Menstrual cycle

Duration: 24-38 days (28 average), with first day of bleeding = day 1 of cycle
Menses last 3-7 days with average of 35-50mL blood loss
Cycle involves simultaneous changes in ovaries (ovarian cycle) and uterus (uterine cycle)

normal menstrual cycle

follicular phase 1-14 pre ovulation
- hypothalamus secretes gonadotrophin releasing hormone (GnRH) → ant. pit. to secrete gonadotropins leutinising hormone (LH) and follicle stimulating hormone (FSH) in equal amounts
- LH and FSH travel to follicle in ovary
  - follicles are small clusters of theca and granulosa cells which protect developing oocyte.
  - Theca cells have LH receptors, when stimulated, result in secretion of adrestenedione
  - granulosa cells have FSH receptors, when stimulates, result in secretion of aromatase, which converts andrestenedione into 17beta estradiol (oestrogen)
  - follicular growth → increased 17B estradiol in blood → negative feedback on ant pit → decreased FSH release (only enough for one follicle)
  - follicle with most FSH receptors grows the fastest to become the dominant follicle. at this point, granulosa cells also develop LH receptors, while the follicle continues to release eostrogen. this increases sensitivity of ant pit to GnRH from hypothalamus
  - estrogen in blood declines, and estrogen from dominant follicle becomes positive feedback (+++FSH and LH release from ant. pit. responding to GnRH) 1-2 days before ovulation. the FSH and LH bind to the theca and granulosa cells to facilitate rupture and release of the oocyte to travel down the fallopian tube to the uterus for the luteal phase
luteal phase 15-28 post ovulation

Untitled

cycle	Phase	Description	Mechanism
ovarian 1-14 days	Follicular	Day 1 of menses – day before LH surge
Most variable length phase	FSH → follicular development → granulosa cells release oestrogen (-ve feedback on FSH)
Dominant (Graafian) follicle selection → oestrogen spike and LH surge
ovarian 14-15	Luteal	Until beginning of next menses	Graafian follicle rupture -release oocyte
LH converts follicle into corpus luteum → progesterone production and LH suppression (CL regresses w/o preg)
Uterine 3-7 days	Menses	Menstrual bleeding, usually 14 days post-ovulation	Decrease in progesterone with CL regression → uterine arterial vasospasm → bleed. (hence progesterone delays menses)
Uterine 10 days	Proliferative	Growth of endometrium under influence of oestrogen	Follicles produce oestrogen (+granulosa cells produce aromatase, converting androgens) → endometrial proliferation
Uterine 10-14 days	Secretory	With progesterone: Functional layer of endometrium prepared for implantation, with increased cervical mucus and raised body temp
Without: endometrial apoptosis and menstruation