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Life Cycle > Menstrual Cycle > Flashcards

Menstrual Cycle Flashcards

1
Q

What are the main aims of the menstrual cycle?

A
  • selection of a single oocyte
  • regular spontaneous ovulation
  • maintaining haploid chr in gametes
  • cyclical changes in vagina, cervix and fallopian tube
  • prep. of uterus for potential pregnancy
  • support for fertilised zygote
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2
Q

What is the HPG axis control of the menstrual cycle?

A

hypothalamus -> pulsatile GnRH
anterior pituitary -> pulsatile LH/FSH
Ovaries: oestrogen and progesterone

Feedback on to H and P of HPG (positive/negative depending on concentration of E2 and P)

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3
Q

Why does GnRH have a pulsatile secretion pattern?

A

needs to be pulsatile to trigger release of LH/FSH
If GnRH has continuous secretion, then the HPG axis is switched off

[LH responds better to GnRH than FSH]

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4
Q

Where is the GnRH-R located?

A

GnRH-R is a GPCR

located at the PM surface of anterior pituitary cells

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5
Q

What is the menstrual cycle?

A

2 phases: proliferative/follicular phase and luteal/secretory phase
Separated by ovulation

cycle begins on D1 of bleeding (menses)

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6
Q

What is the follicular phase of the menstrual cycle?

A

growth of follicles up tot ovulation

Driven by E2 production from the follicles

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7
Q

What is the luteal phase of the menstrual cycle?

A

formation of a corpus luteum from the remaining empty follicle post-ovulation

Driven by progesterone secretion from the corpus luteum

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8
Q

What is the HPG axis control of the luteal phase of the menstrual cycle?

A

Progesterone causes negative feedback onto HPG axis

-> reduced GnRH and no LH/FSH

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9
Q

What is the HPG axis control of the follicular phase phase of the menstrual cycle?

A

[VARIABLE]

  • negative feedback brake on HPG axis is released (pseudo-positive feedback, due to lack of inhibition)
  • therefore, negative feedback is reinstated initially (low LH/FSH)
  • As [E2] > 300pmol then switch to positive feedback (LH surge)
  • ovulation
  • corpus luteum makes P which causes negative feedback to reduce FSH
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10
Q

Why is the inter-cycle rise in FSH important in the menstrual cycle?

A

allows selection of a follicle
goes on to become the dominant follicle to ovulate

rise in FSH here is caused by lack of inhibition on the HPG axis, therefore no brake on FSH release

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11
Q

How does follicle selection occur?

A

raised FSH = window for antral follicle recruitment (for those that are sufficiently developed)

FSH threshold hypothesis

E2 levels rise and restore negative feedback and therefore reduce FSH release

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12
Q

What is the FSH threshold hypothesis?

A
  • one antral follicle from group will is @ the correct stage to survive loss of FSH
  • this is the DOMINANT follicle (“selection”) which will be ovulated
  • can come from either ovary
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13
Q

How does the dominant follicle survive the fall in FSH?

A
  • increased sensitivity of FSH-R, better able to cope with lower levels
  • lower [IGFBPs] in DF allows more free IGFs to sensitise DF to FSH
  • increased number of FSH-R
  • increased number of granulosa cells
  • slower turn over of bound FSH on cells to mediate effects
  • acquisition of LH-R
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14
Q

What hormones are at play during dominant follicle selection? How does this aid its survival?

A
  • DF creates E2, which inhibits FSH release causing a fall in FSH
  • At the same time, LH increases
  • DF acquires LH receptors on its granulosa cells
  • other follicles in the selected group of antral follicles DO NOT acquire LH receptors
  • this causes them to undergo atresia and die
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15
Q

When does oestrogen synthesis occur in the dominant follicle?

A
  • E2 is produced in the DF (granulosa cells)
  • produced from cholesterol
  • only starts to occur when LH-R are present
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16
Q

Where does androgen synthesis occur in the female?

A

in theca cells only

17
Q

Where does oestrogen synthesis occur in females?

A

in granulosa cells only

18
Q

What is the LH surge? What are the mechanisms causing this?

A

E2 feedback in the follicular phase is negative

at end of follicular phase, E2 reaches 300pmol and switches to positive feedback mechanism

causes LH release/surge from pituitary and therefore increased serum LH

triggers the ovulation cascade

19
Q

What is the ovulation cascade?

A
  • egg is released
  • this causes LUTENISATION of of the follicle cells
  • Corpus luteum has both luternised granulosa and theca cells
  • E2 production falls, but is still produced
  • P is stimulated and predominates the luteal phase
20
Q

What is lutenisation of the follicle?

A

change of the (granulosa and theca) cells in follicle to form the corpus luteum

21
Q

Where does DF selection occur in the ovary?

A

movement of selected antral follicles from cortex to medulla of ovary

=> better vascular supply

Medulla: where DF selection occurs

22
Q

What local changes occur before ovulation?

A
  • increased blood flow to follicle
  • appearance of apex or stigma on ovary wall
  • local release of proteases and inflammatory mediators
  • enzymatic breakdown of ovary wall

ovulation = release of cumulus-oocyte complex (COC)
this occurs 12-18hr after LH surge

23
Q

What happens after the release of the cumulus-oocyte complex from the ovary?

A
  • extrusion of cumulus-ovary complex from ovary
  • follicular fluid may fill much of Douglas
  • egg is collected by fimbrae of fallopian tube
  • Progression down tube by peristalsis and ciliary action
24
Q

What (unequal) division steps occur post-ovulation?

A

LH surge -> oocyte in DF completes meiosis I
(50% of nuclear contents are transferred to a polar body)
=> secondary oocyte

  • meiosis II is started
  • arrests in prophase
  • will only complete meiosis II after fertilisation
25
Q

Why is the division of the ovulated oocyte described as unequal?

A

2 species are formed from the division: secondary oocyte and polar body

each of which receives half of the nuclear material

all of the cytoplasmic content is transferred into the secondary oocyte

Polar body is extruded to periphery of perivitelline space

26
Q

What is critical about the (secondary) oocyte?

A
  • only want a single oocyte

- oocyte needs to support all of the early cell divisions in embryo until placenta forms

27
Q

How is the corpus luteum formed after ovulation?

A
  • after ovulation: follicle collapses
  • CL forms (fatty, cholesterol and yellow)
  • increased Progesterone and E2 production
  • CL has large numbers of LH receptors
  • CL survival is supported by LH and hCG (for an implanted embryo) - which bind to the LH-R
28
Q

Where is hCG produced?

A
  • produced by trophoblast cells in the implanted embryo
  • functions to stabilise CL structure after fertilisation and implantation
  • binds to LH-R
29
Q

When are the levels of hCG at its highest?

A
  • during the first trimester of pregnancy (peak at ~8wks)

- once CL regresses, hCG falls and placenta takes over

30
Q

What are the functions of progesterone post-ovulation?

A
  • supports oocyte in its journey
  • maintains the CL and inhibits FSH (during luteal phase of MC)
  • prepares and maintains endometrium
  • controls cells in uterine tubes
  • alters cervical secretions
31
Q

What is the main function of E2 in the uterus?

A
  • production and preparation of the endometrium
32
Q

How is CL regression after the luteal phase?

A
  • when fertilisation does not occur, CL has a finite lifespan of 14 days
  • CL must be removed in order for new cycle to begin
  • cell death occurs, vasculature breakdown, CL shrinks
33
Q

What are the 2 main important secretions of CL?

A
  • progesterone (predominantly)

- E2

34
Q

What pharmacological agents can be used to modulate the HPG axis clinically?

A

Infertility: LH/FSH injections

Contraception: exogenous E2/P to inhibit LH/FSH release

Infertility: Clomid to disrupt negative feedback on LH/FSH release

Central precocious puberty: continuous GnRH infusion

35
Q

What are the physical signs of ovulation?

A
  • slight rise in basal body temp (+0.5-1C, measured by thermometer)
  • tender breasts
  • abdominal bloating
  • light spotting
  • changes in cervical mucus
  • slight pain on one side of abdo
36
Q

What hormone is being detected in ovulation prediction kits?

A
  • urinary metabolites of E2 (e.g. E3G)

- good indication of LH (surge) to indicate when ovulation is imminent

Life Cycle (29 decks)

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