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Science of reproduction > Menstrual Cycle I > Flashcards

Menstrual Cycle I Flashcards

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1
Q

What controls the menstrual cycle?

A

The HPG axis and pulsatile release of GnRH

  • Pulsatile release of GnRH and gonadotrophins is fundamental for maintaining the HPG axis and fertility
  • No pulsatile release = cessation of menstrual cycle
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2
Q

What is the normal length of a menstrual cycle?

A
  • The length of a menstrual cycle is the number of days between the first day of menstrual bleeding of one cycle to the onset of menses of the next cycle
  • 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
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3
Q

What is it called if the menstrual cycle is less than 21 days?

A

polymenorrheic

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4
Q

What is it called if the menstrual cycle is more than 35 days?

A

oligomenorrheic

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5
Q

What phases is the menstrual cycle made up of?

A

Follicular phase and luteal phase

Luteal phase&raquo_space; ovulation&raquo_space; follicular phase

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6
Q

How long is the luteal phase?

A

Always 14 days - it’s independent from the life cycle of the corpus luteum

If there’s any variation in a MC, it’s usually in the follicular phase

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7
Q

What hormone is predominantly high during the luteal phase?

A

Progesterone

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8
Q

What is the rise in FSH called?

A

The intracycle rise in FSH

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9
Q

What happens to FSH when oestrogen levels rise?

A

FSH goes down

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10
Q

Why do different cells make different steroids?

A

Different enzymes are present in different cellular compartments

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11
Q

Where is aromatase found and what does it do?

A

Granulosa cells and it can convert androgens into oestrogens - this process is driven by FSH

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12
Q

What receptors are found on granulosa cells ONLY?

A

FSH receptors

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13
Q

Where are androgens made?

A

In the theca cells because they have the specific enzymes that convert cholesterol into androgens

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14
Q

What happens in the late luteal/early follicular phase in the menstrual cycle?

A
  • Progesterone declines because of the death of corpus luteum (no pregnancy)
  • Progesterone inhibits FSH (through negative feedback on the hypothalamus and the pituitary) during pregnancy
  • Only FSH is raised (aka the inter-cycle rise)
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15
Q

What happens in the mid-follicular phase of the of the MC?

A
  • FSH stimulates follicles, they grow and become antral follicles
  • Antral follicles produce oestrogen
  • Rise in oestrogen means FSH falls due to negative feedback, this allows the selection of the dominant follicle
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16
Q

What happens during the mid of the cycle?

A
  • Dominant follicles produces a lot of oestrogen
  • Oestrogen has to be sustained and exceed the threshold of 300pmol
  • Feedback switches from negative to positive, causes LH surge
  • LH surge causes ovulation of the dominant follicle
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17
Q

What happens during the mid-luteal phase?

A
  • After ovulation the follicle becomes a corpus luteum
  • Corpus luteum makes a lot of progesterone which exerts negative feedback and keeps LH/FSH low
  • There’s a peak of oestrogen coming from the CL but progesterone dominates and negative feed back is maintained
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18
Q

What is the different between oestrus and the menstrual cycle?

A
  • All mammals have cyclical ovarian function & the similar reproductive system (in terms of HPG axis) to produce a mature egg(s) and the necessary sex steroids
  • Menstrual cycles occur only in humans, primates (apes and monkeys) & is named for the regular appearance of menses i.e. shedding of then endometrial lining
  • Oestrus cycle is when animals go into heat and it’s characterised by the animals behaviour
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19
Q

What is the oestrus cycle?

A
  • The cyclic appearance of behavioural sexual activity (heat or oestrus)
  • They do not menstruate – the endometrium is reabsorbed if fertilization does not occur
  • Day 0 of the oestrous cycle is the day of beginning of sexual receptivity
  • Ovulation usually occurs early in cycle as high oestrogen levels stimulate sexual behaviour as well as exerting positive feedback
  • Different species have different lengths of cycles
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20
Q

What are the different types of breeding cycles in animals?

A
  • Polyestrus
  • Monoestrus
  • Seasonal polyestrus
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21
Q

What is a polyestrus breeding cycle?

A

The animal can cycle during the entire year, independently of environmental cues.

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22
Q

What is a monoestrus breeding cycle?

A
  • These are animals which have only one cycle per year ie the wolf, and the fox.
  • Aim to give birth in the spring because there’s more food availability, longer days and less chance of predators
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23
Q

What is a seasonal polyestrus breeding cycle?

A

These animals only cycle during a determined season, in response to specific environmental cues, such as an increase or decrease of light hours.

  • Short day breeders: such as the ewe, nanny and doe, start cycling as the days get shorter in the fall.
  • Long day breeders: on the other hand, are animals such as the mare that start cycling as the days are getting longer in the spring
24
Q

What 2 proteins can also give feedback like oestrogen and progesterone?

A

Inhibin and activin

25
Q

What produces inhibin?

A
  • Sertoli cells in the testis

- Granulosa cells in the ovary

26
Q

What is the structure of inhibin?

A
  • Disulphide-linked protein dimers

- Common α-subunit with different β-subunits giving two forms of Inhibin (inhibin A and B)

27
Q

What do inhibin A and B do?

A
  • Both forms specifically suppress FSH secretion by pituitary without affecting LH secretion
  • Acts at the level of pituitary (not hypothalamus)
28
Q

What does activin do?

A
  • Stimulates FSH secretion
29
Q

What is follistatin?

A

FSH-suppressing protein from follicular fluid – binds to activin with high affinity » neutralizes FSH-stimulating ability of activins

30
Q

How many genes make the precursor protein for me inhibin and activin?

A

3 genes

31
Q

How many proteins are made from the precursor protein of inhibin/activin?

A

3 different proteins made:

  • alpha protein: specific for inhibin
  • Beta A protein: can form either activin or inhibin
  • Beta B protein: can form either activin or inhibin

They are all part of the TGF-beta super family of proteins

32
Q

How many forms can activin take?

A

3 forms depending on the beta chain composition:

  • Activin A (Beta-A homodimer)
  • Activin B (Beta-B homodimer)
  • Activin AB (Beta-A Beta-B heterodimer)
33
Q

Do activins have an alpha subunit?

A

no

34
Q

Why are these so many subtypes of inhibin and activin?

A

Inhibin will always inhibit FSH and activins will always stimulate/activate FSH but different sub-types are made at different stages of the menstrual cycle

35
Q

Which protein (out of activin and inhibin) is made more of during the early antral phases?

A
  • In early antral phases, more activins are made compared to inhibin. correlates with an increase in FSH during that phase
  • As folliculogenesis progresses, inhibin increases in concentration, correlates with a decrease in FSH during that phase
36
Q

Which protein is found during the pre-antral follicle stage?

A

Activin B

37
Q

How is there an increase inhibin B as the follicle grows?

A
  • Activin makes FSH
  • FSH stimulates the production of the alpha subunit as the follicle grows
  • There’s an increase in Inhibin B
38
Q

What proteins are found when the dominant follicle is chosen?

A

There’s an increase in Inhibin A and a decrease in Inhibin B

39
Q

What proteins are found when there is a corpus luteum?

A

Inhibin A and no Inhibin B

40
Q

What is AMH (Anti-Mullerian hormone)?

A

A glycoprotein and also a member of the TGFβ superfamily

41
Q

When is AMH expressed in males?

A

from week 8 of development - causes regression of the Müllerian ducts by a wave of apoptosis

42
Q

Where is AMH expressed in ovaries?

A
  • expressed by ovarian granulosa cells with levels peaking in selectable follicles (large preantral and small antral follicles) » then decreasing
  • AMH production in preantral follicles is variable, but has been detected from the primary stage onwards – different in different species
43
Q

How is AMH distributed in follicles?

A
  • Around the small early antral follicles, there’s (staining) AMH
  • AMH (staining) decreases in the large pre-ovulatory follicles
  • No staining in the theca at all
44
Q

What is the purpose of AMH in females?

A

A regulator of normal follicle growth and development

45
Q

How does AMH act during folliculogenesis?

A
  1. Inhibits transition from primordial to primary follicles – prevents mass activation and growth of follicles
    • However, there’s no AMH receptors on the primordial follicles
  2. Inhibits FSH-dependent cyclical recruitment of antral follicles by inhibiting FSH-stimulated aromatase and FSH receptor expression → in the normal cycle would act to prevent over-recruitment of growing follicles
    • At some point AMH levels falls and FSH recruits follicles
46
Q

What is preantral follicle growth?

A

FSH independent

47
Q

What does the inter-cycle rise of FSH do?

A
  • only the follicles at the right stage at the right size will respond to the FSH and they will be recruited to the menstrual cycle to grow exponentially.
  • Activin is also made by the granulosa cells in the eggs to stimulate FSH and aid this process
48
Q

What causes the intercycle rise in FSH to decrease?

A

Granulosa cells also make oestrogen (gives negative feedback) and inhibin later on so the FSH levels fall – so only one dominant follicle survives, the rest can’t grow further and die.

49
Q

How is a dominant follicle selected?

A
  • One follicle from the group of antral follicles in ovary is just at the right stage at the right time
  • This becomes the dominant follicle which survives fall in FSH and goes onto ovulate
  • The DF follicle will have more FSH receptors, binds to FSH better, has a lower threshold for FSH and develops LH receptors
50
Q

What happens to LH as FSH decreases?

A

LH increases

51
Q

What receptors are found in granulosa cells and what steroids are produced as a result of these receptors?

A
  • FSHr, only the dominant follicle granulosa cells will have LHr from the mid-follicular phase onwards
  • Oestrogen and progesterone once LHr has been acquired
52
Q

What receptors are found in theca cells and what steroids are produced as a result of these receptors?

A
  • LHr only

- Progesterone and androgens

53
Q

Describe Inhibin B levels throughout the cycle.

A

highest in early-mid follicular phase and declines in LFP (small peak at LH surge), zero in luteal phase.

54
Q

Describe Inhibin A levels throughout the cycle.

A
  • increases in late follicular phase with highest levels in luteal phase (being made by CL) – contributes to inhibition of FSH in this phase.
  • The dramatic decline in Inhibin A at the end of the luteal phase allows for increase in FSH.
55
Q

What do the frequency and amplitude of GnRH decide?

A
  • Frequency decides whether LH or FSH is released

- Amplitude decides whether synthesis or release is phase

56
Q

What parts of the reproductive tract do oestrogen and progesterone affect?

A
  • Endometrium
  • Oviduct/Fallopian tubes
  • Cervix
  • Vagina - changes in vaginal epithelial cells

Science of reproduction (32 decks)

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