Physiology of the Menstrual Cycle Flashcards
What are the two functions of ovaries?
1) Hormone production
2) Oocyte release - menstruation, reproduction
Describe the hypothalamic-pituitary-ovarian axis
- Hypothalamus → GnRH → anterior pituitary → LH, FSH → gonads → sex hormones → negative feedback to hypothalamus and pituitary
What causes cramps?
The drop in progesterone
Describe how the number of eggs drops over time
1) Before birth - 7 million
2) At birth - 2 million
3) First period - 0.5 million eggs
4) Menopause - no eggs
What is menarche?
The onset of menstruation
What do ovulation kits measure?
The level of LH (LH surge) in urine to predict when ovulation occurs
What does the corpus luteum secrete?
Progesterone (and oestrogen)
Why is LH called so?
Bc it acts on the corpus luteum
How much thicker does the lining get during the follicular/proliferative phase?
11-14mm
Describe the ovarian cycle
1) Follicular phase (D1-10) - hormones signal 10-20 follicles to grow in ovaries, oestrogen matures one oocyte
2) Ovulatory phase (D11-14) - oocyte undergoes cell division, follicle wall thins and ruptures, oocyte enters abdominal cavity near timbre of fallopian tube
3) Luteal phase (D14-28) - oestrogen levels drop, egg travels through the fallopian tube towards the uterus
How does oestrogen feedback change in the cycle?
First half of the cycle oestrogen is associated with positive feedback, but in the second half, it is associated with negative feedback
What can you see in a transvaginal scan?
The follicle/corpus luteum
Describe the motility of the oocyte during ovulation
1) Shortly before ovulation, fimbriae begin to sweep over the surface of ovaries
2) Once the oocyte is in the tube, it is propelled by cilia and fallopian tube contractions
When does implantation begin?
By day 6-7 after ovulation (D21)
Where do gonadotropins act?
Directly on ovaries
What are the roles of oestrogen?
1) Promotes maturation of reproductive organs
2) Development during puberty
3) Regulates the menstrual cycle
4) Pregnancy
What are the roles of progesterone?
1) Promotes breast development
2) Maintains the uterine lining
3) Regulates the menstrual cycle
4) Sustains pregnancy
What is the FSHR?
Follicle stimulating hormone receptor - transmembrane receptor that interacts with FSH and represents a GPCR
What are the 4 phases of the menstrual cycle?
1) Menstrual phase
2) Proliferative phase
3) Late proliferative phase
4) Secretory phase
Describe the menstrual phase of the menstrual cycle
D1-5 → the functional layer of the endometrium becomes detaches from the uterine wall, resulting in bleeding (menses)
Describe the proliferative phase of the menstrual cycle
D6-14 → GnRH, FSH, LH
→ Follicular development
→ Increasing levels of oestrogen
→ Endometrium begins to proliferate and thicken, tubular glands and spiral arteries form
→ Stimulation of progesterone receptor synthesis in endometrial cells
Describe the late proliferative phase of the menstrual cycle
D13-14 → the glands are long and tortuous due to the active growth
→ Stroma is gradually becoming oedematous
Describe the secretory phase of the menstrual cycle
D15-22 → rising levels of progesterone
→ Enlargement of glands which begin secreting mucus and glycogen in preparation for implantation of the fertilised ovum
→ Increased fluid (oedema) in the stroma
What is the menstrual cycle?
Series of cyclic changes in the endometrium in response to changes in levels of ovarian hormones
What % of women with regular cycles are on their fertile window on any given day of their cycle between days 6-21?
At least 10%
What is defined as a regular cycle?
A cycle that has no more than 4 days variation from month to month
What is day 1 of the menstrual cycle?
The first day of bleeding
How long does menstruation last?
3-8 days, written as 7/28 or 5-6/27-32
Describe oogenesis before ovulation
- At birth all primordial oocytes are arrested at prophase I of meiosis I
- The majority of the follicles/oocytes undergo atresia from birth to menopause
- Meiosis I generates a secondary oocyte and the first polar body dies
- Secondary oocyte arrests at metaphase II of meiosis II (just before it ovulates)
When does meiosis II occur in oogenesis?
Meiosis II only completes when the egg is fertilised at which poiunt the second polar body is extruded to prevent triploidy
What happens alongside oogenesis?
Folliculogenesis (follicular development) which can take 300+ days and ovulation
What are the 3 stages of the ovarian cycle?
1) Follicular phase
2) Ovulation
3) Luteal phase
What are the 3 stages of the uterine cycle?
1) Menstruation
2) Proliferative phase
3) Secretory phase
