07 - THE PHASES

Question 1

(I. The Luteal Phase)

1. What is high?
2. What are being produced by the developing cohorts of small follicles?

Answer 1

1. progesterone
2. Some estradiol and inhibin (the follicular waves)

Question 2

(I. The Luteal Phase)

(Hypothalamus)

1. HIgh progesterone from the CL feeds back on the hypothalamus to do what?
2. Small amounts of estrogen from developing follicles synergize with progesterone to do what?

Answer 2

1. decrease the frequency of GnRH pulses
2. decrease GnRH secretion

Question 3

(I. The Luteal Phase)

(Anterior Pituitary)

1. Low frequency GnRH pulses “favor” what over what?
2. Low levels of estradiol from developing waves follicles do what to FSH and LH?
3. Inhibin from these developing follicles also functions to do what?

Answer 3

1. FSH secretion over LH secretion
2. suppress release of FSH and LH (in response to GnRH)

(so estradiol has inhibitory action at pituitary as well as hypthalamus)

3. inhibit FSH secretion

Question 4

(I. The Luteal Phase)

(Overall Effect)

1. GnRH pulses exist but are kept in check (low frequency)
2. LH secretion is pulsatiel in response to GnRH… does it reach high levels?
3. FSH levels fluctuate as a result of what?

Answer 4

2. no
3. its own more autonomous secretion and the feedback effects of ovarian hormones that it induces by promoting follicle growth (estradiol and inhibin)

Question 5

The luteal phase

Answer 5

Question 6

(2. Early Follicular Phase)
1. Depending of species… this will include what stages?
2. Is there a CL in this stage?
3. What is the only feedback?
4. Levels of what are increasing?

Answer 6

1. proestrus and early estrus
2. no (thus no progesterone)
3. estradiol and inhibin being produced from follicles
4. estrogen levels

Question 7

(2. Early Follicular Phase)

(Hypothalamus)

1. In the absence of progesterone (no CL) there is the potential for what?
2. So what keeps GnRH secretion in check?

does it stop GnRH secretion?

BUT - the ihibitory estrogen feedback prevents what?

Answer 7

1. increased frequency of GnRH pulses
2. increasing (but still moderate) estrogen levels

doesn’t stop - just keeps in check (still get some LSH and FSH secretion)

surge release of GnRH (thus no surge release of gonadotropin)

Question 8

(2. Early Follicular Phase)

(Anterior Pituitary)

1. Estrogen also feeds back negatively on the pituitary gonadotropes to do what?
2. What feeds back directly on gonadotropes to specifically reduce FSH secretion?

Answer 8

1. reduce LH and FSH secretion in response to GnRH “pituitary clamp”
2. inhibin (this is espeically true as we from early to mid and late follicular phase)

Question 9

(2. Early Follicular Phase)

(Overall Effect)

1. You have enough GnRH from the hypothalamus and enough FSH and LH secretion from the pituitary to permit what?
2. BUT and LH surge is prevented by what?

Answer 9

1. continued follicle growth (particularly of dominant follicles) and increasing estrogen concentrations
2. negative estrogen actions on both hypothalamus and pituitary (the estrogen level has not yet reached the threshold level needed to switch from negative to positive feedback)

Question 10

(3. Late Estrus - The LH Surge)
1. What is very high?

what can also be high?

Answer 10

1. estradiol

inhibin

Question 11

(3. Late Estrus - the LH surge)

(Hypothalamus)

1. The exposure to high levels of estradiol (in the absence of progesterone) for a certain time (~36 hrs) does what?
2. Positive estradiol feedback does what?

Answer 11

1. switches feedback from negative to positive

(The increasing estrogen has now crossed a threshold level to become postive and/or activate the GnRH “surge center” if one exists)

2. gives a great increase in GnRH secretion.

Question 12

(3. Late Estrus - the LH surge)

(Anterior Pituitary)

1. Further elevation in levels of estradiol does what?
2. Subsequently, the elevated estrogen appears to faciclate the GnRH self-priming mechanism that does what?

Answer 12

1. removes the inhibtion that it previously exerted on the pituitary gonadotrope response to GnRH (ie removes the pituitary clamp)
2. increases the amount of LH release in response to each unit of GnRH

(i.e. the high estrogen first removes the inhibitory effect and then actually increases the sensitivity
of the pituitary gland to GnRH)

Question 13

(3. Late Estrus - the LH surge)

(Overall Effect)

1. What allows the LH surge to occur?
2. Is there also an FSH surge?
3. Note: The decline of the LH surge (i.e. why it stops) is not due to slowing GnRH secretion in most species (in these the GnRH surge continues for longer than the LH surge) but rather to loss of what?

Answer 13

1. the increased GnRH and increased pituitary sensitivty
2. depends on species - but most do - so really a “gonadotropin” surge
3. sensitivity to GnRH

Question 14

(4. Immediately Post Ovulation - early metestrus)
1. with ovulation of large follicle we have lost what?
2. How much progesterone is the CL (currently a corpus hemorrhagicum) producing?

Answer 14

1. estradiol and inhibin
2. not much

(ie we are in an ovarian hormone vacuum)

Question 15

(4. Immediately Post Ovulation - early metestrus)

(Hypothalamus)

1. Levels of GnRH eventually become depleted after the surge and while pulses exist there is no huge increase in release despite the absence of negative steroid feedback

Question 16

(4. Immediately Post Ovulation - early metestrus)

(Anterior Pituitary)

1. LH secretion is often briefly abolished following the surge. It usually returns in less than a day but levels are reduced, perhaps due to a combination of what?
2. In the absence of inhibition from ovarian estradiol and inhibin there is often a what in the days following ovulation?

Answer 16

1. down-regulation of GnRH receptor mechanism, reduced GnRH, and depleted LH stores (the exact mechanism probably varies between species)
2. a surge release of FSH

(overall effect)

This FSH release may function to recruit a new cohort of follicles to the growing pool for the new cycle.