Repro 2: Control Of Reproductive Processes Flashcards
In males, how does testosterone affect GnRH secretion?
Reduces it (negative feedback)
In females, how does varying levels of oestrogen affect GnRH secretion?
Moderate levels of oestrogen has a negative feedback effect, reducing GnRH secretion
High levels of oestrogen has a positive feedback effect, increasing GnRH secretion to promote a surge
In females, how does progesterone affect GnRH secretion?
Progesterone increases the inhibitory effect of oestrogen (when oestrogen is at moderate levels)
Progesterone reduces the frequency of pulses, oestrogen (at moderate levels) reduced the amount released per pulse
Which cells secrete FSH and LH?
Gonadotrophs, in the anterior pituitary
What does inhibin do?
Reduces the amount of FSH secretion
Where is inhibin released from?
Granulosa cells of developing ovarian follicles, and Sertoli cells in seminiferous tubules
What is the action of LH in the testes?
Acts on Leydig cells to produce testosterone
What is the action of FSH in the testes?
Acts on Sertoli cells of seminiferous tubules to enable spermatogenesis
(Makes the, responsive to testosterone which promotes spermatogenesis)
What is the action of LH and FSH in females?
Antral phase:
LH binds to theca cells and stimulates them to produce androgens
FSH binds to granulosa cells and stimulates them to convert the androgens into oestrogens
Pre-ovulatory phase:
Granulosa cells develop receptors for LH
LH surge stimulates ovulation
After ovulation:
LH stimulates CL to secrete oestrogen and progesterone
How long is the luteal phase of the menstrual cycle?
14 days (CL only lives for 14 days)
What are the phases of the menstrual cycle (in the ovaries and endometrium)?
Ovarian cycle has the follicular phase and luteal phase
Endometrial cycle has menses, proliferating phase then secretory phase
Describe the follicular phase of the ovarian cycle.
Lack of negative feedback from hormones means GnRH can start to rise, so FSH and LH can also start to rise
FSH stimulates some of the follicles in the ovary to mature (from primary to secondary follicles)
As the follicles mature, they produce oestrogen
When oestrogen is at a high level, it exerts a positive feedback effect on LH causing the LH surge
This surge triggers ovulation of the most mature, dominate follicle (releases its oocyte)
(Meanwhile FSH remains lower as it is inhibited by inhibin to prevent recruitment of further follicles)
Describe the luteal phase of the ovarian cycle.
LH surge triggers the remnants of the follicle to form the corpus luteum
The CL secretes progesterone, oestrogen and inhibin
Progesterone has negative feedback on GnRH (so levels of LH and FSH fall)
Inhibin also has negative feedback on FSH
If there is no fertilisation, CL degenerates and hormone levels all fall and trigger the menses
How does the luteal phase end if there is fertilisation of the oocyte?
The implanting embryo will develop a placenta
The syncytiotrophoblast layer secretes hCG which signals to preserve the corpus luteum
CL can continue to produce oestrogen and progesterone to support early pregnancy up until approx 4 months when the placenta can take over
What hormones does the CL produce?
Progesterone, oestrogen and inhibin
What happens to the endometrium at the end of the luteal phase when oestrogen and progesterone suddenly fall when there is no fertilisation?
Apoptosis of the endometrium so the tissue is shed as a menstrual bleed
Spiral arteries contract in an attempt to reduce the bleeding
What is the actions of oestrogen in the proliferative phase?
Thickening of endometrium
Increased growth and motility of myometrium
Stimulate muscular contraction of Fallopian tube
Stimulate thin and alkaline cervical mucus secretion (for sperm transport)
Anabolic metabolic change
What is the actions of progesterone in the secretory phase?
Thickening of endometrium and myometrium
Reduces Fallopian tube and myometrium motility
Stimulates thickening and acidification of cervical mucus to inhibit sperm transport
Catabolic metabolic changes
What is the actions of testosterone in a male?
Reversible regulatory effects eg maintenance of internal genitalia
And partially reversible deterministic effects ie secondary sexual characteristics eg deeper voice, body hair, increased muscle and bone mass
What is the effect of the menopause on LH, FSH and GnRH when the ovary produces much less oestrogen?
Less negative feedback effect so LH, FSH and GnRH levels will all rise (especially FSH due to lack of inhibin)
These hormones all return to lower levels several years after the menopause
In pregnancy once the placenta has developed, what will happen to levels of GnRH, LH and FSH?
These will all fall, because placental oestrogen Dan progesterone will exert negative feedback on both the hypothalamus and the pituitary
What hormone inhibits prolactin secretion?
Dopamine
How long does a mature oocyte remain viable for fertilisation after it has ruptured from the follicle?
24 hours
