Male Reproductive System I Flashcards
What are the major functions of the adult testes and where do they occur?
Spermatogenesis - seminiferous tubules
Steroid hormone synthesis - Leydig cells
Where is the majority of estradiol in males made?
Made in adipose tissue from testosterone
How is testosterone transported in the blood?
40-60% bound to sex hormone-binding globulin
Bound to albumin
2-3% free
What is bioavailable testosterone?
Both free and albumin-bound
Albumin-bound testosterone readily dissociates
What pathway is activated in Leydig cells to activated steroidogenesis via what hormone?
LH activates a G-protein coupled receptor
cAMP signaling pathway
What is the function of 5a-reductase and what is a clinical use for inhibitors of this enzyme?
Converts testosterone to dihydrostestosterone
5a-reductase inhibitors are useful in treating benign prostatic hyperplasia and prostate cancer
What is the action of aromatase?
Metabolizes testosterone to estradiol
Occurs in certain CNS nuclei, Leydig and Sertolie cells, and adipose tissue
What are excretory metabolites?
Biologically inactive forms of testosterone
Occurs primarily in the liver
What is the reason for the rise in testosterone production at puberty?
Rise in plasma LH concentrations
LH promotes androgen biosynthesis in Leydig cells
Describe the plasma testosterone profile during the male lifespan
Childhood - testosterone relatively low
Adolescent - increase in production during puberty
Older adult - age related decline at 4th or 5th decade
Age-related increase in SHBG also contributes to decrease in bioavailability
What is the mechanism of action for androgen hormones?
Receptor is a ligand-activated transcription factor
Binds testosterone and DHT, but has higher affinity for DHT
Does NOT bind to DNA until the hormone is bound
What are the actions of androgens in the fetus?
Promote differentiation of the male reproductive tract and external genitals
What are the actions of androgens at puberty?
Growth and maturation of reproductive structures
Development of male secondary sexual characteristics
What are specific androgen actions?
Growth of hair and sebaceous glands
Enlargement of larynx and thickening of vocal cords
Behavioral effects
Erythropoietin
Promote bone growth and formation (conversion to estradiol)
What is the blood-testis barrier?
Tight junctions between adjacent Sertoli cells that limits the exchange of material between interstitial fluid and the lumen of the seminiferous tubules
What substances do Sertoli cells secrete?
Androgen Binding Protein
Estradiol
Glycoproteins (AMH, Inhibins, Activins)
What is androgen binding protein and its function?
Secreted into the seminiferous tubules
Serve to keep testosterone levels elevated
Sertoli cell function can be assessed by measuring ABP production
What is Anti-mullerian hormone?
Secreted by Sertoli cells, promotes the regression of mullerian ducts in male embryo
What is Inhibin B?
Principal circulating inhibin in males
Plays important role in feedback regulation of FSH secretion
What is the feedback loop involving FSH and inhibin B?
FSH and testosterone stimulate inhibin B production
Inhibin B feeds back to the pituitary where it blocks GnRH-stimulated FSH release
What is the function of estradiol in sertoli cells?
Formed by the aromatization of androgens in response to FSH stimulation
What is spermatogenesis?
Process that produces a continuous supply of functional germ cells in males
Begins at puberty
Requires functional sertoli cells and intact HPG axis
What is the role of GnRH in spermatogenesis?
Delivery of GnRH in a pulsatile fashion is essential for appropriate FSH and LH
What are the actions of FSH on Sertoli cells?
Promotes proiferation and differentiation
Maintains function
Promotes proliferation and development of spermatogonia
Stimulates release of ABP and Inhibin
What pathway does FSH activate in Sertoli cells?
G-protein/cAMP pathway
What is the role of LH in spermatogenesis?
LH is required
Indirect effects via stimulation of testosterone from Leydig cells
How does testosterone act as a local regulator of spermatogenesis?
Activates genes in Sertoli cells
Works synergistically with FSH
What is decapacitation?
While in the epididymis, spermatozoa are coated with substances that block the acrosomal reaction
