Lec 5 - The menstrual cycle Flashcards
Describe the female HPG axis.
- Hypothalamus
- –> GnRH is released. - The anterior pituitary
- –> LH and FSH is released. - Ovaries
- –> Oestrogen and progesterone is released.
Describe the male HPG axis.
- Hypothalamus
- –> GnRH is released. - The anterior pituitary
- –> LH and FSH is released. - Testes
- –> Testosterone is released.
What are the three stages of the menstrual cycle?
- Preparation
- Ovulation
- Waiting
Briefly summarise each stage of the menstrual cycle.
- Preparation
- of the gamete —> done in the ovarian cycle.
- of the endometrium —> uterine cycle. - Ovulation
- Release of the gamete - Waiting
- pause, maintaining the endometrium until a signal is received to indicate that fertilisation has happened.
What do the gonadotrophins act on?
- They act on the ovary.
What do the ovarian steroids act on?
- They act on tissues of the reproductive tract.
- They act to control the cycle.
What are the gonadotrophins released by the anterior pituitary?
FSH and LH
How do the gonadotrophins act on the ovary?
- by promoting follicular development.
- by producing ovarian hormones:
- –> steroid hormones
- –> inhibin
Why is it important for the GnRH release to be pulsatile?
- If the GnRH receptors are exposed to continuous presence of GnRH they would become desensitised.
- This means FSH and LH production would stop.
- The gonadal steroid production would also stop.
What is endometriosis?
It is the ectopic deposits of endometrium outside the uterus.
How is endometriosis treated?
It is treated with GnRH agonist to relieve symptoms.
- This works by stopping FSH and LH production.
- Resulting in the ectopic endometrium deposits to stop responding to FSH and LH so they will disappear.
Describe the start of the cycle.
- There is no ovarian hormone production.
- Early development of follicles begins.
- there are low steroid and inhibin levels.
- There is little inhibition at the hypothalamus or anterior pituitary.
- The start of the cycle is free from inhibition.
- FSH levels are rising.
What are the effects of FSH?
- FSH binds to granulosa cells.
- follicular development continues.
- The Theca interna appears.
- The follicle is now capable of oestrogen secretion.
- Inhibin secretion begins.
What are granulosa cells?
They are cells that support the development of the oocyte and carry out the endocrine functions of the ovary.
What happens in the mid-follicular phase?
- here there is the adding of subtle control.
- In this phase the recruitment of any further follicles needs to be prevented.
- The follicular oestrogen is now at a concentration when it can exert positive feedback at the hypothalamus and anterior pituitary.
- there is only an effect seen in LH —> massive increase.
- Follicular inhibin is also rising
- –> There is selective inhibition on FSH production by anterior pituitary.
What is done to prepare for ovulation?
- Circulating oestradiol and inhibin rise rapidly.
- Oestradiol production is no longer dependent on FSH.
- There is a surge in LH production.
- Progesterone production begins.
- –> Granulosa cells become responsive to LH. - There is also the modulation of GnRH pulse generator.
What does a high oestradiol concentration do?
It enhances the sensitivity of the anterior pituitary gonadotrophs to GnRH.
What does the LH surge indicate?
- It is the point which we have maturation of female gametes and we’re ready to release them.
- It promotes ovulation.
What happens during ovulation?
- Meiosis I completes and meiosis II starts.
- The mature oocyte is extruded through the capsule of the ovary.
What are the effects of LH during ovulation?
- After ovulation the follicle is luteinised.
- LH secreted oestrogen and progesterone in large quantities.
- Inhibin is still being produced.
- But LH is now also suppressed because of negative feedback due to the presence of progesterone.
- This suspends further gamete development establishing the waiting phase.
What happens during the waiting phase?
- Nothing is happening to the follicles in this phase.
- in this phase, waiting for signals such as a signal to indicate that fertilisation has taken place.
What happens during the luteal phase?
- The corpus luteum produces the following:
- produces progesterone and oestrogens from androgens.
- produces inhibin - When LH stops rising, then the luteal phase regresses spontaneously.
How long the luteal phase last?
- It is autonomous.
- The luteal phase can support itself for 14 days.
What is the endometrium?
- It is the inner lining of the uterus.
- It is a specialised epithelium which becomes highly glandular and the glands start to secrete.
What happens to the endometrium in the uterine cycle?
- It is responsive to the hormones produced by the ovary.
- Each month the endometrium thickens and renews itself preparing for pregnancy.
- In the uterine cycle it responds to oestrogen by proliferating.
- In the uterine cycle it responds to oestrogen and progesterone.
What are the two layers of the endometrium and what do they do?
- Functional layer
- –> It is hormone responsive and is shed if no pregnancy occurs. - Basal Layer
- –> It provides the source from which a new functional layer is developed.
Describe the changes of the uterus over the menstrual cycle.
- Early proliferative —> glands sparse and straight.
- Late proliferative —> functional layer has doubled and the glands are now coiled.
- Early secretory —> endometrium max thickness and very pronounced coiled glands
- Late secretory —> Glands adopt characteristic saw tooth appearance.
What happens at the end of the menstrual cycle if fertilisation has not occurred?
- The Corpus Luteum regresses due to the absence of a further rise in LH.
- There is a dramatic fall in gonadal hormones such as progesterone and oestrogen.
- There is negative feedback and it resets to start again.
What happens at the end of the menstrual cycle if fertilisation has occurred?
- In this case the syncytiotrophoblast produces human chorionic gonadotrophin.
- –> which is made by the embryo to signal its presence.
- A luteinising effect is also exerted.
- The corpus luteum is intact.
What does the corpus luteum do in pregnancy?
- The corpus luteum supported by placental hCG produces steroid hormones to support the pregnancy.
- –> Eventually the placenta is capable of production of sufficient quantities of steroid hormones to control the HPO axis throughout pregnancy.
- ——-> The HPO axis needs to be in the depressed state as you don’t want to have any more follicles/ oocytes as already pregnant.
What are the stages in the ovarian cycle?
- follicular
2. luteal
What are the stages in the uterine cycle?
- proliferative
2. secretory
What are the gonadal steroids?
- oestrogen
- progesterone
What are the actions of oestrogen in the follicular phase?
- Fallopian tube function.
- Thickening of the endometrium.
- Growth and motility of the myometrium.
- Thin alkaline cervical mucus.
- vaginal changes
- changes in skin, hair and metabolism.
What are the actions of progesterone in the luteal phase?
- further thickening of the endometrium into secretory form.
- Thickening of myometrium but reduction in motility.
- Thick, acid cervical mucus.
- Changes in mammary tissue
- Increased body temperature
- metabolic changes
- electrolyte changes.
What is the normal length of the cycle and why may it vary?
- normal duration is 21-35 days.
- –> variation in the length of the follicular phase cause the variations in the cycle duration.
- the Luteal phase is strictly controlled to 14 days.
What factors affect the menstrual cycle?
- Physiological factors:
- pregnancy
- lactation - emotional stress
3 low body weight
