HP Axis in Repro

Question 1

What does the HPA control overall in the body?

Answer 1

Thyroid
Adrenal
Gonadal
Somatic growth
Lactation
Milk secretion
Water metabolism

Question 2

What are the general characteristics of the control mechanisms of one of the HP axis?

Answer 2

Secretion in pulses tied to internal biological clock usually synchronised to external lights
Act on specific membrane receptors
Transduce signals via second messengers
Stimulate release of stored pituitary hormones
Stimulates hyperplasia and hypertrophy of target cells
Regulates its own receptor

Question 3

Describe the location of the pituitary gland?

Answer 3

Pituitary gland (hypophysis lies immediately beneath the hypothalamus in a depression of the base of the skull called the pituitary fossa, also called the Turkish saddle (Sella turcica). The gland sits at the base of the brain and is roughly the size and shape of a bean.

There are two parts to the pituitary gland – the pars nervosa (posterior) or neurohypophysis and the pars distalis (anterior) or Adenohypophysis

Question 4

Describe the anterior pituitary gland

Answer 4

Anterior pituitary (adenohypophysis)
Glandular tissue. Connected to the hypothalamus by the superior hypophyseal artery. The anterior pituitary in a mixture of hormone producing glandular cells. The anterior pituitary produces six peptide hormones: Prolactin, GH, TSH, ACTH, FSH and LH.

All these hormones are involved in reproduction but mostly LH and FSH.

Question 5

Describe the hormone involved in the HPG axis

Answer 5

Gonadotrophin releasing hormone is released by the hypothalamus and stimulates the anterior pituitary gland to secrete two hormones – LH and FSH.

Question 6

Describe GnRH, its release and where goes

Answer 6

GnRH is 10 amino acids long. Released in pulses every 1-3 hours. Intensity of GnRH stimulus is affected by the frequency of release and intensity of release. GnRH travels to pituitary in hypophysial portal system.

Question 7

Describe where LH and FSH are produced and what they act on?

Answer 7

Gonadotrophs in the anterior pituitary (make up 5-10%). They produce both LH and FSH.
After stimulation of the Gonads by FSH and LH they release Gonadal steroids (which are different in males and females). These steroids have a negative effect on release of GnRH and LH and FSH.

Question 8

How do FSH and LH exert their affects on the gonads?

Answer 8

FSH and LH act via G alpha S - adenylate cyclase on testicular sertoli cells and leydig cells. Stimulates sex hormone synthesis (steroidgenesis) and controls gamete production.

Question 9

What does LH cause to happen in the testis?

Answer 9

LH acts on leydig cells to produce testosterone. Testosterone reduces LH and FSH (by inhibiting GnRH release).

Question 10

What does FSH cause to happen in the testis?

Answer 10

FSH acts on receptors of sertoli cells of the seminiferous tubules. This results in them growing and secreting Spermatogenic substances. Sertoli cells also release ABP Androgen binding protein which binds testosterone, testosterone is needed for the process to take place and so ABP holds it in the seminiferous tubules. Inhibin is also released from the sertoli cells and reduces FSH secretion selectively as well as inhibiting GnRH and so regulates the production of sperm cells.

Question 11

How do Testosterone levels vary in males?

Answer 11

In the medium to long term testosterone levels are constant but it still has a circadian rhythm and is highest early morning. Effects of environmental stimuli both driven by the brain.

Question 12

How does FSH and LH exert their affects on the ovaries?

Answer 12

Act primarily on gonads via G alpha S - adenylate cyclase. They target ovarian granulose cells and theca interna cells. This stimulates sex hormone synthesis (steroidogenesis – oestrogen, progesterone and inhibin. This control gamete production folliculogenesis and ovulation.

Question 13

Which cells do FSH and LH act on in females?

Answer 13

FSH acts only on granulosa cells whilst LH acts on both theca interna cells and granulosa cells.

Question 14

What does FSH and LH stimulate in the antral phase of follicular development?

Answer 14

During the antral phase of follicular development LH stimulates theca cells to produce androgens. FSH stimulates granulosa cells to develop enzymes which convert those androgens into oestrogens.

Question 15

What does LH stimulate in the pre-ovulatory phase?

Answer 15

In the pre-ovulatory phase, LH receptors form in the outer granulosa cells and a surge of LH stimulates ovulation.

Question 16

What does LH stimulate after ovulation?

Answer 16

After ovulation LH stimulates the corpus luteum to secrete progesterone and oestrogen. As the corpus luteum grows more steroids are produced per LH.

Question 17

How does oestrogens action depend on its concentration?

Answer 17

Moderate amounts of oestrogen reduce GnRH secretion – negative feedback. High amounts of oestrogen alone promote GnRH secretion – positive feedback – LH surge (only in absence of progesterone).

Question 18

How does progesterone interact with the function of oestrogen?

Answer 18

Progesterone increases inhibitory effects of moderate oestrogen. Progesterone prevents positive feedback of high oestrogen (No LH surge). Oestrogen reduces GnRH per pulse, progesterone decreases the frequency of pulses.

Question 19

What is the role on inhibin in females?

Answer 19

Inhibin is released from granulosa cells of the corpus luteum. It inhibits secretion of FSH but only a small inhibitory effect on LH.