1. + Hormones of the Male Reproductive System Robin Gottschalk BIO 260 Professor Tonini December 9, 2015

2. + HormoneSourceAction TestosteroneMostly testes, but some is produced in the adrenal glands -Stimulates formation/regulation of male reproductive ducts, glands, and genitalia -Responsible for libido in males, promotes aggressiveness -At puberty it stimulates: spermatogenesis, growth and maturation of internal and external genitalia, secondary sex characteristics Gonadotropin-releasing Hormone (GnRH) HypothalamusPrompts gonadotropin release Gonadotropins (FSH and LH)Anterior Pituitary-FSH stimulates production of sperm -LH promotes production of testosterone InhibinTestesServes as “barometer” of the normalcy of spermatogenesis

3. + The Hypothalamic-Pituitary-Gonadal (HPG) Axis Established at puberty Regulates the testosterone and sperm production Hormonal interactions between the hypothalamus, anterior pituitary gland, and gonads The hormones involved include GnRH, FSH, LH, Testosterone, and Inhibin

4. + Sequence of HPG Axis Events 1. The hypothalamus releases gonadotropin-releasing hormone (GnRH)

5. + Sequence of HPG Axis Events 2. GnRH binds to pituitary cells (gonadotropic cells), prompting them to secrete FSH and LH

6. + Sequence of HPG Axis Events 3. FSH stimulates the sustenocytes to release androgen-binding protein (ABP). -ABP makes the spermatogenic cells receptive to testosterone by keeping the concentration of nearby testosterone high

7. + Sequence of HPG Axis Events 4. LH binds to the interstitial endocrine cells surrounding the seminiferous tubules, leading to their release of testosterone. -Locally rising testosterone levels serve as the final trigger for spermatogenesis.

8. + Sequence of HPG Axis Events 5. Testosterone entering the bloodstream exerts a number of effects at other body sites.

9. + Sequence of HPG Axis Events 6. Feedback inhibition on the hypothalamus and anterior pituitary -Rising levels of testosterone will feed back to inhibit the release of GnRH and the Gonadotropins (both FSH and LH) -Inhibin releases when sperm count is high, stops the release of FSH and GnRH (until sperm count falls below 20million/mL)

10. + HPG Axis balance: The 3 interacting sets of hormones (Overview) -GnRH indirectly stimulates the testes via its effects on FSH and LH release -FSH and LH directly stimulate the testes -Testosterone and Inhibin exert negative feedback controls on the hypothalamus and anterior pituitary