2.1 - hypothalamic-pituitary-gonadal axis

Question 1

what does successful reproduction require?

Answer 1

each sex to produce the right number of gametes at the right time
an effective apparatus to transfer sperm from male to female
creation of opportunities to use apparatus
sperm transport and fertilisation
support of the conceptus, embryo and foetus
successful parturition
support of the neonate

Question 2

what controls successful reproduction?

Answer 2

hormones

Question 3

where are major controlling structures located?

Answer 3

in the brain:
hypothalamus
pituitary gland (esp. anterior)
gonads

Question 4

in a pregnant female, important control is exerted by what?

Answer 4

the placenta

Question 5

how do structures interact with each other?

Answer 5

primarily by secretion of hormones

Question 6

where does the pituitary gland arise from embryologically?

Answer 6

Rathke’s pouch

Question 7

describe Rathke’s pouch

Answer 7

has a number of cell types secreting different hormones, most of which are involved in, or affected by, reproduction

Question 8

what type of hormone are most pituitary hormones?

Answer 8

trophic hormones

Question 9

what are trophic hormones?

Answer 9

hormones which act on other tissues to promote the secretion of further hormones

Question 10

what is each secretory cell type in the anterior pituitary controlled by? produced by? passes to?

Answer 10

controlled by releasing (sometimes inhibitory) hormones
produced by nerve cells in the hypothalamus
which pass to the anterior pituitary

Question 11

where do the hormones travel through from the nerve cells in hypothalamus (production) to anterior pituitary?

Answer 11

hypophyseal portal vessels

Question 12

what is the significance of the hypophyseal portal vessels?

Answer 12

carries blood from median eminence of the hypothalamus (hypothalamic releasing and inhibiting hormones are secreted) to the anterior pituitary (hormones act here, resulting in further secretion - trophic hormones)

Question 13

what are the main reproductive anterior pituitary hormones?

Answer 13

FSH and LH

Question 14

which cell type produces FSH and LH?

Answer 14

gonadotrophs

Question 15

which hormone controls gonadotrophs? (release of FSH + LH)

Answer 15

gonadotrophin releasing hormone (GnRH)

Question 16

where is GnRH released from? under the influence of?

Answer 16

several groups of hypothalamic neurones secrete GnRH

under influence both of other structures in brain and circulating hormones

Question 17

what does ‘environment via sensory input’ for release of GnRH mean?

Answer 17

under influence of other structures in brain (from sensory input)

Question 18

how is GnRH secreted?

Answer 18

in pulsatile fashion

a burst of secretion about once an hour

Question 19

what is secreted and from where in response to pulses of GnRH from hypothalamic neurones?

Answer 19

gonadotrophs in the anterior pituitary secrete gonadotrophins

Question 20

what happens in the absence of GnRH?

Answer 20

no LH or FSH is secreted (gonadotrophins)

secreted by gonadotrophs in ant. pit. by GnRH

Question 21

where do FSH and LH primarily act?

Answer 21

on the gonads (sex organs - testis, ovaries)

Question 22

what do FSH and LH do?

Answer 22

influence the production of gametes (ova / sperm) and to promote the secretion of gonadal steroids

Question 23

what are the gonadal steroids?

Answer 23

females: oestradiol and progesterone
males: testosterone

Question 24

what do gonadal steroids (oestradiol, testosterone) influence?

Answer 24

GnRH-secreting neurones (which secrete GnRH - direct feedback control)

Question 25

what is GnRH also affected by aside from gonadal steroids?

Answer 25

affected by GnRH’s connection to other parts of the brain influenced by factors e.g. body weight + changes in environment (environment via sensory input?)

Question 26

in males, how does testosterone affect secretion of GnRH?

Answer 26

testosterone reduces secretion of GnRH

Question 27

in females, how does INTERMEDIATE levels of oestrogen affect secretion of GnRH?

Answer 27

oestrogen (principally oestradiol) at intermediate concentration REDUCES secretion of GnRH

Question 28

in females, how does progesterone affect secretion of GnRH?

Answer 28

increase the INHIBITORY effect of oestrogen

enhance inhibition on GnRH with oestrogen - amplify

Question 29

in females, how does MODERATE levels of oestrogen affect secretion of GnRH?

Answer 29

REDUCES the amount of GnRH secreted per pulse, progesterone…?

Question 30

how does oestrogen ALONE at HIGH concentration affect GnRH?

Answer 30

promotes the release of GnRH - producing a surge

Question 31

how does progesterone affect oestrogen? further affecting?

Answer 31

prevents high levels of oestrogen producing a GnRH surge (releases LH + FSH)

Question 32

what are gonadotrophs secreted by? in response to?

Answer 32

secretes FSH + LH

in response to pulsatile secretion of GnRH by the hypothalamus

Question 33

the amount of FSH + LH secreted in response to each pulse of GnRH by hypothalamus is affected by what?

Answer 33

1. oestrogen at intermediate concentration which reduces it

2. oestrogen at high concentration which promotes it

Question 34

what reduces the amount of FSH secreted in response to GnRH?

Answer 34

inhibin

Question 35

what is inhibin?

Answer 35

a protein hormone produced by ovarian follicles (granulosa cells) and in seminiferous tubules (Sertoli cells)

Question 36

in the testis, LH stimulates the secretion of what? by?

Answer 36

LH stimulates secretion of testosterone by Leydig cells

Question 37

how much testosterone is secreted by Leydig cells each day? (stimulated by LH)

Answer 37

4-10mg