S4) The Menstrual Cycle Flashcards
Group the menstrual cycle into 3 general stages
- Preparation – ovarian & uterine cycle
- Ovulation – release of gamete
- Waiting – maintaining endometrium until fertilisation occurs
Identify 2 types of hormones which control the menstrual cycle
- Gonadotrophins
- Ovarian steroids
Where do gonadotrophins act?
Gonadotrophins act on the ovary
What two actions do ovarian steroids do?
- Act on tissues of the reproductive tract
- Act to control the cycle
In the HPO axis, what substance is produced by the hypothalamus?
GnRH
In the HPO axis, what does GnRH do?
Acts on anterior pituitary to release gonadotrophins:
- FSH
- LH
In the HPO axis, what do gonadotrophins do?
- Promoting follicular development
- Promote production of ovarian hormones
Identify two types of ovarian hormones
- Steroid hormones
- Inhibin
How is the HPO axis controlled?
- Controlled by effects of gonadal hormones
- Negative and positive feedback control
Why is the pulsatile release of GnRH important?
- Intermittent GnRH receptor is required for fertility
- If GnRH receptors are exposed continuously to GnRH, they become desensitised and gonadotrophin production stops
What are the two stages in the ovarian cycle?
- Follicular phase
- Luteal phase
Describe the layout and actions of the HPO axis
What is the main event occuring at the start of the menstrual cycle?
Early development of follicles begins
Describe the hormone levels at the start of the menstrual cycle
- No ovarian hormone production
- Low steroid and inhibin levels
- FSH levels rising
Describe the amount inhibition observed at the start of the menstrual cycle
Free from inhibition as there is little inhibition at the hypothalamus or anterior pituitary
Describe the effects of FSH on the developing follicles
FSH binds to granulosa cells:
- Follicular development continues
- Theca interna appears
- Follicle now secretes oestrogen & inhibin
What two things need to occur in the mid follicular phase?
- Nominate a dominant follicle
- Prevent recruitment of any further follicles
Explain how FSH is inhibited by the developing ovarian follicle
- [Follicular oestrogen] now exerts positive feedback on the hypothalamus and anterior pituitary
- Selective inhibition of FSH occurs as only [LH] rises
- [Follicular inhibin] rises
What 4 events occur in preparation for ovulation?
⇒ [Oestradiol and inhibin] rise rapidly
⇒ Oestradiol production independent of FSH
⇒ Surge in LH production
⇒ Modulation of GnRH pulse generator
What causes the surge in LH?
- High [oestradiol] enhances sensitivity of anterior pituitary gonadotrophins to GnRH
- Selective inhibition of FSH leads to surge in LH
What is the relationship between the surge in LH and the production of progesterone?
- Granulosa cells become responsive to LH
- Progesterone production begins
Identify 3 processes which occur during ovulation
- Mature oocyte extruded through the ovary capsule
- Meiosis I is completed
- Meiosis II commences
Describe the hormone levels immediately after ovulation
- Follicle is luteinised and secretes oestrogen and progesterone in large quantities
- Inhibin continues to be produced and LH is suppressed
Why is LH production suppressed after ovulation?
Negative feedback due to the presence of large quantities of progesterone
What characterises the waiting phase?
Further gamete development is suspended
In the luteal phase, what hormones are produced by the corpus luteum?
- Progesterone
- Oestrogen
- Inhibin
What causes the corpus luteum to regress?
The absence of a surge of LH
What are the two stages in the uterine cycle?
- Proliferative phase
- Secretory phase
What is the endometrium and what does it do?
- The endometrium is the lining of the uterus
- It is a specialised epithelium that responds to ovarian hormones
Explain how the uterus responds to the following:
- Oestrogen
- Progesterone
- Responds to oestrogen by proliferating
- Responds to oestrogen and progesterone by secreting
Label the following structures on the uterus below:
- M = myometrium
- E = endometrium
- F = functional layer
- B = basal layer
What is the myometrium?
The myometrium is the muscular wall of the uterus
What does the functional layer of the uterus do?
The functional layer is hormone responsive and sheds if no pregnancy occurs
What does the basal layer of the uterus do?
The basal layer provides the source from which a new functional layer is developed
Describe the appearance of the uterus during the early proliferative phase
Early proliferative: glands sparse, straight
Describe the appearance of the uterus during the late proliferative phase
Late proliferative: functional layer has doubled, glands now coiled
Describe the appearance of the uterus during the early secretory phase
Early secretory: endometrium max thickness, very pronounced coiled glands
Describe the appearance of the uterus during the late secretory phase
Late secretory: glands adopt characteristic “saw-tooth” appearance
What effect does the regression of the corpus luteum have?
- Dramatic fall in gonadal hormones relieves negative feedback
- Menstrual cycle starts again
What happens when fertilisation occurs?
- Syncytiotrophoblast produces human chorionic gonadotrophin
- Corpus luteum is maintained by placental hCG
What is the significance of human chorionic gonadotropin?
- Made by the embryo to signal its presence
- Exerts a luteinising effect
What role does the corpus luteum have in pregnancy?
- It produces steroid hormones to support the pregnancy
- It regresses once the placenta can produce sufficient quantities of steroid hormones to control the HPO axis
Illustrate the relationship between the ovarian and menstrual cycles
Identify the 6 effects of oestrogen in the follicular phase
- Fallopian tube function
- Thickening of endometrium
- Growth & motility of myometrium
- Thin alkaline cervical mucus forms
- Vaginal changes
- Changes in skin, hair, metabolism
Identify the 6 effects of progesterone in the luteal phase
- Further thickening of endometrium into secretory form
- Thickening of myometrium & reduction of motility
- Thick acidic cervical mucus forms
- Changes in mammary tissue
- Increased body temperature
- Metabolic and electrolyte changes
What is the normal duration of the menstrual cycle?
21 – 35 days
What causes variations in the menstrual cycle?
- Variation in the length of the follicular phase
- Luteal phase strictly controlled to 14 days (± 2)
Identify 4 factors which affect the menstrual cycle
- Pregnancy
- Lactation
- Emotional stress
- Low body weight
