HPG Axis Flashcards
Name the main hormones of reproduction released from the HPG axis
Hypothalus: GnRH
Pituitary: LH, FSH, prolactin oxytocin
Gonads: Inhibin ,(Oestrogen, Progesterone), testosterone
Describe the anatomy of the pituitary gland
Lies in the cavity of the sphenoid bone
Connects to the hypothalamus via the pituitary stalk/infindibulum.
Encapsulated by dura mater
Communication between the pituitary and hypothalus is via neural and hormonal signalling.
Brieftly describe the HPG axis in males
Initiation of spermatogenesis at puberty requires LH and FSH.
GnRH induces the anterior pituitary to produce LH anf FSH.
FSH binds to receptors on Sertoli cells which induces the expression of androgen receptors, which increases the tissue responsiveness to andgrogens. Also stimulates inhibin.
LH binds to receptors on Leydig cells and induces tehm to increase the production of testosterone. Testosterone translocates to the tubules and binds to androgen receptors on Sertoli cells.
Inhibin has a negative feedback effect on pituitary release of FSH. Testosterone acts at the hypothalamus and pituitary to inhibit LH
Describe the endocrine changes that occur at menopause
Oocyte numbers decline with age
Concentrations of oestrogen and progesterone fall, and the loss of negative feedback causes a rise in GnRH, FSH and LH
Highest rise in FSH due to loss of inhibin from follicles.
What are the cell types in the anterior pituitary gland and what hormones do they produce?
Corticotrophs: ACTH, b-lipotrophin, MSH
Thyrotrophs: TSH
Gonadotrophs: LH, FSH
Somatotrophs: GH
Lactotrophs: PRL
Describe the release of GnRH
Synthesised by neurons in the hypothalamus as a prepropeptide which is cleaved to GnRH and secreted into the hypophyseal vessels to the anterior pituitary.
Release is pulsatile: every 1-2 hours.
Increased pulsatile activity or continuous release acts as an analogue and blocks the gonadotrophs.
Stimulates gonadotrophs to produce FSH and LH from the anterior pituitary. Stimulates hyperplasia and hypertrophy of target cells.
Describe the control of GnRH secretion by the hypothalamus
Before ovulation high levels of oestrogen act on the hypothalamus to release high frequency, low amplitude pulses of GnRH. This increases sensitvity of gonadotrophs to GnRH and stimulates LH and FSH release.
Continous/non-pulsatile release of GnRH has an inhibitory effect and reduces secretion of gonadotrophins.
Describe the effects of testosterone in the male
Some converted to di-hydrotestosterone by Sertoli cells (more potent)
Promotes spermatogenesis, secondary sexual characteristics
Affects behaviour by acting on the hypothalamus.
Enhances long bone growth, muscle protein synthesis, male pattern hair,
What are the action of gonadal steroids in males?
GnRH released from the hypothalamus stimulates FSH and LH from the anterior pituitary
LH receptors in Leydig cells stimulate testosterone biosynthesis and secretion. Promotes spermatogenesis.
FSH binds to Sertoli cells and enhances sensitivity to testosterone
Inhibin produced by Sertoli cells in response to the gonadotropin exerts negative feedback inhibition on FSH release from pituitary gonadotropes.
Testosterone from Leydig cells exerts negative feedback inhibition on LH release from pituitary gonadotropes.
Describe the action of gonadotrophins on the ovaries
Follicle stimulating hormone (FSH)
Stimulates development of follicles
Luteinising hormone (LH)
Stimulates ovulation
and development of
the corpus luteum
FSH + LH stimulates secretion of oestradiol
LH stimulates secretion of progesterone
Action of LH in males
binds to LH receptors on Leydig cells
induces them to increase production of testosterone
Testosterone translocates to the tubules and binds to the Androgen Receptor on Sertoli Cells
Action of FSH in males
Binds to the FSH receptor on Sertoli cells
This induces the expression of Androgen Receptor increasing responsiveness to androgens
Also stimulates inhibin
High levels of inhibin exert negative feedback on FSH
