiol. A response that strengthens its own stimulus is positive feedback; it ends when the surge turns the follicle's cells toward making progesterone.

  • Negative feedback: Negative feedback would make high estradiol lower LH, as it does for most of the cycle, not raise it.
  • Correct: Positive feedback: Correct. Estradiol and the gonadotropins drive each other higher until the surge turns the follicle toward progesterone.
  • A neural reflex with no hormone involved: The loop runs through hormones: estradiol in the blood, GnRH in the portal system and LH in the blood.
  • Antagonism between two hormones: Antagonistic hormones oppose each other's effects; here estradiol and LH reinforce each other.

2. According to the graph, which hormone reaches the highest level of its whole cycle around day 21, and what makes it?

  1. LH, from the anterior pituitary
  2. Progesterone, from the corpus luteum
  3. FSH, from the anterior pituitary
  4. Estradiol, from the dominant follicle
Show the answer

Around day 21 the corpus luteum is at its most active, and progesterone is at the peak of its curve.

  • LH, from the anterior pituitary: LH is low throughout the luteal phase, held down by progesterone.
  • Correct: Progesterone, from the corpus luteum: Correct. Progesterone from the corpus luteum peaks in the middle of the luteal phase.
  • FSH, from the anterior pituitary: FSH is at its lowest in the middle of the luteal phase.
  • Estradiol, from the dominant follicle: Estradiol has only a smaller second hump then, lower than its peak before ovulation, and it comes from the corpus luteum; no dominant follicle exists in the luteal phase.

3. During the luteal phase, why does no follicle go on to ovulate?

  1. All remaining follicles have already been used up
  2. Progesterone, estradiol and inhibin from the corpus luteum keep FSH and LH low
  3. The corpus luteum physically blocks the rest of the ovary
  4. Oxytocin from the posterior pituitary holds back the growth of every remaining follicle
Show the answer

The corpus luteum's hormones give strong negative feedback. Progesterone slows GnRH pulses, FSH and LH fall to their lowest levels, and progesterone blocks the LH surge, so no follicle can mature and be released until the corpus luteum dies.

  • All remaining follicles have already been used up: Tens of thousands of follicles or more remain in a young woman's ovaries; hormones, not a lack of follicles, are the reason.
  • Correct: Progesterone, estradiol and inhibin from the corpus luteum keep FSH and LH low: Correct. Negative feedback from the corpus luteum keeps the gonadotropins low and prevents an LH surge.
  • The corpus luteum physically blocks the rest of the ovary: The corpus luteum occupies only the old follicle's site; its effect on other follicles is through hormones.
  • Oxytocin from the posterior pituitary holds back the growth of every remaining follicle: Oxytocin acts on the uterus and breast, not on follicle growth.

4. Maya's cycles are regular and 32 days long. On about which day does she ovulate?

  1. Day 14
  2. Day 16
  3. Day 18
  4. Day 25 or later
Show the answer

The luteal phase is fairly steady at about 14 days, so ovulation comes about 14 days before the next period: 32 − 14 = day 18.

  • Day 14: Day 14 applies only to a 28-day model cycle.
  • Day 16: Day 16 is half of 32. Ovulation is not at the midpoint; it is set by counting back from the next period.
  • Correct: Day 18: Correct. 32 − 14 = 18.
  • Day 25 or later: Day 25 would leave a luteal phase of only about a week, far shorter than the usual 12 to 14 days.

5. Priya, 19, runs 100 km a week on too little food. Her periods stopped eight months ago and her estradiol is low, but her FSH and LH are low rather than high. Where is the problem?

  1. Her ovaries have run out of follicles
  2. Her uterus can no longer build a functional layer
  3. Her hypothalamus has slowed GnRH release
  4. Her anterior pituitary is making too much FSH
Show the answer

Low estradiol with low FSH and LH means the ovaries are not being driven. Too little energy for her training slows GnRH pulses, so the pituitary releases little FSH and LH and no follicle matures.

  • Her ovaries have run out of follicles: If her ovaries had failed, the loss of negative feedback would make FSH and LH high, as after menopause.
  • Her uterus can no longer build a functional layer: A uterine problem would not lower her estradiol, FSH and LH.
  • Correct: Her hypothalamus has slowed GnRH release: Correct. Low gonadotropins with low estradiol point above the ovary, to the hypothalamus.
  • Her anterior pituitary is making too much FSH: Her FSH is low, not high.

6. Early in the follicular phase, estradiol and inhibin rise and FSH falls. How does the dominant follicle keep growing while its neighbors undergo atresia?

  1. It has the most FSH receptor proteins, and its granulosa cells also respond to LH
  2. It makes its own FSH
  3. It is the only follicle that contains an oocyte
  4. It is protected by progesterone from the corpus luteum left over from the last cycle
Show the answer

As FSH falls, only the follicle most sensitive to it keeps growing. The dominant follicle has the most FSH receptor proteins, the best blood supply, and granulosa cells that also respond to LH.

  • Correct: It has the most FSH receptor proteins, and its granulosa cells also respond to LH: Correct. Greater sensitivity lets it keep growing on less FSH, and LH receptor proteins give it a second source of support.
  • It makes its own FSH: FSH comes only from the anterior pituitary.
  • It is the only follicle that contains an oocyte: Every follicle contains an oocyte.
  • It is protected by progesterone from the corpus luteum left over from the last cycle: There is no active corpus luteum in the follicular phase; the old one has died.

7. According to the graph, FSH starts rising in the last days of the cycle, before the next period begins. What causes this rise?

  1. The LH surge of the next cycle has started early
  2. The dominant follicle of the next cycle is already releasing some FSH
  3. The endometrium releases FSH into the blood as it breaks down
  4. The dying corpus luteum's hormones fall, easing negative feedback
Show the answer

In the last days, the corpus luteum is dying, so its progesterone, estradiol and inhibin fall. With less negative feedback, the pituitary releases more FSH, which rescues the next group of follicles.

  • The LH surge of the next cycle has started early: LH is flat at that time; the next surge comes about two weeks later.
  • The dominant follicle of the next cycle is already releasing some FSH: Follicles respond to FSH; they do not make it.
  • The endometrium releases FSH into the blood as it breaks down: The endometrium is not an FSH source; FSH comes from the anterior pituitary.
  • Correct: The dying corpus luteum's hormones fall, easing negative feedback: Correct. Less negative feedback lets FSH rise.

8Summary

The ovarian cycle (follicular phase, ovulation, luteal phase) and the uterine cycle (menstrual, proliferative and secretory phases) run in step, and ovulation comes about 14 days before the next period. When the corpus luteum dies, falling progesterone sheds the functional layer and lifts negative feedback, so FSH rises and recruits follicles. Their estradiol and inhibin lower FSH, leaving one dominant follicle, and estradiol rebuilds the endometrium. High, sustained estradiol then switches the hypothalamus and pituitary to positive feedback, producing the LH surge, which triggers ovulation and forms the corpus luteum. Its progesterone makes the endometrium secretory, thickens cervical mucus, raises body temperature and, with estradiol and inhibin, holds FSH and LH down for 12 to 14 days. Puberty starts when the GnRH pulse generator reawakens; menopause follows when the follicles run out, estradiol and inhibin fall and FSH rises; hormonal contraception keeps negative feedback on so there is no LH surge and no ovulation.

9What comes next

10Connections