Lecture 6: Menstrual Cycle Flashcards
What axis controls the menstrual cycle?
- HPO axis (Hypothalamic-Pituitary-Ovarian)
What are the key requirements to maintain the HPO axis?
Pulsatile release of both:
- GnRH
- Gonadotrophins
What are the gonadotrophins?
- LH
- FSH
(-hCG)
Describe the HPO axis
What determines the length of a menstrual cycle?
- number of days between first day of menstrual bleeding of one cycle to the onset of menses of the next cycle
What are the characteristics of the normal menstrual cycle (MC)
- Median duration of MC is 28 days with most cycles between 25-30 days
- Menstruation lasts 3-8 days, written as 7/28 or 5-6/27-32
What is the term when the MC is less than a certain number of days?
- MC<21 days
- polymenorrheic
What is the term when the MC is greater than a certain number of days?
- MC>35 days
- oligomenorrheic
When is MC most irregular typically?
- around extremes of reproductive life
- i.e menarche and menopause
Describe/Draw the levels of steroids and gonadotrophin during a normal MC
Describe the follicle and its size during a normal MC
What is the corpus luteum?
Mass of cells (leftovers of the follicle), secreting mainly P and some E2:
- Granulosa cells
- Theca cells
An increase in E2 levels leads to a …
- decrease in FSH levels
- negative feedback
Where do we get the Progesterone in MC?
- Corpus luteum
Where do we get the E2 from in the MC?
- Granulosa cells
What causes recruitment of early antral follicles?
- intercycle rise of FSH
What is the 2 cell 2 gonadotrophin theory?
Different enzymes in different cellular compartments
- Theca: enzymes predominately responsible for P & Androgen families
- Granulosa: production of E2
- LHR only - Theca
- FSHR (& LHR) - Granulosa
Where and what drives androgen & progesterone production?
- Theca cells
- LH
Where is E produced?
- Granulosa cells
When is E produced?
- Follicular phase
What drives E production?
- FSH
Which steroids are made where?
When is LHR acquired in the granulosa cell?
- From mid-follicular phase onwards in DF only
- After selection of dominant follicle; allows Dominant follicle to make P
What is the function of the LHR in the granulosa cell?
- To produce Progesterone only
- after MF phase, will drive P and E production as well
Between Theca and GC, where is aromatase only found?
- Granulosa
What happens during the LL and EF phase of the MC?
- P declines
-> Selectively raises FSH (break from HPO axis is released due to decline in P)
= Inter-cycle rise in FSH
Why does P decline in the LL and EF phase?
- CL dies if no pregnancy thus decline in P
What is the significance of the inter-cycle rise in FSH?
- recruitment of the antral-follicles into the MC
What happens during the MF phase of the MC?
- E2 increases
-> Negative Feedback
= FSH falls -> Selection of dominant follicle
What causes the increase in E2?
- Antral follicles grow (i.e. GC grow) -> release E2 -> E2 levels increase -> exerts negative feedback -> FSH levels decrease
When does the LH surge occur?
- Mid-cycle
What requirements need to be fulfilled for the LH surge to occur?
- 2 days of E2; >300 pmol
What does 2 days of E2 > 300 pmol cause?
- Positive Feedback
- Thus LH surge
What causes the high E2 levels mid cycle?
- Dominant follicle
- As it grows quickly and expansively it pumps out E2
What does the LH surge cause?
- Ovulation
- Formation of CL
What happens to the antral follicles when FSH decreases?
- Die
- EXCEPT for dominant follicle
What occurs during the ML phase?
- High P due to High LH (LH surge)
-> Negative feedback - P overcomes E2 (P always dominates of E2)
=Low FSH/LH
What is the cause of the high P in the ML stage?
- CL
What is the Follicular Phase?
- growth of follicles up to ovulation
- dominated by E2 production from follicles
What is the luteal phase?
- formation of CL from the empty follicle
- dominated by P production from CL
Which phase has a set number of days?
- Luteal phase
- 14 days
- Dominated by CL
What feedback occurs during Follicular Phase?
- VARIES
- Release of -ive feedback from CL
- -ive feedback then reinstated, then
- Switch from -ive to +ive feedback
What feedback occurs during Luteal phase?
- Negative feedback
-> Progesterone
When does ovulation occur?
- end of Follicular phase
What becomes of the reminent of the follicle after ovulation?
- Becomes CL
What occurs at the end of Luteal Phase if no pregnancy?
- CL dies
-> Increase in FSH & MC restarts
What happens during the rise and fall of FSH during the follicular phase?
Rise:
- “window of opportunity”: Recruitment of the growing follicles into the follicular stage
Fall:
- Increase in E2 -> Selection of one of these follicles (others die) -> DOMINANT FOLLICLE -> PREOVULATORY FOLLICLE -> OVULATES
Do all animals have the MC?
No, only:
- Humans
- Primates (apes and monkeys)
What do all mammals have in common?
- cyclical ovarian function
- Same reproductive system when it comes to HPG axis
- Produce mature egg and necessary sex steroids
What is the oestrus cycle?
- cyclic appearance of behavioural sexual activity (heat or oestrus)
Do animals that undergo oestrus menstruate?
No - endometrium reabsorbed if no fertilisation
What is day 0 of the oestrus cycle?
- First day of sexual receptivity
When does ovulation occur in animals that undergo oestrus?
- Early in cycle
- High E2 levels = stimulation of sexual behaviour & +ive feedback
Do all species that undergo oestrus have the same cycle length?
- No
What is poly-oestrus?
- In heat several times/year
- cats, cows, pigs
What is di-oestrus?
- Twice/year
What is mono-oestrus?
- One breeding season/year
- usually in spring (daylight)
- Bears, foxes, wolves
Which animal has no oestrus cycle?
- Rabbits
- Induced ovulates
- Induced to ovulate by mating & can conceive at any moment
HPO axis
What gonadal protein inhibits FSH directly? Where is it produced?
- Inhibin
- Sertoli cells in testes
- Granulosa cells in ovary
What is the structure of inhibin?
- Disulphide-linked protein dimers
- Common α-subunit with different β-subunits giving two forms of Inhibin
- Both forms suppress FSH secretion by pituitary
- Not affecting LH secretion
What gonadal protein stimulates FSH secretion?
- Activins
- Isolated from follicular fluid
What gonadal protein indirectly suppresses FSH secretion?
- Follistatin
- FSH-suppressing protein
- From follicular fluid
How does Follistatin work?
- binds activin with high affinity » neutralizes FSH-stimulating ability of activin
How do we know inhibin only works on FSH and not LH?
- Experiment: Using ovariectomized (ovx) sheep, GnRH agonist was injected in the presence and absence of inhibin
Why did they use ovx sheep?
- Mono-ovulators
- similar to humans
- Sheep are a good model for reproduction
- otherwise mice due to limitations when it comes to sheep (expense/storage etc.)
What does overiectomised (ovx) mean?
- Removal of ovaries
Why were ovx sheep used?
- Prevent the endogenous feedback effects interfering
Why did the experiment inject GnRH agonist?
- stimulate gonadotrophin production
Biosynthesis of inhibins and activins occurs from how many genes?
- 3
What are the TGFβ- superfamily of precursor proteins involved in?
3 genes encoding for:
- α- protein (specific for inhibin)
- βA- protein,
- βB- protein,
These alpha and beta subunits are all members of TGF-beta superfamily of proteins.
The genes encode for larger precursor proteins which are then processed… (activin/inhibin genes)
proteolytically
What are the isoforms of inhibin?
- Inhibin A
- Inhibin B
What are the isoforms of activin?
- Activin A (βA-homodimer),
- Activin B (βB-homodimer)
- Activin AB (βAβB-heterodimer)
How are inhibin and activin produced?
- The gene products are the sub-units which will combine at the time of release from the cell.
How is the amount of activin and inhibin determined during MC and folliculogenesis?
- Ratio of Activin:Inhibin
Where is inhibin and activin produced in females?
- Granulosa cells
When do activins dominate in the MC?
- EFP
- Correlates with high FSH
When do inhibins dominate in the MC?
- LFP
- Correlates with decreased FSH
Which hormone aids the inter-cycle rise in FSH?
- Activin
What does the inter-cycle rise in FSH cause?
- recruits early antral follicles
What is the significance of inhibin on the MC?
- Rise in E2
- Fall in FSH
What happens to the levels of activin and inhibin as we go through folliculogenesis?
- Inhibin production increases
- Activin produced decreases
Describe the experimental procedure that questioned what happens when inhibin is blocked?
- (Red) Inject rats in the late antral phase with Inhibin anti-serum
- = blocks inhibin from working as it has antibodies that bind to and prevent it from working
- Red = increase in FSH
- (Green) Inject with normal serum = no peak of FSH (as you’d expect at this stage of MC)
= thus showed that in Late Antral phase, inhibin needed for fall in FSH; NOT ONLY E2!!
? + ? = Fall in FSH
E2 + Inhibin = Fall in FSH
What are the main hormones in the TGFBeta superfamily?
- Inhibin
- Activin
- AMH
What is AMH?
- Anti-Mullerian Hormone
- glycoprotein
- member of TGFBeta superfamily
What is the importance of AMH in males?
- expressed from week 8 of development
- causes regression of Müllerian ducts by a wave of apoptosis.
Do females express AMH?
Yes
Where is AMH expressed in females?
- ovarian granulosa cells
Do AMH levels peak in females, and if so, where?
- levels peaking in selectable follicles (large preantral & small antral follicles)
–> then decreases
Has AMH been detected elsewhere? (other than preantral and small antral follicles)
Yes
- Prenatal follicles
- However variable and may be species variable
What is the role of AMH in the MC?
- regulator of normal follicle growth and development
What is the function of AMH?
- Inhibits transition from primordial to primary follicles
- Inhibits FSH-dependent cyclical recruitment of follicles
-> by inhibiting FSH-stimulated aromatase & FSHr expression → in normal MC, prevents over-recruitment of growing follicles
What is the function of FSH?
- To drive follicle growth
- Recruiment of antral follicles into MC
- stimulates aromatase
What does AMH inhibit?
- The function of FSH
What does AMH prevent in the normal MC?
- over-recruitment of growing follicles
= AMH maintains a balance
What is the problem with over-recruitment of follicles?
- Depletion of follicles quicker
-> early menopause
Where do inhibins and alctivins act on?
- Anterior Pituitary ONLY
Where to E2 and P act on?
- Hypothalamus
- A. Pituitary
What is the ‘window of opportunity’ & follicle selection?
- Raised FSH present a “window” of opportunity
- FSH threshold hypothesis
- One follicle from the group of antral follicles in ovary is just at the right stage at the right time….
- Becomes DOMINANT follicle -> survives fall in FSH & goes onto ovulate
- Known as “selection”
- Can be in either ovary
- E2 levels rise reinstating -ive feedback at pituitary -> FSH levels fall = prevents further follicle growth
How does the dominant follicle survive the fall in FSH?
- As FSH falls, LH increases.
- Dominant follicle acquires LH receptors on granulosa cells.
- Other follicles do not, so they loose their stimulant and die.
How are the gonadotrophin receptors distributed in follicles? Thus, what steroids are produced?
Granulosa:
- FSHr
- LHr from mid-follicular phase onwards in DF
- FSH drives E2 production in FP; LH drives P ONLY (no androgens) in LP
Theca:
- LHr ONLY
- P and androgen production
- Inhibin B: highest in early-mid FP (ratio of activin: inhibin) and declines in LFP (small peak at LH surge), zero in luteal phase.
- decline in Inhibin A at end of L phase allows for increase in FSH.
Inhibin A: increases in late FP with highest levels in L phase (being made by CL) – contributes to inhibition of FSH in this phase.
- Decline in Inhibin A at end of L phase allows for increase in FSH.
- Activin High = FSH High
- E2 rises via -ive feedback = inhibit B which contrinbutes to this -ive feedback
Are E2 and P the only factors affecting the HPO axis?
No!!
Are the gonadal steroid hormones only acting on the follicles?
NO!!
- Acting elsewhere in the female reproductive tract - everything has to be ready for both transport of fertilised egg and sperm!
Other than the follicles, what else are the steroid hormones acting on?
E2, P and (Androgens?) are acting on:
- Endometrium
- Oviduct/Fallopian tubes
- Cervix
- Vagina - changes in vaginal epithelial cells
