The Gonads 2 (11)

Question 1

what is androstenedione

Answer 1

a weak precursor molecule to androgens. is produced in both gonads and adrenals

Question 2

what happens if testosterone undergoes reduction using 5a- reducatase

Answer 2

it forms a more potent androgen called dihydrotestosterone (DHT) that still works on the same androgen receptors

Question 3

what happens if testosterone undergoes aromatisation using aromatase

Answer 3

it forms oestrogens

Question 4

where is testosterone and DHT produced

Answer 4

prostate, seminiferous tubules, seminal vesicles, skin, brain and adenohypophysis

Question 5

where is 17-b-oestradiol (main circulating oestrogen) produced

Answer 5

adrenals, sertoli cells, liver, skin and brain

Question 6

how are testosterone and DHT transported in the blood

Answer 6

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

Question 7

how are testosterone and DHT transported in the seminiferous fluid

Answer 7

bound to androgen binding globulin (ABG)

Question 8

What are the principal actions of androgens in foetus’

Answer 8

Development of male internal and external genitalia. General growth (acting with other hormones, stimulates e.g. protein anabolism). Behavioural effects during development

Question 9

what are the principal actions of androgens in adults

Answer 9

spermatogenesis. Growth and development of male genitalia, secondary accessory sex glands, secondary sex characteristics. Protein anabolism. Pubertal growth spurt. Behavioural effects.

Question 10

define an oestrogen

Answer 10

any substance (natural or synthetic) which induces mitosis in the endometrium

Question 11

what is the main hormone produced in the menstrual cycle

Answer 11

17b-oestradiol

Question 12

what is the precursor to 17b-oestradiol

Answer 12

oestrone

Question 13

what is the main oestrogen produced in pregnancy

Answer 13

oestriol

Question 14

What are the principle actions of oestrogens (reproductive)

Answer 14

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.

Question 15

What are the principle actions of oestrogens (other)

Answer 15

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

Question 16

define progestogens

Answer 16

any substance (natural or synthetic) which induces secretory changes in the endometrium

Question 17

examples of progestogens

Answer 17

Progesterone and

17a-hydroxyprogesterone

Question 18

what are the principal actions of progestogens

Answer 18

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

Question 19

describe the hypothalamo-pituitary-testicular axis

Answer 19

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.

Question 20

describe the negative feedback loop in the hypothalamo-pituitary-testicular axis

Answer 20

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

Question 21

summary of androgen production in leydig cells

Answer 21

stimulated by GnRH/LH system. is reduced by -ve feedback by testosterone

Question 22

summary of spermatogenesis in sertoli cells

Answer 22

stimulated by GnRH/FSH system. requires GnRH / LH / testosterone system for complete spermatogenesis.
limited by inhibin negative feedback (direct and indirect)

Question 23

describe the hypothalamo-pituitary ovarian axis

Answer 23

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

Question 24

what are the phases of the menstrual cycle

Answer 24

early follicular. early mid follicular. mid follicular. late follicular. luteal

Question 25

what is the early follicular phase

Answer 25

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.

Question 26

what is the early mid follicular phase

Answer 26

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

Question 27

what is the mid follicular phase

Answer 27

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.

Question 28

what is the late follicular phase

Answer 28

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.

Question 29

what is the luteal phase

Answer 29

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.

Question 30

what is the local (autocrine) positive feedback loop during the early mid follicular phase

Answer 30

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.

Question 31

what do high levels of FSH and LH cause

Answer 31

a rise in oestrogen levels

Question 32

how does the negative feedback loop switch to positive feedback loop (oestrogen in hypothalamo-pituitary ovarian axis)

Answer 32

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.

Question 33

what happens if fertilisation occurs

Answer 33

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

Question 34

what happens if fertilisation does not occur

Answer 34

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).

Question 35

define amenorrhoea

Answer 35

absence of menstrual cycles. primary if a women never had a single period. secondary if a women has had periods which then stop

Question 36

define oligomenorrhoea

Answer 36

infrequent menstrual cycles. could be due to absence of LH surge

Question 37

define infertility

Answer 37

inability to get pregnant or inability to impregnate.

Question 38

causes of infertility

Answer 38

pituitary failure, prolactinoma (causes excess prolactin), testicular failure, ovarian failure, polycystic ovarian syndrome