Menstrual Cycle I

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

• 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

polymenorrheic

oligomenorrheic

Follicular phase and luteal phase Luteal phase»_space; ovulation»_space; follicular phase

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

Progesterone

The intracycle rise in FSH

FSH goes down

Different enzymes are present in different cellular compartments

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

FSH receptors

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

• 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)

• 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

• 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

• 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

• 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

• 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

• Polyestrus • Monoestrus • Seasonal polyestrus

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

• 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

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

Inhibin and activin