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Reproduction (Physiology 2) > The Menstrual Cycle > Flashcards

The Menstrual Cycle Flashcards

1
Q

What are the aims of the menstrual cycle?

A
  • selection of a single oocyte
  • regular spontaneous ovulation
  • correct number of chromosomes in eggs
  • cyclical changes in the vagina, cervix and Fallopian tube
  • preparation of the uterus
  • support of the fertilised dividing egg
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2
Q

What is the significance of the menstrual cycle?

A

The cyclical changes permit the transport and implantation of the egg

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3
Q

Describe how the HPG cycle is regulated?

A

Feedback is crucial for the regulation of the menstrual cycle.
GnRH secreted in a pulsatile fashion stimulating varying LH/FSH pulsatile secretion to the ovary. Oestrogen and Progesterone are secreted in the ovary which feedbacks to the hypothalamus to regulate the cycle

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4
Q

Describe what occurs in the follicular phase of the menstrual cycle

A

growth of follicles up to ovulation = dominated by oestradiol production from dominant follicle

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5
Q

What happens in the luteal phase?

A

The formation of corpus luteum from the empty follicle = dominated by progesterone production from the corpus luteum

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6
Q

How long does the menstrual cycle last?

A

Most women’s cycles are 30 days

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7
Q

How many phases are there in the menstrual cycle?

A

2 phases separated by ovulation
Cycle begins on day 1=first day of bleeding
Next 14 days are follicular phase i.e. growth of follicle
Ovulation occurs at end of follicular phase

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8
Q

What is the corpus luteum?

A

Empty follicle becomes the corpus luteum

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9
Q

What is the regulatory hormone of the HPG axis during the luteal phase?

A

Progesterone = Negative feedback

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10
Q

What is the regulatory hormone during the Follicular phase?

A

variable

  1. Release (stop) of negative feedback
  2. Negative feedback then reinstated, then
  3. Switch from negative to positive feedback
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11
Q

Outline the steps of the menstrual cycle

A
  1. Late Luteal / Early Follicular
    P declines, selectively raises FSH = intercycle rise
  2. Mid Follicular
    E2 increases -ve feedback , FSH falls
  3. Mid Cycle
    2 days of E2 >300 pmol causes +ve feedback and LH
    surge
  4. Mid Luteal
    High P causes -ve feedback = low LH/FSH, P overcomes
    E2
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12
Q

What is the result of the menstrual cycle?

A

At the end the corpus luteum (is the left-over follicle after ovulation) is dying and the progesterone that it was making falls

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13
Q

What causes the favourable growth of follicles during the menstrual cycle?

A

High Progesterone was exerting negative feedback at level of hyp/pit ∴keeping LH/FSH low.
As levels fall, the brake on negative feedback is lost but in such a way that it causes FSH levels to increase preferentially. This stimulates follicles to grow and make oestradiol.

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14
Q

What is the effect of E2 in the menstrual cycle?

A

E2 feeds back to hyp-pit and inhibits FSH release, so levels fall again.

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15
Q

Explain how a dominant follicle arises

A

In the meanwhilst LH levels have been rising a little across the follicular phase and this allows a single follicle to grow to become the dominant follicle. This DF makes massive amounts of E2.

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16
Q

What causes positive feedback to occur in the menstrual cycle?

A

After 2 days of E2, (reach level of >300pmol) the negative feedback becomes positive and there is a huge release of LH causing the egg to be released

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17
Q

What happens to the remaining follicle?

A

The remaining follicle becomes the CL which makes prog. & this causes negative feedback again

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18
Q

What is the significance of the intercycle rise in FSH?

A

The inter-cycle rise and fall in FSH (due to negative feedback of oestrogen) is very important because it allows selection of a single dominant follicle

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19
Q

Outline how a dominant follicle is chosen

A
  1. Small follicles grow in ovaries eventually developing
    theca, blood supply and early antrum - need FSH
  2. FSH rise causes continued follicle growth of the follicles
    recruited into the cycle
  3. Recruited follicles produce E2 as they grow
  4. E2 reinstates negative feedback to HPO causing FSH
    levels to drop
  5. One of the follicles will remain growing => selected
    dominant follicle
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20
Q

How is further dominant follicle growth prevented?

A

Oestradiol levels rise reinstating negative feedback at pituitary causing FSH levels to fall prevents further follicle growth

21
Q

Describe the gonadotrophin levels during the follicular phase

A

As FSH falls, LH increases

22
Q

Why does the dominant follicle survive?

A

Dominant follicle acquires LH receptors on granulosa cells.

DF also increases sensitivity to FSH by increasing FSH receptors & increases number of granulosa cells.

Other follicles do not, so they lose their stimulant and die.

23
Q

What is the significance of the dominant follicle increasing FSHR and LHR sensitivity?

A

Increases sensitivity to FSH → increased FSH receptors

Increased no. of granulosa cells
2.5 million GC in EFP and 50-100 million at ovulation

Increases E2 production due to

increased aromatase levels
200x more E2 in dominant follicle compared to others

Acquisition of LH receptors
LHR gene is switched on by FSH

24
Q

Briefly outline steroidogenesis

A

All steroids originate from cholesterol and are made by sequential removal of C atoms by a series of enzymes (in blue) which are distributed through the cellular compartments of the follicle

25
Q

Which cells are responsible for androgen production?

A

Androgens produced in theca ONLY
- Theca always has LHr, never FSH
- LH drives androgen and progesterone production from
theca

26
Q

Which cells produce oestrogens?

A

Oestrogens produced in GC ONLY
Granulosa have FSHr, then LHr acquired from mid-follicular phase onwards
FSH and LH then drives oestrogen production in follicular phase

27
Q

What causes the switch from negative to positive feedback ?

A

At the end of the follicular phase E2 levels raised for long enough → feedback switches from negative to positive

28
Q

What is the result of positive feedback of the HPO axis?

A

Causes massive release of LH from pituitary
Exponential rise in LH in serum
Triggers ovulation cascade

29
Q

Outline the ovulation casacde

A
  1. Egg is released
  2. Above result in changes in follicle cells = luteinisation ie.
    formation of the corpus luteum
  3. Corpus luteum has both luteinised granulosa and theca
    cells
  4. E2 production falls and P is stimulated
30
Q

Why does blood flow to the follicle increase in ovulation?

A

The blood flow to the follicle increases dramatically to increase vascular permeability which increases intra-follicular pressure

31
Q

What characteristic changes occur in the ovaries during ovulation?

A

Appearance of apex or stigma on ovary wall

Local release of proteases - Enzymatic breakdown of protein of the ovary wall

12-18 hrs after peak of LH, hole appears in follicle wall and ovulation occurs

32
Q

What does a rise in intra-follicular pressure cause?

A

Oocyte with cumulus cells is extruded from the ovary under pressure
follicular fluid may pour into Pouch of Douglas - ectopic

33
Q

Where is the egg collected?

A

The egg is ‘collected’ by fimbria of Fallopian tube due to signal from fluid
egg progresses down tube by peristalsis and action of cilia

34
Q

What initiates the egg to undergo completion of meiosis I?

A

In response to the LH surge, the nucleus of the oocyte in the dominant follicle completes the first meiotic division

35
Q

Why is the egg split evenly?

A

Majority of the cytoplasm & mitochondria is retained within the egg as it will have to carry the early fertilised embryo. The sperm only carries its DNA

36
Q

Describe the division of the egg

A

½ the chromosomes are put into a small “package” in the egg called the 1st polar body
The egg is now a secondary oocyte
The 1st polar body plays no further part in the process and does not divide again
Oocyte begins the 2nd meiotic division, but arrests again.

37
Q

Why is the oocyte division so significant

A

The oocyte has to support all of the early cell divisions of the dividing embryo until it establishes attachment to the placenta

38
Q

What happens to the follicle after ovulation?

A

After ovulation the follicle collapses
Corpus luteum is formed, ‘yellow body’
Progesterone production increases greatly, also E2

39
Q

What does the Corpus luteum contain?

A

CL contains large nos. LH receptors

CL supported by LH and hCG (from implanting embryo if a pregnancy occurs

40
Q

What are the 2 secretions of the CL?

A
  • progesterone

- oestradiol

41
Q

What is the significance of Corpus luteum secretions?

A

These 2 secretions of CL are vital because they will determine if pregnant.

42
Q

What is the role of the progesterone secreted form CL?

A
  • supports oocyte in its journey
  • prepares the endometrium
  • controls cells in Fallopian tubes
  • alters secretions of cervix
43
Q

What is the role of the oestradiol secreted from CL?

A

for endometrium thickening

44
Q

What happens to the CL if fertilisation doesn’t occur?

A

If fertilisation does not occur, CL has finite lifespan of 14 days.
removal of CL essential to initiate new cycle
Cell death occurs, vasculature breakdown, CL shrinks
Process is not well understood

45
Q

What are the signs of ovulation?

A

A slight rise in basal body temperature, typically 0.5 - 1 degree, measured by a thermometer - would need to keep a record of basal body temp from day 1 of last menstrual period as body temp varies day to day
Tender breasts
Abdominal bloating
Light spotting
Changes in cervical mucus
Slight pain or ache on one side of the abdomen

46
Q

What do ovulation prediction kits identify?

A

Ovulation prediction kits detect LH, as ovulation precedes LH surge
E3G is a urinary metabolite of oestradiol, allowing women to identify high fertility leading up to ovulation

47
Q

What is the fertile period?

A

Fertile period spans 6 days and is affected by:
Lifespan of the egg → upto 24h after ovulation
Lifespan of sperm → median = 1.5 days

48
Q

How long does sperm last within the female body?

A

Sperm can survive <5 days in the sperm supportive mucus of fertile days of cycle → sperm survival is dependent on the type & quantity of mucus within the cervix and the quality of the sperm

Reproduction (Physiology 2) (12 decks)

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