Reproductive hormones & HPG axis Flashcards
What is GnRH are where is it synthesised?
GnRH: gonadotrophin releasing hormone.
Synthesised in the hypothalamus
Where is prolactin synthesised?
Anterior pituitary gland
Where is oxytocin synthesised?
Posterior pituitary gland
Where are FSH and LH synthesised?
Pituitary gland
Where are the following hormones produced:
oestrogens, progesterone, inhibin, oxytocin, relaxin, some testosterone
Ovary
Name some hormones synthesised in the ovary
- Oestrogens
- Progesterone
- Inhibin
- Oxytocin
- Relaxin
- (some) testosterone
Where are the following hormones produced:
testosterone, inhibin, oestrogen, some other androgens
Testis
Name hormones synthesised in the uterus/placenta
- Prostaglandin F2alpha
- progesterone
- oestrogen
- eCG (equine chorionic gonadotrophin)
- = hCG = human version
How do gonadotrophins have an effect?
- e.g. FSH, LH, CG
- These are glycoprotein heterodimers with a shared alpha chain
- Biological activity is conferred by different beta chains
- They bind to G-protein linked receptors
- They then undergo a series of metabolic conversions and are excreted in urine (hence pregnancy testing works)
How do steroids have an effect?
- These are organic compounds produced by a series of enzymatic conversions
- These enzymes are present in some cells but not others (hence steroid synthesis can be controlled)
- These steroids at via androgen, progesterone and oestrogen receptors
Describe steroid metabolism
- Steroid is secreted by gonad
- Steroid enters blood, travels to target tissue
- Causes a change in target tissue
- Steroid in blood passes through liver
- Liver renders the steroid water-soluble
- Steroid re-enters the blood and enters kidney/bile
- Steroid is excreted in urine and/or faeces as glucuronide or sulphate
- This is how we track performance-enhancing steroids
- Also, we know about repro cycles of exotics species through faeces collection
What is the hypothalamo-hypophyseal-portal system?
- Portal system comprising of hypophyseal artery, primary portal plexus, hypophyseal portal veins and secondary portal plexus
- This system transports neurohormones e.g. GnRH from the hypothalamus to the anterior pituitary
- Here the neurohormones bind to receptors on endocrine cells in the anterior pituitary and regulate the secretion of other hormones (E.g. FSH, LH)
Describe the HPT axis
HPT axis: hypothalamic-pituitary-testicular axis
- Male hypothalamus releases frequent intermittent bursts of GnRH that occur day and night
- Pulse of GnRH –> pulse of LH –> pulse of testosterone
- The HPT axis forms a simple negative feedback loop
Leydig cells are _________________________ so respond to ________________
Leydig cells are LH receptor +ve so respond to testosterone
Sertoli cells are ______________________ so respond to _______ which ________________________
Sertoli cells are FSH receptor +ve so respond to inhibin which inhibits FSH
What is the corpus luteum?
- Transient endocrine gland formed from the remnants of the follicle
- Secretes progesterone to support pregnancy
What is a follicle on an ovary?
- A structure inside which the oocyte develops
- Multiple stages
- Secretes oestradiol
Describe GnRH production in the female vs male
- In females GnRH production in 2 groups of hypothalamic neurones
- Surge (preovulatory) centre
- Tonic centre
- In males, surge centre is inactive
- Pattern of production: there is basal secretion of small regular pulses. When stimulated by oestrogen, secretion increases and there is a large preovulatory surge before ovulation.
Describe the hypothalamic-pituitary-ovarian axis during the follicular phase
- For most of the follicular phase, low level of E2 has negative feedback
- Approaching ovulation there is a critical switch.
- Rising E2 switches to positive feedback, leading to LH surge and ovulation
- Inhibin inhibits FSH production in later stages of follicle growth especially
Describe the hypothalamic-pituitary-ovarian axis during the luteal phase
- Luteal progesterone (P4) exerts strong negative feedback on GnRH neurones
- This acts on mammary and uterine tissue
- Luteolysis will remove this negative feedback
- In the absence of fertilisation, the CL undergoes luteolysis
- This remove progesterone production (its own negative feedback)
