Male Reproductive Endocrinology Flashcards
Axons of GnRH neurons
Extend into pituitary stalk and terminate on hypothalamic-hypophyseal portal system (capillary network) into which they release GnRH
Portal system
Blood from capillaries of one organ is transported to capillaries of another organ by a connecting vein or veins (returns to systemic circulation instead of pulmonary)
-allows small quantities of releasing hormones to act on anterior pituitary before dilution through general circulation
What pattern do GnRH hormones secrete GnRH?
An episodic pattern
How is GnRH captured?
Captured in primary portal plexus (PPP) and transferred to anterior pituitary through portal veins into secondary portal plexus (SPP)
Posterior pituitary
Does not have a portal system, axons of paraventricular nucleus (in hypothalamus) extend into posterior pituitary and hormones are captured directly by capillaries
Do GnRH hormones have receptors?
GnRH neurons lack receptors for testosterone and estradiol so negative feedback is indirect through presynaptic steroid-sensitive neurons
Release of GnRH
Pulsatile and waves of FSH, then LH, then testosterone follow
What does FSH act on?
Sertoli cells which release inhibin and provide negative feedback to anterior pituitary to directly suppress FSH release
What does LH act on?
Leydig cells which release testosterone that provides negative feedback to GnRH neurons in hypothalamus
Testosterone feedback/diffusion
Through blood, or diffuses across basement membrane to sertoli cells where it is converted to dihydrotestosterone (DHT) and/or E2 which feedbacks to hypothalamus reducing GnRH production