4.1 The Menstrual Cycle Flashcards
Describe the HPO axis
GnRH produced by hypothalamus
Acts on anterior pituitary to produce gonadotropins (LH and FSH)
Gonadotropins act on ovary
- Promote follicular development
- Production of ovarian hormones (steroid hormones and inhibin)
Why is the nature of GnRH release important?
Pulsatile
If it wasn’t, GnRH receptors would become desensitised
FSH and LH production would stop
Gonadal steroid production stops
What happens at the start of the cycle?
FSH
No ovarian hormone production Early development of follicles Low steroid and inhibin levels Little inhibition FSH rises
What are the effects of FSH?
Binds to granulosa cells Follicular development continues Theca interna develops Follicle can now secrete oestrogen so oestrogen levels increase Inhibin also secreted by follicle
What happens during the mid-follicular phase?
Follicular oestrogen exerts positive feedback
Increase in gonadotropins
Increase only seen in LH
As follicular inhibin inhibits FSH production at anterior pituitary
What happens in preparation for ovulation?
Increase in eostradiol and inhibin
Eostradiol not dependent on FSH causing a LH surge
Progesterone production begins as granulosa cells become responsive to LH
How is the waiting phase established?
Lutenised follicle after ovulation
Oestrogen and progesterone secreted in large amounts
Inhibin produced
Presence of progesterone inhibits LH via negative feedback
So further development of gamete is suspended
What happens in the luteal phase?
CL producing oestrogen, progesterone and inhibin
CL regresses after 14 days if no further rise in LH
How does the endometrium respond to oestrogen and progesterone?
Oestrogen by proliferating
Oestrogen and progesterone by secreting
Describe the structure of the uterus
Myometrium
- Muscular wall
- Responds to oestrogen and progesterone
- Not shed
Endometrium
- Functional layer Shed
- Basal layer Replaces functional layer
Describe changes to uterus over menstrual cycle
Early proliferative - sparse straight glands with thin endometrium
Late proliferate - coiled glands
Early secretory - max thickness of endometrium, pronounced glands
Late secretory - glands have saw-tooth appearance
What happens at end of cycle if there is no fertilisation?
No rise in LH so CL regresses
Fall in Gonadal hormones
There is no longer negative feedback
What happens if fertilisation occurs?
Syncytiotrophoblast produces human chorionic gonadotropin
Lutenising effect
What do the gonadal steroids do during the follicular phase of the ovarian cycle?
Oestrogen
Thicken endometrium
Thin, alkaline cervical mucus
Growth and motility of myometrium
What are the actions of the gonadal steroids during the luteal phase?
Progesterone
Thicken endometrium to secretory form Thicken myometrium but reduce motility Thick, acidic cervical mucus Mammary tissue changes Increased body temp metabolic and electrolyte changes
