Physiology Flashcards
describe the hypothalamic-pituitary-gonadal axis (HPG)
- hypothalamus= GnRH
- anterior pituitary= LH (high frequency pulses) and FSH (low frequency pulses)
- ovaries= oestrogen and progesterone
- testes= testosterone
how is GnRH released from the hypothalamus?
pulsatile manner
what does GnRH act on in the anterior pituitary?
kisspeptin receptors
what does oestrogen concentration above a certain threshold cause?
increases GnRH pulses driving release of LH
what does an increase in progesterone cause?
reduces the frequency of GnRH pulses causing release of FSH
what does LH surge underpin?
ovulation
what is corpus luteum formed from?
FSH (lipids) which produces progesterone
what is hCG the same as in terms of hormones from the anterior pituitary?
LH
what happens if a developing embryo is not present?
levels of progesterone decrease
FSH (follicle stimulating hormone) causes what in males?
spermatogenesis
FSH causes what in females?
oogenesis (growth of ovarian follicles)
secretion of oestrogen
what happens if oestrogen reaches a certain concentration?
it no longer does negative feedback on the pituitary and hypothalamus but instead exerts positive feedback resulting in LH surge leading to ovulation and formation of the corpus luteum (progesterone)
role of progesterone
decreases pulsatility decreasing LH secretion
needed for expression of enzymes that break down the follicular wall leading to release of the oocyte
which anterior pituitary hormone acts on the theca cell?
LH
which anterior pituitary hormone acts on the granuloma cell?
FSH
role of LH (luteinising hormone) in males
secrete testosterone
role of LH in women
ovulation
progesterone production by corpus luteum
how early does the LH surge precede ovulation?
24-36 hours
when does progesterone peak?
following ovulation produced by the corpus luteum
what is the onset of puberty due to?
steady acceleration in GnRH pulsatility
two phases in the menstrual cycle
follicular/proliferative phase
luteal/secretory phase
what does an ovarian follicle consist of?
an oocyte surrounded by granuloma and theca cells
role of theca cells
produce androgens (can be overstimulated by hyperinsulinaemia leading to increased free androgens) that are converted to oestrogen by granuloma cells via the enzyme aromatase
when does the follicular phase of the menstrual cycle begin?
menstruation
hone does levels of GnRH increase?
when the follicular phase begins (menstruation) causing an increase in FSH and LH
LH role
causes theca cells to produce androgens
FSH role
stimulates granuloma cells to convert androgens to oestrogen
does oestrogen levels rise in the follicular phase
yes
what does FSH do during menstruation
causes follicles to mature until a dominant follicle emerges which is released from the ovary and the others undergo atresia
what happens during ovulation (day 12-14)
levels of oestrogen peak triggering a surge in LH which triggers ovulation and the oocyte to be released into the Fallopian tube
what happens during the luteal phase (day 15-28)
the follicle develops into the corpus luteum in response to LH which produces progesterone and activates secretion from the endometrium (thickens for implantation of the fertilised egg)
increase in basal body temperature and thickness of cervical mucus
what do high levels of progesterone do?
inhibit FSH/LH
describe menstruation
if fertilisation does not occur, the corpus luteum degenerates causing levels of progesterone and oestrogen to drop
the arterioles undergo vasospasm which leads to necrosis and bleeding
levels of FSH and LH increase allowing cycle to begin again.
what secretes oestrogen
ovaries
adrenal cortex
placenta in pregnancy
what does oestrogen do to the body?
increases thickness of the vaginal wall
regulates LH surge
reduces vaginal pH through increased lactic acid production
decreases viscosity of cervical mucus to facilitate sperm penetration
what does high oestrogen inhibit?
secretion of FSH and prolactin (negative feedback)
what secretes progesterone?
corpus luteum
role of progesterone
maintain early pregnancy until the placenta (HCG) takes over around 5-10 weeks
maintains thickness of endometrium
responsible for infertile thick mucus (infection control)
relaxes myometrium (risk of GI)
increases basal body temperature
inhibits secretion of LH
where does spermatogenesis take place?
seminiferous tubules
what do the seminiferous tubules contain?
Sertoli cells and maturing germ cels surrounded by peritubular hyoid cells
what do Sertoli cells do?
hold and respond to FSH
testosterone is released into the circulation and taken up by Sertoli cells
what does testosterone do?
maintain blood-testes barrier
spermatogenesis
releases mature spermatozoa from Sertoli cells by influencing peritubular myoid cells
what does LH do in males?
it stimulates testosterone production from Leydig cells (stimulation of enzymes required for synthesis of cholesterol to testosterone)
