HPO axis and menstrual cycle Flashcards
Describe the physiology of the HPO axis and
normal menstrual cycle.
LOs
What is the other name for gonadotrophin-releasign hormone (GnRH)
Lutenising hormone releasing hormone
What does Gonadotrophin-releasing hormone do?
Binds to gonadotrophic cells of the anterior pituitary gland, leading to release of follicle stimulating hormone (FSH) and lutenising hormone (LH)
What does HPO axis stand for?
Hypothlamic-pituitary-ovarian axis
When is the follicular phase
Day 0-14 of menstrual cyclez
When is the luteal phase
Day 14-28 of menstrual cycle
Why does FSH hormone concentration initially rise at the start of the follicular phase?
It stimulates maturation of the follicle from primary to secondary
What do follicles produce as they mature to secondary follicles?
Oestrogen
What effect does oestrogen have in the forst 10 days of the menstrual cycle? (low conc)
Negative feedback on pituitary gland, inhibiting release of leutenising hormone
When is FSH primarily secreted
In response to low oestrogen concentrations
What other systemic impacts does oestrogen have on women
Stimulates bone and muscle growth, stimulates endometrial (uterus) growth, female secondary characteristics, maintains glands
What effect does oestrogen have after 10 days
Oestrogen levels continue to rise, higher levels of oestrogen have a positive feedback effect, stimulating release of lutenising hormone
How does GnRH production vary across the follicular period
Increases
What triggers the ovulation of the most mature follice
The massive spike in LH
What happens to LH levels post ovulation
They drop, as does GnRH. FSH has a small spike due to LH mirroring LH
What is the oocyte
The female egg, released from follicle post ovulation
What happens to the follicle post ovulation
It becomes the corpus luteum/dead follicle
Ie luteal phase
What does the corpus luteum generally do during the luteal phase
Slowly degrades and secretes hormones
What hormones does the corpus luteum secrete?
Oestrogen, inhibin, and progesterone
What does inhibin do?
Negative feedback, inhibits FSH secretion because inhibin is produced in the luteal phase, we dont need any more follicles to mature yet
What does progesterone do?
Negative effect on hypothalamus, reducing secretion of GnRH, as oestrogen level slowly decreases
STIMULATES ENDOMETRIAL GROWTH
What happens to LH, FSH, and GnRH in the luteal phase
They decrease concentration
What happens when corpus luteum degenerates at the end of the luteal phase?
Progesterone level decreases, can no longer inhibit GnRH, levels increase, leading to a new cycle. No progesterone means endometrium cannot be sustained, so sheds as period
What is oestradiol?
A form of oestrogen
What does oestradiol do?
Prepares female reproductive tract for fertilisation/implantation,
Induces progesterone receptor expression,
Stimulates the proliferation of epithelial lining of tubal epithelium, sugar rich fluid
Stimulates endometrial hyperplasia and hypertrophy of epithelial lining.
When is GnRH used in a therapeutic sense?
Endometriosis (endometrial tissue grows outside uterine cavity), continued GnRH inhibits gonadatrophin secretion, reduces oestrogen levels, reducing endometriotic tissue
does GnRH exposure reduce gonadatrophin secretion?
If exposure is continous
What does fixed combination OCP mean
Dosage of oestrogen and progestin is the same
What does varying-dose ocp mean
2-3 different dosages of oestrogen and progestin
What is the minipill
Progestin only
How do contraceptive steroids work
Supress LH and FSH, no follicular development or LH ovulation
Progestin causes cervical mucous thickening, increases viscosity,
Which cells in the follicle respond to LH
Theca cells
Which cells in the follicle respond to FSH
Granulosa cells
