The Gonads 2 (11) Flashcards
what is androstenedione
a weak precursor molecule to androgens. is produced in both gonads and adrenals
what happens if testosterone undergoes reduction using 5a- reducatase
it forms a more potent androgen called dihydrotestosterone (DHT) that still works on the same androgen receptors
what happens if testosterone undergoes aromatisation using aromatase
it forms oestrogens
where is testosterone and DHT produced
prostate, seminiferous tubules, seminal vesicles, skin, brain and adenohypophysis
where is 17-b-oestradiol (main circulating oestrogen) produced
adrenals, sertoli cells, liver, skin and brain
how are testosterone and DHT transported in the blood
They are bound to specific plasma proteins as the androgens are lipophilic therefore need to be bound to prevent excess effects. 60% are bound to sex hormone binding globulin (SHBG). 38% bound to albumin - 2& are free/bioactive
how are testosterone and DHT transported in the seminiferous fluid
bound to androgen binding globulin (ABG)
What are the principal actions of androgens in foetus’
Development of male internal and external genitalia. General growth (acting with other hormones, stimulates e.g. protein anabolism). Behavioural effects during development
what are the principal actions of androgens in adults
spermatogenesis. Growth and development of male genitalia, secondary accessory sex glands, secondary sex characteristics. Protein anabolism. Pubertal growth spurt. Behavioural effects.
define an oestrogen
any substance (natural or synthetic) which induces mitosis in the endometrium
what is the main hormone produced in the menstrual cycle
17b-oestradiol
what is the precursor to 17b-oestradiol
oestrone
what is the main oestrogen produced in pregnancy
oestriol
What are the principle actions of oestrogens (reproductive)
Stimulates growth and proliferation (mitosis) of the endometrium. Final maturation of follicle during follicular phase of menstrual cycle. Induces LH surge resulting in ovulation. Effects on vagina + cervix. Stimulates growth of ductile system of breast. Decreases sebaceous gland secretion.
What are the principle actions of oestrogens (other)
Increases plasma protein synthesis (hepatic effect). Increases salt and water reabsorption. Influences release of other hormones (e.g. prolactin, thyrotrophin) Feedback regulation. Behavioural influences Stimulates osteoblasts. Metabolic actions e.g. on lipids
define progestogens
any substance (natural or synthetic) which induces secretory changes in the endometrium
examples of progestogens
Progesterone and
17a-hydroxyprogesterone
what are the principal actions of progestogens
stimulates secretory activity in endometrium and cervix. stimulates growth of alveolar system in breast. decreases renal NaCl reabsorption due to competitive inhibition of aldosterone. associated with an increase in body temperature. negative feedback regulation on hypothalamic GnRH
describe the hypothalamo-pituitary-testicular axis
GnRH (gonadotrophin releasing hormone) is released in pulsatile patterns from the hypothalamus. GnRH passes down to the gonadotrophs in the adenohypophysis which produce LH and FSH. These enter the general circulation and work on the testes. LH stimulates leydig cells to produce testosterone which stimulates virilisation and spermatogenesis. FSH stimulates sertoli cells to produce inhibin which also stimulates spermatogenesis.
describe the negative feedback loop in the hypothalamo-pituitary-testicular axis
testosterone directly inhibits FSH and LH release from the adenohypophysis. it also indirectly inhibits the pulse generator releasing GnRN from the hypothalamus, which in turn also reduces gonadotrophin release. Inhibin has the same effect
summary of androgen production in leydig cells
stimulated by GnRH/LH system. is reduced by -ve feedback by testosterone
summary of spermatogenesis in sertoli cells
stimulated by GnRH/FSH system. requires GnRH / LH / testosterone system for complete spermatogenesis.
limited by inhibin negative feedback (direct and indirect)
describe the hypothalamo-pituitary ovarian axis
same as testicular axis except FSH and LH act on the ovaries. There is a direct negative feedback on the adenohypophysis and an indirect negative feedback on the hypothalamus. The hormones have different effects during the phases of the menstrual cycle
what are the phases of the menstrual cycle
early follicular. early mid follicular. mid follicular. late follicular. luteal
what is the early follicular phase
Beginning of the menstral cycle. Oestrogen and progesterone levels are low so there is little -ve feedback so there is an increase in LH, FSH and GnRH. LH and FSH start stimulating the development of some follicles in the ovaries. There is a little rise in 17a-hydroxyprogesterone during ovulation.
what is the early mid follicular phase
There is no further increase in FSH or LH.
Oestrogen levels are beginning to rise dramatically (increases in the blood and in the ovaries). Progesterone does not change
what is the mid follicular phase
Selective negative feedback loop by rising oestrogen and inhibin on the GnRH/FSH system results in atresia of all the follicles that are still FSH dependent. the largest follicle doesn’t need FSH to develop and proliferate continues to grow and produce large amounts of 17b- oestradiol. This is the graafian follicle. LH is still produced.
what is the late follicular phase
an LH surge is triggered by very extremely high circulating 17b-oestradiol from graafian follicle. The LH surge is sufficient high enough to overcome the FSH -ve feedback so you get a lesser FSH surge.
what is the luteal phase
after ovulation, the cells of the follicle collapse to form the corpus luteum, which still has cells with FSH and LH receptors. as O and P levels fall after ovulation, FSH and LH are initially released. The corpus luteum is stimulated to produce more O and P. O and P levels increase which exerts a -ve feedback effect on the hypo-pitu axis. high levels of P stop any chance of positive feedback so O levels are not as high as follicular phase. FSH and LH levels decrease as they get inhibited by the increased -ve feedback.The corpus luteum is no longer stimulated to produce O and P. So O and P levels begin to fall. As these levels fall, the -ve feedback on the hypo-pitu axis is reduced so LH and FSH levels start to rise again so they can reach a critical level at the start of the cycle again.
what is the local (autocrine) positive feedback loop during the early mid follicular phase
oestrogen produced by the granulosa cells bind to the oestrogen receptor on the same granulosa cell and stimulate the aromatase enzyme via a 2nd messenger system. rising levels of plasma oestrogen stimulates granulosa cell growth. more granulosa cells means more ability to synthesise oestrogen.
what do high levels of FSH and LH cause
a rise in oestrogen levels
how does the negative feedback loop switch to positive feedback loop (oestrogen in hypothalamo-pituitary ovarian axis)
extremely high levels of 17b-oestradiol from the graafian follicle in the absence of progesterone results in the positive feedback on GnRH/LH secretion. This causes an LH surge.
what happens if fertilisation occurs
If fertilisation occurs, a lot of oestrogen and progesterone is needed so the placenta, at an early stage, will start producing hCG which mimics the effects of LH
what happens if fertilisation does not occur
progesterone, oestradiol and inhibin exert a -ve feedback on LH and FSH release leading to luteolysis (breakdown of the corpus luteum) and menstruation (shedding of the endometrial lining).
define amenorrhoea
absence of menstrual cycles. primary if a women never had a single period. secondary if a women has had periods which then stop
define oligomenorrhoea
infrequent menstrual cycles. could be due to absence of LH surge
define infertility
inability to get pregnant or inability to impregnate.
causes of infertility
pituitary failure, prolactinoma (causes excess prolactin), testicular failure, ovarian failure, polycystic ovarian syndrome
