Reproductive Endocrinology in the Male

1
Reminders!

• The FINAL exam is Monday, March 15th, 430 700
  pm.
• 120 pts Covers the material since the last
  midterm.

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Reproductive Endocrinology in the Male

• Organization of the Testis
• Neuroendocrine Control of Testicular Function
• Mechanism of Action of LH FSH on the Testis
• Transport and Metabolism of Testosterone
• Actions of Testosterone
• Meiosis and Spermatogenesis
• Seminal Fluid Formation
• Male Sexual Function

3
Organization of the Testis

• The testis is composed of two main compartments
  the seminiferous tubule and the interstitial
  space.
• Contents of the seminiferous tubule
• - developing sperm cells
• - Sertoli cells
• Contents of the interstitial space
• - Leydig cells
• - peritubular cells
• - blood vessels

4
Functional Organization of the Testis

• Seminiferous Tubules
• - site of spermatogenesis, spermiogenesis
• - site of inhibin and androgen-binding protein
  production (Sertoli cells)
• - Sertoli cells also nurture developing sperm
  cells
• Interstitial space
• - site of testosterone production (Leydig cells)

5
Neuroendocrine Regulation of Testicular Functions


GnRH

T,
Testis
Spermatogenesis
inhibin
6
The Structure of LH and FSH

• LH and FSH are composed of two subunits.
• The ? subunit is shared by FSH, LH, TSH, and hCG
• Distinct ß subunits confer specific receptor
  binding and biological activity of these
  glycoproteins
• Each subunit is glycosylated, folded, and joined
  to its corresponding subunit

7
Regulation of LH and FSH Synthesis and Release by
GnRH

• GnRH stimulates both the synthesis and release of
  gonadotropins.
• GnRH is released from the hypothalamus in a
  pulsatile manner (once ever 30-45 minutes in
  adult men).
• Constant, high levels of GnRH will DECREASE
  gonadotropin release, by down-regulation of GnRH
  receptors.

8
Mechanism of GnRH Action on Pituitary Gonadotroph
Cells

• GnRH binds to a G protein-coupled receptor.
• Binding of GnRH to receptor results in activation
  of phospholipase C.
• Get increased production of IP3 and DAG.
• Increased IP3 leads to release of LH, FSH.
• DAG activates PKC, which increases synthesis of
  LH and FSH.

9
Mechanism of GnRH Action on Pituitary Gonadotroph
Cells
Calcium release
GnRH
R
synthesis
Protein Kinase C
LHb mRNA
10
Feedback Regulation of LH and FSH by Gonadal
Steroids

• Testosterone (and estradiol, from peripheral
  conversion) exerts negative feedback effects on
  LH and FSH synthesis and release in the male.
• - decreased pulsatile release of GnRH
• - decreased pituitary response to GnRH
  stimulation
• Males do NOT show positive feedback responses of
  LH to high levels of estradiol.

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Regulation of LH and FSH by GnRH Alone?

• To date, only one hypothalamic releasing factor
  controlling release of LH and FSH has been found
  (GnRH).
• There are cases when LH release is different from
  FSH release. How can this happen?
• Two possibilities
• - influence of GnRH pulse frequency on LH versus
  FSH levels
• - other factors which control FSH levels exist

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Influence of GnRH Pulse Frequency on LH versus
FSH Release

• GnRH is released from the hypothalamus as
  discrete pulses.
• High frequency pulses (once every 30 minutes)
  result in stimulation of LH release, with less
  release of FSH.
• Low frequency pulses (once every 60 minutes)
  result in stimulation of FSH release, with less
  release of LH.
• Thus, regulating the pulse frequency of GnRH
  release can give preferential release of LH or
  FSH.

13
Influence of Gonadal Peptides on Synthesis and
Release of FSH

• The testis and ovary produce two hormones which
  influence FSH, but not LH release.
• Inhibin preferentially inhibits the synthesis
  and release of FSH from the anterior pituitary
  (no effect on LH).
• Activin preferentially stimulates the synthesis
  and release of FSH from the pituitary (no effect
  on LH).
• Both act at the level of the pituitary (do not
  influence GnRH release).

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Structures of Inhibin and Activin
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Structures of Inhibin and Activin
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Regulation of Inhibin Production

• Inhibin production from Sertoli cells is
  stimulated by FSH.
• However, a real role of inhibin and activin from
  the testis on FSH release is not clear in adult
  males (blocking inhibin action doesnt increase
  FSH).
• There is evidence that inhibin and activin are
  also produced locally from the pituitary (and
  possibly hypothalamus), providing local control
  of FSH release.

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Mechanism of Action of LH and FSH on Testicular
Cells

• Leydig cells have receptors for LH, but not for
  FSH.
• Sertoli cells have receptors for FSH, but not for
  LH.
• Both the LH and FSH receptor are G
  protein-coupled receptors, with a fairly high
  degree of homology.
• Specificity of hormone binding is determined by
  the amino terminus extracellular domains.

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Binding Specificity Due To Extracellular Domains
of the LH and FSH Receptors
LH
FSH
19
Mutant LH/FSH Receptor

LH
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Mechanism of Action of Gonadotropins

• Gonadotropin receptors are coupled to Gs
• - increased cyclic AMP
• - leads to activation of protein kinase A
• - phosphorylation of cyclic AMP response
  element binding protein (CREB), a transcription
  factor

21
Actions of FSH on the Testis

• FSH acts on Sertoli cells
• - stimulate spermatogenesis
• - increase production of inhibin
• - increase production of androgen-binding
  protein, which binds to testosterone to increase
  local levels in the testis and epididymis
• FSH also augments effects of LH on testosterone
  production (possibly through peritubular cells?)

22
Actions of LH on the Testis

• Primary action stimulate testosterone production
• - binds to LH receptors on Leydig cells
• - increases conversion of cholesterol esters to
  free cholesterol
• - increases expression of steroidogenic acute
  regulatory protein (cholesterol transfer)
• - increases expression of P450 side chain
  cleavage to stimulate steroidogenesis
• - increases expression of 17a-hydroxylase

23
Transport and Metabolism of Testosterone

• Testosterone is bound to binding proteins in the
  plasma
• - 45 bound to TeBG (not biologically
  available)
• - 35 bound to albumin, 16 to other proteins
• - only 4 free in blood
• T can be metabolized to other hormones, or
  conjugates
• - peripheral conversion to E2 or DHT
• - conjugation in liver, excretion via kidney

24
Actions of Testosterone

• Testosterone acts on a wide variety of tissues
  during sexual differentiation and puberty, and in
  the adult (see last lecture).
• In the testis, a major action of testosterone is
  the maintenance of spermatogenesis, via
  stimulation of Sertoli cells.
• The actions of T are mediated through an
  intracellular androgen receptor, which acts as a
  transcription factor.

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Development of Gametes Meiosis

• Recall that mitosis is the production of two
  identical daughter cells through DNA replication
  and division of a cell.
• Meiosis production of four unique haploid cells
  from one diploid cell.
• Mechanism for producing haploid sex cells
  (spermatozoa and oocytes).

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Meiosis Two Stages

• There are two stages to meiosis meiosis I and
  meiosis II.
• Each stage has prophase, metaphase, anaphase, and
  telophase.
• Begin with a diploid (2n) stem cell
• - 23 pairs of chromosomes
• - one pair maternal
• - one pair paternal
• Duplicate the DNA copy each paternal and
  maternal chromosome (92 chromosomes)
• Now begin meiosis I

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Meiosis I

• Prophase I nuclear membrane dissolves, spindle
  apparatus forms, chromosomes duplicated and
  joined, forming a tetrad

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Meiosis I

• Metaphase I tetrads align at middle of cell
  (metaphase plate)
• Anaphase I chromosomes move to different sides
  of the cell (separation of maternal and paternal
  chromosomes, but not all to same cell)

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Meiosis I and Interkinesis

• Telophase I New nuclei form. Each nucleus has
  two identical sets of the 23 chromosomes (some
  paternal, some maternal, randomly assorted).
• Interkinesis the period between first meiotic
  division and second meiotic division.

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Meiosis II

• Prophase II nuclear membranes break down,
  spindle forms, each chromosome consists of two
  chromatids
• Metaphase II chromosomes align at metaphase
  plate
• Anaphase II chromatids separate into separate
  chromosomes
• Telophase II new nuclear membranes form,
  followed by cytokinesis

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Meiosis II
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Summary of Meiosis(for a single chromosome)
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How does Meiosis give Genetic Diversity?

• While the stem cell had all paternal and maternal
  chromosomes, each daughter cell receives only
  some maternal and some paternal chromosomes
  (random assortment)
• During prophase I, there is crossing over of DNA
  in tetrads, giving unique chromosome

34
Spermatogenesis

• The process by which spermatids are produced.
• Occurs in seminiferous tubule of the testis.
• In addition to developing sperm cells,
  seminiferous tubule contains Sertoli cells.
• Functions of Sertoli cells
• - nurture developing sperm
• - produce inhibin
• - produce androgen-binding protein

35
Spermatogenesis

• Stem cells (spermatogonia) constantly undergo
  mitosis, to produce more spermatogonia (2n)
• Spermatogonia can differentiate (mature) to form
  primary spermatocytes (still 2n)
• Primary spermatocytes enter meiosis.
• At the end of the first meiotic division, each
  primary spermatocyte forms two secondary
  spermatocytes (each has two identical copies of
  23 chromosomes)
• Each secondary spermatocyte undergoes second
  meiotic division to form two spermatids (1 n).

36
Spermiogenesis

• Spermatids are round cells.
• To be functional, spermatids must mature into
  spermatozoa, by the process of spermiogenesis.
• During spermiogenesis, sperm develop distinct
  head and tail regions.
• Head contains acrosomes (enzymes required to
  fertilize egg)
• Tail required for motility

spermiogenesis
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Maturation of Sperm

• Sperm leaving the testis and entering the
  epididymis are nonmotile and not capable of
  fertilization.
• Sperm acquire motility and final maturation as
  they travel through the epididymis.

38
Time Span of Sperm Development

• It takes about 70 days to develop from
  spermatogonia to spermatozoa.
• It takes another 14 days to travel through
  epididymis to the ejaculatory duct.
• Illness and exposure to toxic agents can have a
  delayed effect on quality of sperm produced.

39
Production of Semen

• Semen is composed of spermatozoa and the
  secretory products of the seminal vesicle (60)
  and prostate (30).
• Products of the seminal vesicle and prostate
  include
• - fructose (metabolized for energy by sperm)
• - prostaglandins (uterine contractions)
• - coagulating and decoagulating factors
• - antibacterial agents
• - pH adjusters (acids and bases)
• The bulbourethral gland secretes an alkaline
  mucus, neutralizes acidity in the urethra and
  provides lubrication

40
Capacitation of Sperm

• In order to fertilize an egg, the spermatozoa
  must undergo capacitation in the female
  reproductive tract following ejaculation.
• Capacitation results in
• - increased velocity of sperm movement
• - release of enzymes which allow sperm to reach
  the oocyte and penetrate it (acrosome reaction)
• - requires 2 to 6 hours
• (sperm may remain alive in the reproductive tract
  for days)

41
Male Sexual Response

• Three distinct phases have been identified
  arousal, emission, and ejaculation.
• Arousal erotic thoughts or physical stimulation
  result in activation of the parasympathetic
  system (via pelvic splanchnic nerves).
• - increased production of nitric oxide
• - nitric oxide activates soluble guanylate
  cyclase, resulting in increased production of
  cyclic GMP
• - cyclic GMP causes vasodilatation of blood
  vessels in the penis, resulting in increased
  blood flow and erection

42
Male Sexual Response

• Emission Sympathetic stimulation causes
  peristaltic contractions of the ampulla of the
  ductus deferens, the seminal vesicles, and the
  prostate. Thus, spermatozoa and seminal fluids
  enter the prostatic urethra.
• At the same time, the internal urethral sphincter
  closes off the bladder to prevent retrograde
  ejaculation

43
Male Sexual Response

• Ejaculation Contractions of two skeletal
  muscles
• - ischiocavernosus contractions against the
  erectile tissue of the penis
• - bulbocavernosus contractions push semen from
  base of penis to urethral opening
• (note that these are skeletal muscles under
  sympathetic control)

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Role of Testosterone in Male Sexual Response

• Testosterone increases libido (sexual thoughts
  and desires).
• However, testosterone plays little role in
  capacity of men to have sexual intercourse.
• Impotence (inability to achieve or maintain
  erection) can be due to physical causes
  (circulatory problems, drugs, alcohol, trauma,
  illness) or psychological state (depression,
  anxiety, stress).
• Viagra inhibitor of cyclic GMP-specific
  phosphodiesterase (increases cyclic GMP levels).

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Mechanism of Action of Viagra
arousal parasympathetic stimulation nitric
oxide cyclic GMP increased penile blood flow
Viagra
PDE5
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NEXT (AND LAST) LECTURE...

• Reproductive Endocrinology of the Female