Menstrual cycle & sex steroid axis Flashcards
List the key reproductive hormones
- GnRH
- LH
- FSH
- Oestrogen
- Progesterone
- Testosterone
Where is GnRH secreted from and what does it stimulate the secretion of ?
It is released from the hypothalamus and stimulates the release of FSH & LH from the anterior pituitary
Describe the manner in which GnRH is released
- It is released in a pulsatile manner
- In males GnRH is secreted in pulses at a constant frequency
- Whereas in females the frequency of the pulses varies during the menstrual cycle, pulses every 1-2 hrs during the early follicular phase, decreasing in frequency late follicular phase and decrease frequency to every 4 hours during the luteal phase
What do high and low frequency GnRH pulses stimulate ?
- High-frequency GnRH pulses stimulate LH
- Low-frequency pulses are required for FSH release
What effect does oestrogen and progesterone have on GnRH pulses ?
- Oestrogen concentrations above a certain threshold will increase GnRH pulsatility driving the release of LH
- Increase in progesterone concentrations reduces the frequency of GnRH pulses
The onset of pubertal maturation is associated with what?
A steady acceleration in GnRH pulsatility (peak at night; In boys, this diurnal rhythm results in peak testosterone in the early morning leading to erections)
GnRH pulsatility is regulated by oestrogen and progesterone however GnRH producing neurons do not contain a receptor for these hormones ==> what neurones do oestrogen and progesterone indirectly act on to regulate GnRH pulsatility ?
Kisseptin neurones
What is the function of FSH & LH in males ?
- FSH - causes the testes to produce sperm (spermatogenesis)
- LH - causes the testes to secrete testosterone
What is the function of FSH & LH in females ?
- FSH - stimulates ovarian follicular development (oogenesis), thickens the endometrium & it causes the ovary to secrete oestrogen
- LH - LH peak causes ovulation and stimulates corpus luteum development and production of progesterone by the corpus luteum. Also thickens the endometrium
What is the menstrual cycle ?
It is a complex series of physiological changes occuring in a women on a monthly basis resulting in prodicution of an ovum & thickening of the endometrium to allow for implantation if fertilisation should occur
How long does one entire menstrual cycle last ?
- 28days
- Note - normal range is 28-35
When does the first day of the menstrual cycle begin ?
On the first day of menstruation (when they see blood)
How long does bleeding typically last ?
3-8 days
When does ovulation occur in the menstrual cycle ?
On day 14
What controls the menstrual cycle ?
- The hypothalamus - it regulates the hormones released from the anterior pituitary
- It releases gonadotropin releasing hormone (GnRH) which stimualtes the anterior pituitary gland to release FSH & LH
Describe the stages of development of the ovum
- FSH & LH travel to the female ovaries
- In each ovary there are many follicles (primordial), each month only some of the follicles begin to mature due to the rise in FSH levels at the start of the cycle
- While these follicles are maturing they produce oestrogen
- Only one of the developing follicles will reach full maturity (Graafian follicle) & ovulate
- During the first 10 days of the cycle oestrogen has a -ve feedback on the pituitary gland where it inhibits LH but after 10 days its levels have increased where the opposite effect happens and it & GnRH stimulate LH secretion causing a LH surge triggering ovulation
- During ovulation the oocyte is released
- The oocyte turns into the corpus luteum (under the influence of LH which then starts to slowly degrade but it will also secrete hormones - oestrogen, progesterone & inhibin
- If no fertilisation (like in a normal menstrual cycle) occurs, the corpus luteum degrenerates and the hormones it secretes decrease, eventually allowing GnRH levels to rise
- GnRH levels increase due to not being inhibited by progesterone ==> new cycle occurs
- Also at the same time as the new cycle initiating because of the decreased oestrogen & progesterone levels it means the endothelial linning will no longer be maintained and thus will shed off (known as the period)
What are the 2 phases of the menstrual cycle ?
- Follicular phase: days 0-14
- Luteal phase: days 14-28
What is the effect of oestrogen concentrations on FSH & LH and how does this affect their levels during the start of the menstrual cycle?
- Low concentrations of oestrogen will inhibit LH secretion and will stimulate FSH secretion ==> LH levels do not change at the start due to oestrogen inhibiting and GnRH stimulating cancelling eachother out. Whilst FSH levels initially increase & then decrease as oestrogen levels increase as cycle progresses
- High concentrations of oestrogen will stimulate LH secretion
Why does the concentration of oestrogen increase from start of menstrual cycle to ovulation (14 days) ?
Because as the follicles in the ovaries mature they produce more oestrogen
What happens to oestrogen levels following ovulation ?
They will decrease slightly
What happens to inhibin concentrations during the menstrual cycle ?
It was not present prior to ovulation, afterwards it begins to increase due to the corpus luteum
What happens to progesterone levels during the menstrual cycle ?
Prior to ovulation they were low, afterwards they increase during the luteal phase
Describe the stages of the menstrual cycle
- Initially at the start of the menstrual cycle there is a steady increase in GnRH, which should cause an increase in FSH &LH, but instead a increse then decrease of FSH & steady level of LH is seen (due to effects of oestrogen)
- After 10 days the increase in GnRH & oestrogen (now high enough to stimulate LH) both stimulate LH secretion. Resulting in a massive spike in LH levels.
- The massive spike in LH levels triggers ovulation og Graafian follicle
- FSH also has a slight spike due to side effect of the massive LH surge
- After ovulation LH, FSH & GnRH levels all drop due to the action of progesterone & inhibin
What is the effect of inhibin during the menstrual cycle ?
Produced by the corpus luteum it inhibits FSH stimulation because during the luteal phase we do not want anymore maturation of follicles in the ovaries ==> as the corpus luteum develops, inhibin secretion suppresses FSH secretion (-ve feedback)
What are the overall effects of oestrogen on the body (not just talking about menstrual cycle)?
- At low conc. it inhibits LH secretion, at high concs. it stimulates LH secretion
- High estrogen concentration inhibits secretion of FSH and prolactin (-ve feedback)
- It stimulates bone & muscle growth
- It stimulates endometrial thickening (endometrial glands & stroma) (occurs during the follicular phase)
- Maintains female secondary characterisitics & maintains glands (e.g. breast)
- Reduce vaginal pH through increase in lactic acid production
- Decrease viscosity of cervical mucus to facilitate sperm penetration
What are the overally effects of progesterone in the body (not just talking about the menstrual cycle) ?
- Its main effect is stimulating endometrial thickening (oestrogen also does this)
- It has a -ve feedback effect on the pituitary where it inhibits the secretion of LH
- Responsible for infertile thick mucus (prevent sperm transport and help prevent infection)
- Relaxes the myometrium (smooth muscle). Functional progesterone withdrawal thought to regulate parturition (birth)
- Has thermogenic effect (increases basal body temperature)
What do decreasing FSH levels cause ?
Atresia of all developing follicles except the dominant one
What happens during the luteal phase ?
- Progesterone induced glandular secretory activity
- Deciualisation (endometrium changes to structure called decidua, important role in promoting placenta formation)
- Decidualised endometrial lining is shed if implantation does not occur i.e. period
Go over this overview of the phases of the menstrual cycle and the hormones secreted
