Testosterone-the primary male sex hormone. Androgen

1. TestosteroneITS ROLE, ITS SYNTHESIS & SECRETION

2. Introduction:
 Secreted by Leydig (interstitial) cells which are situated in the spaces b/w adjacent seminiferous
tubules.
 This steroid hormone is synthesized from cholesterol in the testes & is the principal androgen.
 It is lipid-soluble & readily diffuses out of Leydig cells into interstitial fluid & then into blood.
 Only differs from Estradiol in the number of double bonds & the types of functional groups
attached to Ring A.
 Testosterone production by testes decreases with age, so effects are not usually apparent until
old age.
 An Androgen is a hormone that promotes the development of masculine characteristics.
 Leydig cells begin to secrete testosterone during the 8th week.
 DHT stimulates development of the urethra, prostate, & external genitals.

3. Secretion:
 LH (from ant. pituitary gland) stimulates Leydig cells to secrete testosterone.
 Via negative feedback, testosterone suppresses secretion of LH by anterior pituitary gonadotrophs
& suppresses secretion of GnRH by hypothalamic neurosecretory cells.
 In some target tissues (external genitals & prostate), enzyme 5-alpha-reductase converts
testosterone into another androgen called Dihydrotestosterone (DHT).
 ABP binds to testosterone, keeping its concentration high in the seminiferous tubules.

5. ROLE OF TESTOSTERONE:
 Stimulates descent of the testes before birth.
 Regulates production of sperm.
 Stimulates protein synthesis. Is an anabolic steroid.
 Stimulates development and maintenance of male secondary sex characteristics, such as beard
growth & deepening of the voice.
 Stimulates synthesis of erythropoietin (EPO), the hormone that in turn stimulates the production of
RBCs. Thus, contributes to higher hematocrit in males.
 Also increases formation of 2,3-biphosphoglycerate (BPG) which decreases affinity of Hemoglobin
for O2 & thus helps unload O2 from Hb.
 Promotes a man’s libido.
 Acts synergistically w/FSH on Sertoli cells to stimulate secretion of androgen-binding protein (ABP)
into the lumen of the seminiferous tubules.
 Stimulates (with FSH) the final steps of spermatogenesis. Promotes production of spermatozoa.
 Stimulates secretion from the male accessory sex glands.

6. Additional:
 At about age 55, a decline in testosterone synthesis leads to reduced muscle strength, fewer viable
sperm, & decreased sexual desire.

7. Sertoli cells:
 A.k.a Sustentacular cell. NB: Spermatogenic cells produces sperm
 A supporting cell in the seminiferous tubules that secretes fluid for supporting nutrients to sperm
and the hormone inhibin.
 Removes excess cytoplasm from spermatogenic cells.
 Mediates the effects of FSH & testosterone on spermatogenesis.

8. Leydig cells:
 A.k.a interstitial cell of Leydig or interstitial endocrinocyte.
 A type of cell that secretes testosterone in the presence of LH; located in the connective tissue
between seminiferous tubules in a mature testis.

9. The End:
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