HPO axis and the Menstrual Cycle Flashcards
How is menstrual cycle regulated?
Hypothalamus + anterior pituitary gland + ovaries
work together to regulate menstrual cycle
I.e HPG axis
- Hypothalmic Pituaitry Ovarian Axis
What is the role of the hypothalamus in menstrual cycle?
Releases gonadotrophin releasing hormone
which stimulates release of
LH & FSH (gonatrophins) from anterior pituitary gland
What is the role of FSH & LH on ovaries?
LH:
- act on theca cells to stimulate production & secretion of androgens
FSH:
- bind to granulosa cells to stimulate follicle growth
- convert androgens (from theca cell) to oestrogen
- stimulate Inhibin secretion
What feedback systems are there in menstrual cycle?
Moderate oestrogen levels- negative feedback loop- willl inhibit secretion of gonadotrophin releasing hormone
Inhibin inhibits secretion of FSH & LH- negative feedback
High levels of oestrogen (w/out progesterone) stimulate release of LH, FSH & gonadotrophin hormone- positive feedback
Low levels of progesterone & oestrogen promote secretion of gonadotrophin release forming, FSH & LH- positive feedback
What are the phases of ovarian cycle?
Follicular phase
ovulation
Luteal phase
How long is ovarian cycle?
28 days
follicular phase- days 1-14
Luteal phase- day 15-28
What is the function of follicles? Describe the development of follicles in the follicular phase.
Follicles= sacs that contain the egg (oocyte)
- Primordial follicles → primary follicles:
- zona pellucida & theca foliculi are formed.
- Theca folliculi produce oestrogens during maturation process - Primary follicles → secondary follicles:
- Follicular antrum - a fluid filled space develops around oocyte. - Graafian follicle - ready to release secondary oocyte.
- Follicular antrum gets bigger.
What is the follicular phase?
Marks beginning of new cycle
- Gonadotrophic releasing hormone (GnRH) is secreted from hypothalamus
- stimulates the anterior pituitary to secrete FSH & LH. - At the same time, low levels of inhibin means there is a negative feedback at HPG axis
- results in increase in FSH & LH levels too
- FSH= stimulates primary follicle into Graafian follicle (contains secondary oocyte) & forms thecal cells
- FSH= stimulates thecal cells to produce oestrogen
- oestrogen forms endometrium - As oestrogen levels rise, negative feedback reduces FSH levels
- results in only 1 follicle maturing & others forming polar bodies - As oestrogen levels continue to rise, positive feedback increases levels of gonadathrophin release hormone, FSH & LH
- however effect is only seen in LH- LH levels surge since increased inhibin selectively inhibits FSH production. - Granulosa cells becomes leutenised & express receptors for LH
- LH stimulates ovulation
What happens during ovulation?
Follicular phase ends w/ ovulation
- Due to LH surge, follicle ruptures & mature ooctye is expelled into abdominal cavity
- oocyte enters to Fallopian tube
- remains visible for fertilisation for 24H
Usually occurs in alternative ovaries.
What occurs in the luteal phase?
- ruptured follicle caves in & fills w/ blood clot.
- Granulosa & theca cells increase- form Corpus luteum- a tissue in ovary that forms at site of follicle rupture
- produces oestrogen, progesterone & inhibin- maintain conditions of fertilisations - If fertilisation occurs, corpus luteum is maintained by HcG & LH
- produces hormones to support pregnancy
- & results in further negative feedback of FSH & LH release. - In absence of fertilisation, corpus luteum degenerates after 12 days
- fall in hormones (LH, thus Progestrone & oestrogen), relieving -ve feedback, resetting HPG axis to being cycle again
- area where it degenerated becomes scar tissue= called corpus albicans - Low levels of progesterone & oestrogen removes inhibition & promotes secretion of GnRH, FSH & LH.
NOTE: to help remember how FSH & LH levels vary- learn the feedback systems- makes it easier!
What are the phases of the endometrium cycle (uterine cycle)?
Proliferative phase
Secretory phase
Menses
What occurs in the proliferative phase?
Runs alongside the follicular phase
- starts approx day 5 when bleeding stops
Prepares reproductive tract for fertilisation & implantation!
- Oestrogen imitates thickening of endometrium
- via cell proliferation & regeneration of stratum functionalis.
- endometrium approx 2mm thick - production of alkaline cervical mucus to facilitate sperm transport
phase ends w/ maturation of the graffian follicle & ovulation at approx day 14.
What occurs in the secretory phase?
Runs alongside luteal phase
- 14 days in duration
- Progesterone stimulates further thickening of endometrium into a glandual form
- glands enlarge & secrete glycogen, to nourish the embryo at implantation. - Spiral arteries in endometrium increase in size & become coiled. - endometrium is now approx 5mm thick.
- Cervical mucus changes to a thick plug, blocking the cervical canal & protecting the embryo.
- thickening of myometrium,
- if fertilisation doesn’t occur, corpus luteum hormones (O & P) drop
- results in spiral artery spasm & endometrium degeneration due to deprivation of nutrients & autodigestion by lysosomes.
Followed in approx 24 hrs by menstruation
What is menses?
Marks beginning of new menstrual cycle
Occurs in absence of fertilisation once corpus lute has broken down & internal lining of uterus is shed
- spiral arteries dilate & bleed into necrotic stratum functionalis
- uterus contracts in response to prostaglandins to expel the blood- contraction can result in pain - dysmenorrhoea.
Bleeding lasts 4-7 days
Clotting of menstrual blood flow is prevented by release of fibrinolysins, ensuring the endometrium is completely expelled.
Describe body temperature variations through the menstrual cycle.
Temp falls - corresponds w/ LH surge approx 24 hrs prior to ovulation
Progesterone secreted by corpus luteum results in rise in metabolic rate & temperature by 0.5 °c.
- This rise persists until menstruation
