Menstrual cycle Flashcards
What is the super top master hormone in the menstrual cycle?
GnRH, released in a pulsatile manner from the hypothalamus into the portal vessels, binding onto receptors on gonadotrope cells in the anterior pituitary gland.
What does GnRH do?
Drives LH and FSH secretion in a pulsatile manner
Changing the pulsality of GnRH changes LH and FSH secretion from anterior pituitary
Slow pulsing = FSH release
Fast pulsing = LH release
What is the menstrual cycle?
A cycle of changes in females involving the hypothalamus, pituitary, ovaries and uterus that prepares a secondary oocyte for fertilisation
What drives the menstrual cycle?
GnRH, FSH, LH, oestrogen, progesterone
How long is the menstrual cycle?
28-32 days, begins on day 1 of menstruation
Two stages of 14 days with ovulation in the middle
How do GnRH, LH and FSH interact?
GnRH neurons in hypothalamus generate pulsatile GnRH release.
FSH & LH act vie endogenous opioids in the hypothalamus to modulate the frequency and amplitude of these pulses
Amplitude and frequency of GnRH pulsality dictates pattern of LH and FSH
What does LH primarily do?
androsterone production in theca cells
FSH’s main effect?
oestrogen production in granulosa cell and follicle growth
1st half of menstrual cycle
Day 1-14
Menstruation to ovulation
Ovaries in follicular phase
Endometrium in menstrual and proliferative phases
What is the follicular phase?
Development of the preantal follicle into the antal follicle into the Graafian follicle to prepare for ovulation
Hormone changes in the follicular phase?
During menstruation:
LH and FSH rise as oestrogen and progesterone decrease
FSH stimulates antral follicles to mature
Granulosa and theca cells make oestrogen, which rises exponentially over the first 12 days.
Oestrogen stimulate LH receptor growth in granulosa cells -> LH surge
LH surge -> one follicle (dominant follicle) released, others regress
What does the dominant follicle contain?
Primary oocyte - female gamete
Zona pellucida - glycoprotein layer surrounding oocyte (egg shell)
Granulosa cells- cuboidal cells surrounding oocyte, secrete oestrogens
Antrum - fluid filled cavity within granulosa cells
Basement membrane/lamina
Theca interna - androgen-secreting stromal cells
Theca externa - non-secretory stromal cells
Endometrial changes in the menstrual phase of 1st half of cycle?
Days 1-4
Loss of functional layer of endometrium which is ischaemic and necrotic
Sloughed tissue passes through vagina with blood from degenerating spiral arteries
Endometrial changes in proliferative phase of 1st half of cycle?
Rising oestrogen levels cause:
- proliferation of cells in the basal layer of the endometrium to form a new functional layer. Glands are formed but not yet active
- Secretion of a watery cervical mucous to facilitate sperm transport across the cervix.
What is ovulation?
Expulsion of the oocyte (with the zona pellucida and some granulosa cells) from the Graafian corpuscle into the peritoneal cavity. It is then wafted by cilia of the fimbriae into the fallopian tubes.
What hormonal changes cause ovulation?
The dominant antral follicle secreting loads of oestrogen so that negative feedback turns positive and there is a DRAMATIC LH SURGE (kweeen) (also FSH to a lesser extent).
LH surge causes follicle to complete first meiotic division and rupture.
2nd half of menstrual cycle
Day 14-28
Ovulation to menstruation
Length determined by lifespan of corpus luteum
Luteal stage of menstrual cycle?
Follicle becomes a corupus luteum
LH surge continues acting on the granulosa and theca cells in the empty follicle after ovulation, and cells change and become yellow to become lutein cells.
Ruptured follicle rump is called corpus luteum.
Over next 10 days, corpus luteum secretes high levels of PROGESTERONE and oestrogen, inhibiting LH and FSH release.
Then low LH causes corpus luteum to spontaneously involute and lose secretory ability, becoming the corpus albicans
What are the effects of the hormones secreted by the corpus luteum?
The corpus luteum secretes progesterone and oestrogen
These inhibit LH and FSH release from the pituitary gland
Falling LH can’t maintain the corpus luteum so undergoes involution
involution makes progesterone and oestrogen levels decrease dramatically, so loss of negative feedback to pituitary gland
FSH and LH secretion increases, causing ovarian follicle growth, thus starting the next cycle.
Endometrial changes during second phase of menstrual cycle?
Secretory phase
After ovulation, progesterone from corpus luteum stimulates differentiation in the endometrium
Nutrients are stored in the cells
Glands become tortuous in preparation for secretion
Endometrial glands start to secrete glycogen-rich secretion
In absence of progesterone following involution of corpus luteum, spiral arteries begin to coil and constrict, becoming ischaemic and necrotic. Blood leaks from capillaries and functional endometrium is shed -> menstruation, start of next cycle.
What happens if fertilisation occurs?
hCG released from implanting conceptus which binds to LH receptors on luteal cells so that the corpus luteum doesn’t involute.
Cervical changes during menstrual cycle?
Watery cervical mucous during ovulation to aid sperm swimming through cervix
Cervical os dilates around ovulation and menstruation
Oestrogen thins cervical mucous
Progesterone thickens cervical mucous
