7. Menstrual cycle and the HPG axis Flashcards
what are the difference in gametogenesis of makes and females ?
gametogenesis in men is continuous.
female = it takes time to prepare the uterus for
implantation of a fertilised oocyte. Therefore, release of this oocyte needs to be periodic, otherwise even if fertilisation occurs, the uterus will not be able to sustain it. A typical cycle lasts between 21-35 days.
What are the 2 types of preparation that need to occur in a female and which cycles are responsible for them?
preparation of the ovary to provide an environment for oogenesis to occur and production of steroid hormones (‘the ovarian cycle’), and preparation of the uterus to receive a fertilised oocyte (‘the uterine cycle’).
in relation to each other, when do the ovarian and uterine cycles occur?
2 cycles in parallel
Average length of 28 days
what are the important hormones for the menstrual cycle
GnRH
LH, FSH
Oestrogen*, Progesterone, Inhibin, activin
why is GnRH released in a pulsatile manner?
Persistent presence of GnRH would lead to desensitisation of it’s receptors on gonadotrophs
FSH and LH production cease
Gonadal steroid production ceases
results in infertility
What are the stages of the ovarian cycle?
Follicular and luteal
What are the stages of uterine cycle?
Proliferative and Secretory
Define endometriosis.
Ectopic endometrial tissue, most commonly in ovaries, fallopian time or pelvic organs
How can symptoms of endometriosis be relieved?
Giving continuous GnRH to such patients can ‘switch the axis off’ to alleviate their symptoms.
What is the first hormone in he menstrual cycle that starts to rise and why?
FSH, low steroid and inhibit levels at start of the cycle, little inhibition of the hypothalamus or AP.
What cells does FSH bind to and what effects does its initial rise have?
Stimulate further follicular development, Theca interna develops,
What are the first hormones produced by the ovaries in the menstrual cycle and what is this in response to?
Oestrogen and inhibin, in response to FSH
what is LH resposnsible for?
‘LH surge’ is the key factor that drives ovulation.
what is the function of the ovarian cycle and what are the two stages?
Preparation of the gamete for release
2 phases:
Follicular phase – Early and Late
Luteal phase
what happens in the early follicular phase?
• No ovarian hormone production = no inhibition on the hypothalamus or AP
• Granulosa cells secrete activin
- ↑ FSH production, ↑ FSH receptors on granulosa cells
- Reduced production of androgens by theca cells – less oestrogen (for now)
• FSH levels increase
- Stimulates follicle growth - number of granulosa cells increases, and causes the development of the
theca interna and externa cells
- The follicle will now produce oestrogen from both the granulosa cells and the theca cells
• Dominant Follicle granulosa cells switch to produce inhibin (from activin)
what happens in the late follicular phase?
• Dominant Follicle granulosa cells produce oestrogen and inhibin.
• Oestrogen
- ↑ FSH receptors on follicle
- Initially exerts negative feedback to reduce FSH production
- ↑ LH receptors on granulosa cells – prep for ovulation
• Inhibin
- decreases FSH production
- Helps LH to ↑ Theca cell androgen production (converted to Oestrogen)
• Oestrogen + Inhibin continue to rise
what happens in the late follicular phase after Oestrogen + Inhibin continue to rise ?
• Oestrogen continues to rise independent of FSH
• High levels of Oestrogen exert positive feedback on
hypothalamus + anterior pituitary → Surge in LH
production
• Ovulation occurs
• Rise is not seen in FSH levels due to presence of inhibin
• Granulosa cells starts secreting progesterone (LH action)
why does inhibin inhibit FSH release?
to prevent FSH stimulating more than one dominant follicle.
once ovulation occurs, is meiosis 1 and 2 complete?
MEIOSIS I completed
MEIOSIS II begun but not completed yet
Mature oocyte travels through the capsule of the ovary into the fallopian tube
what happens in the luteal phase?
• Follicle is luteinised ( granulosa and theca cells become the Corpus Luteum)
• Secretes oestrogen and progesterone in large quantities
• Inhibin also secreted
• Oestrogen + Progesterone exert negative feedback on LH - (The high levels of oestrogen alone would cause positive feedback, however the presence of
progesterone promotes the negative feedback of oestrogen at the hypothalamus and AP)
• FSH levels remain low
• Corpus luteum lifespan 14 ±2
• WAITING PHASE
what is the function of the luteal phase?
This phase happens after ovulation, and it is the stage of the cycle where the reproductive system is waiting to see if fertilization and implantation has occurred.
what are the 3 phases of uterine cycle?
Menstrual, Proliferative and Secretory
define myometrium and endometrium
The myometrium is the muscular wall of the uterus and the endometrium is the epithelial lining of the uterine cavity
what is the endometrium divided into?
Functional layer – responsive to hormone, shed in menstruation
Basal layer – develops into new functional layer
which hormones are responsible for the proliferative and secretory phases respectively?
Oestrogen → Proliferative epithelium
Progesterone + Oestrogen → Secretory epithelium
what happens in the proliferative phase?
At the start of the cycle, the endometrium will proliferate and thicken in response to oestrogen produced by the ovary. There are simple, straight
glands within the endometrium but as this part of the cycle continues and the endometrium continues to develop, the glands become coiled and the functional layer doubles in size
what happens in the secretory phase?
Once ovulation has occurred, the glands that have been produced in the proliferative stage then become secretory under the influence of progesterone. Towards the end of this stage, when the hormones from the corpus luteum begin to fall, the glands then lose their structure and the endometrium becomes ready to shed its functional layer if implantation has not occurred.
what happens at the end of the menstrual cycle?
Corpus luteum → Corpus Albicans
Dramatic fall in oestrogen, progesterone and inhibin
Loss of negative feedback → slow rising FSH
New cycle: Menstruation occurs
what is the action of oestrogen in the follicular phase?
The effect of oestrogen on the reproductive tract is intended to promote fertilisation: • Proliferation of endometrium • Proliferation of myometrium • Fallopian tube motility • Thin, alkaline cervical mucus
what is the action of progestrogen in the luteal phase?
The effect of progesterone on the reproductive tract is intended to sustain a viable pregnancy:
• Secretory function of endometrium
• Reduction of motility of myometrium (assumes fertilisation has occurred)
• Thick, acidic cervical mucus to prevent further entry of sperm
what happens if fertilisation occurs?
Syncytiotrophoblast produces hCG (human chorionic
gonadotropin)(same function as LH)
Extends lifespan of corpus luteum → before placenta
forms
what is a normal cycle length?
Normal duration: 21 to 35 days
what may disrupt the cycle?
Physiological Factors: Pregnancy, Lactation
Emotional stress
Body weight
Infertility – various causes
