Menstrual Cycle I revision Flashcards
What are the 6 aims of the menstrual cycle?
Selection of a single haploid oocyte Regular spontaneous ovulation Correct number of chromosomes in egg Cyclical changes in the vagina, cervix and Fallopian tube Preparation of the uterus Support of the fertilised dividing egg
Which part of the menstrual cycle varies?
The follicular phase
How do we know which part of the menstrual cycle varies?
The luteal phase is relatively constant at 14 days due to the fixed life-span of the corpus luteum .
What are the 5 roles of FSH?
Cyclical recruitment of FSH Granulosa cell multiplication Stimulation of E2 leading to proliferation of endometrium Dominant follicle selection Induction of LH receptors
What are the 3 role of LH?
Completion of meiosis I in oocyte and start of meiosis II
Ovulation
Corpus luteum formation with accompanying progesterone production
What is the key requirement to maintain the HPO axis?
Pulsatile release of both GnRH and gonadotrophins (LH and FSH)
What is the difference between oestrus cycle or menstrual cycle?
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 in animals named because of:
The cyclic appearance of behavioural sexual activity (heat or oestrus)
They do not menstruate – the endometrium is reabsorbed if fertilization does not occur
Where is inhibitin produced?
Sertoli and granulosa cells
What is the purpose of inhibitin?
Suppress FSH secretion by pituitary without affecting LH secretion
What does follistatin do?
Binds activin with high affinity » neutralizes FSH-stimulating ability of activing
What are the precursor proteins for inhibin and activin?
α- protein, specific for Inhibin
βA- protein, can form either Activin/Inhibin
βB- protein, can form either Activin/Inhibin
How many forms can inhibins take?
2: Inhibin A (alpha + beta A), inhibin B (alpha + beta b)
How many forms can activins take?
3: Activin A (beta a + beta a), activin AB (beta a + beta b), activin B (beta b + beta b)
How do activins and inhibins affect follicle production?
Activins correlate with ↑FSH in early follicular phase
Inhibins correlate with ↓FSH in late follicular phase
Where is AMH expressed in females and when do its levels peak?
Expressed by ovarian granulosa cells
Levels peaking in selectable follicles (large preantral and small antral follicles) before decreasing
What are the 2 windows of action of AMH on folliculogenesis?
Inhibits transition from primordial to primary follicles
Inhibits FSH-dependent cyclical recruitment of follicles by inhibiting FSH-stimulated aromatase and FSH receptor expression → in the normal cycle would act to prevent over-recruitment of growing follicles
What is the relationship between AMH production and follicle size?
Reduced AMH secretion as follicle size increases
What is the FSH threshold hypothesis?
Follicle with lowest FSH threshold and survives fall in FSH level will become the dominant follicle.
How does the dominant follicle survive the fall in FSH?
Dominant follicle acquires LH receptors on granulosa cells.
Other follicles do not, so they loose their stimulant and die.
Receptor Distribution in Follicles
Granulosa: have FSHr, then LHr acquired from mid-follicular phase onwards
Theca: always has LHr, never FSHr
Where does oestradiol (E2) act on the HPO axis?
Hypothalamus and pituitary
How do steroids feedback onto GnRH?
Hypothalamus: steroids inhibit GnRH release
Pituitary: steroids sensitise or desensitise the response to GnRH
How does progesterone exert negative feedback on HPO axis?
Overcomes effects of E2
