ENDOCRINE CONTROL of MALE REPRODUCTION
January 6, 2001
8:45 – 9:45 AM
Overall Objectives
Today I will discuss the functions of the adult testis which are the secretion of androgens
and production of spermatozoa. I will also discuss the endocrine and paracrine factors that
regulate sperm production.
Male infertility
Human semen abnormalities:
Infertility affects 10-15% of couples with equal contributions from both partners.
Azoospermia, oligospermia, asthenozoospermia, and/or teratozoospermia are found in over 90%
of all infertile males. Azoospermia, oligospermia, are often due to Y chromosome microdeletions
but this still accounts for less than 5% of male infertility in total suggesting that multiple
mechanisms are responsible for infertility. Potential treatments include in vitro fertilization (IVF)
or intracytoplasmic sperm injection (ICSI). For ICSI, immature germ cells are isolated from the
testis and are injected into the egg. The cost for assisted reproduction procedures is approximately
$8,000 per ovulation cycle in the U.S. Often these procedures result in multiple births and
additional unintended costs. IVF births are surprisingly frequent and account for 4% of births in
Iceland.
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Adult Testis Anatomy and Function Overview
In man and most other mammals the testis is located extra-abdominally in the scrotum. The
marsupial testes shown here is the extreme example of locating the testis away from the body. The
evolutionary advantages of having the testes outside of the body cavity are not clear except that it
appears to be critical for developing sperm to be maintained at lower than body temperature. The
human testis in the scrotal sac is maintained 2°C lower than body temperature (35°C vrs 37°C).
Exposing the testis to above normal temperatures is detrimental to sperm production. For example,
when the testis does not descend correctly and are maintained in the abdomen, a situation called
cryptorchidism, fertility is compromised. Cryptorchidism occurs in 0.7-0.8% of adult men but
10% of testicular cancers arise in undecended testes. Testicular cancer accounts for 1% of all male
cancer deaths. It is the second most common malignancy (after leukemia) in men 20-34.
The next slide shows a testis isolated from a rhesus monkey. The testis is enclosed within
a tough fiberous capsule called the tunica albuginea. Just under the tunica runs a characteristic
spiral artery.
Upon opening the tunica, it is found that the testis is composed largely of tube-like
structures. These are the seminiferous tubules. Between the tubules is the interstitial tissue. Each
of these structures represents a functional unit.
The network of seminiferous tubules used for the production and transport of sperm is the
spermatogenic unit. 2) The system of interstitial cells that produce the steroid hormone called
testosterone is the steroidogenic unit. The anatomic separation of the steroidogenic cells from the
developing germ cells is in contrast to the situation in the ovarian follicle in which steroidogenesis
and development of the oocyte occur in the same structure (the follicle).
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Hormonal regulation of the testis
Testosterone Synthesis
The interstitial tissue consists of loose connective tissue, some blood vessels, extensive
lymph spaces and the highly specialized Leydig cells (432 x 106 per adult male). The Leydig cells
produce the steroid hormone, testosterone. This cell is functionally analogous to the theca cell in
the ovary. A simplified version of the testosterone synthesis pathway is illustrated in this figure.
Testosterone is originally derived from cholesterol. In the Leydig cell, after hydrolysis of
cholesterol esters, cholesterol moves to the mitochondria where p450SSC converts cholesterol to
pregnenolone. Pregnenolone is then converted to testosterone by one of 2 pathways in the
endoplasmic reticulum.
The adult human testis produces about 6-7 mg of testosterone daily which rapidly diffuses
into the seminiferous tubules or into the systemic circulation. In the blood, 2% of testosterone is
free, 68% is bound by albumin and 30% is bound 1000 times more tightly by the 94 Kd TeBG
(testosterone-estrogen binding globulin) protein (also called SHBG –sex hormone binding
globulin). The bioavailable testosterone includes the free fraction and the testosterone that is
easily released from albumin bound fractions. Changes in TeBG levels can dramatically alter
levels of bioavailable testosterone by sequestering more or less of the hormone. Testosterone
levels in peripheral blood are 4 ng/ml but testosterone levels in the testis are 25-125-fold higher
(100-600 ng/ml)
Testosterone levels are high during much of fetal development, peak transiently
again 2-3 months after birth and then rise again during puberty. Testosterone levels remain high
until at least 70 years of age.
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Testosterone Action
This table shows the major physiological effects of testosterone in the human male.
The first impact of testosterone is seen early in fetal development. When the fetal testis
switches on, testosterone is produced and this testosterone is responsible for masculinizing both
the internal and external genitalia. Starting during puberty androgens are responsible for the well
known male characteristics that are listed.
How does androgen work?
Testosterone can exert its action as an androgen in target tissues in two ways. In the first
method, testosterone diffuses passively into the cell and combines with the androgen receptor.
When testosterone binds to the receptor, the testosterone and receptor move together to the nucleus
where they bind to the promoter region of genes and activate gene expression, initiate protein
synthesis and induce androgenic effects. This method of action is seen predominately in muscle
and the testis.
For the second mechanism, testosterone enters the cell in the same manner but is converted
to dihydrotestosterone (DHT) by 5 alpha reductase. DHT has a higher affinity (almost 10-fold) for
the androgen receptor and amplifies the action of testosterone. DHT is important in the external
genitalia, reproductive tract and in skin. Individuals having genetic defects or deficiencies in 5
alpha reductase have female external genitalia at birth. Later, their beard growth is reduced and
they show less of a predisposition to skin conditions such as acne which is androgen dependent.
However, they have nearly normal muscle tone and development.
The dramatic importance of the androgen receptor and testosterone can be visualized in
individuals that do not have androgen receptors or express inactive androgen receptors due to
mutations in the androgen receptor gene on the X chromosome. These mutations cause the
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individuals to be androgen insensitive, resulting in a condition called testicular feminization (tfm)
or androgen insensitivity syndrome (AIS).
The individual shown here is genetically male and gonadally male. The testes are;
however, in the interabdominal cavity. The testes are producing normal levels of testosterone (6-7
mg/day). However, due to a mutation in the androgen receptor gene no androgen receptors are
produced or the receptors that are produced have low affinity for testosterone or for androgen
response elements on the DNA. As a consequence, the biological actions of testosterone are not
manifest, either during fetal development or at the time of puberty. This individual clearly has a
female phenotype. This genetic condition clearly demonstrates the vital importance of the
androgen receptor for eliciting the biological activity of testosterone.
There is a third non-AR-mediated, mechanism whereby testosterone exerts its actions.
That is by the conversion of testosterone into the female sex hormone estrodial. This process is
called aromatization and is catalyzed by P450 aromatase enzymes. The estrodial produced then
interacts with the estrogen receptor to elicit physiological effects.
Why would males need estrogen?
New studies have identified males having mutations in aromatase or the estrogen receptor.
In these cases the patients undergo puberty but there is no pubertal growth spurt. Instead there is
constant linear growth that continues into adulthood without epiphysial fusion. Estrogen therapy
led to rapid skeletal maturation and epiphyseal fusion within 6-9 months for the aromatase
deficient men but not the estrogen receptor deficient men. Therefore, without the aromatization of
testosterone into estrogen or in the absence of the estrogen receptor, growth is altered. These
studies suggest that aromatization of testosterone into estrogen is important for the regulation of
linear growth and bone formation in males. It is possible that estrogen is required to activate VEGF
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which is an essential signal for the ossification process. In males testosterone aromatization to
estrogen is required throughout life to maintain bone mass. (For more information see the
Grumbach and Auchus review article in Dec. 1999 JCEM)
Regulation of Leydig cell function and testosterone secretion
Testosterone secretion from Leydig cells is regulated by the pituitary gland. Specialized
cells called gonadotrophs in the pituitary secrete two hormones that regulate the testes. These
hormones are luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These are the
identical hormones discussed by Dr. Zeleznik that are essential for ovarian function. These
hormones were named gonadotropins because in their absence the gonads (ovary and testis) do not
grow. In addition, male patients with clinically low levels of gonadotropins (hypogonadotropism)
have low levels of testosterone secretion.
The gonadotropin that drives the secretion of testosterone by the Leydig cell is LH. LH
regulates Leydig cell function by binding to G protein-coupled receptors on the surface of Leydig
cells and activating an intracellular signaling pathway. The binding of the hormones to the
receptors causes an increase in the activity of the adenylate cyclase enzyme resulting in increased
levels of the second messenger cAMP. cAMP activates protein kinase A which phosphorylates a
number of proteins including the CREB transcription factor which activates a number of genes.
Through this pathway LH controls testosterone production in Leydig cells by regulating
the production of major rate-limiting enzyme in testosterone synthesis, p450SSC or cholesterol side
chain cleavage enzyme. This is the same enzyme that is induced by FSH in granulosa cells.
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Whether CREB or another cAMP inducible factor regulates p450SSC gene has not yet been
determined.
The impact of gonadotropins and their importance in regulating testosterone in the human
is graphically illustrated by considering the case of a hypogonadal teenage boy that lacks
testosterone. This slide shows a hypogonadal boy of 19-20 years of age. These individuals tend to
be very tall because testosterone at the time of puberty is responsible for terminating linear growth
in males by causing the fusion of the epiphyseal plates. The general appearance of this individual
is not masculine and the musculature is not typical of a young man of 20 years of age. The
external genetalia, testis and penis are small. The pubic hair, beard and body hair are also minimal.
In hypogonal individuals the larynx is not fully developed and the vocal chords do not lengthen
and thicken resulting in a higher voice. Also the general behavior of a teenage pubertal boy may
not occur. These individuals may be less aggressive, perhaps with less sex drive than their cohorts.
All these factors are a consequence of the absence of testosterone. Men who are androgen
deficient often manifest impaired labido and erectile function.
Neuroendocrine Control of Testis Function
As in the case of human females described to you by Dr. Zeleznik, gonadotropin secretion
by the male pituitary is absolutely dependent on the pulsatile secretion of a hypothalamic peptide
known as gonadotropin releasing hormone (GnRH).
GnRH control of gonadotropin secretion is best illustrated by the clinical conditions of the
Kallmann’s syndrome patient who do not have GnRH producing neurons in their hypothalamus.
These individuals are hypogonadotropic, hypogonadal, cryporchid and aspermic. Kallmann’s
syndrome is usually identified by the combination of the absence of puberty and a defective sense
of smell. This condition occurs in 1 in 10,000 males. The KAL gene on the X chromosome has
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been identified and cloned from the Kallman’s syndrome genetic locus. The gene encodes the
anosmin protein, a neural adhesion molecule that may be required during fetal development for the
migration of GnRH neurons from the olfactory placode to the brain. Most affected individuals are
identified because of their failure to undergo puberty. Therapy for this condition may involve
androgen replacement to allow virulization or gonadotropin therapy to induce fertility.
Administration of pulsatile GnRH analogs would be best to restore normal levels of
gonadotropins, testosterone levels, spermatogenesis and mature sperm production but is
technically difficult.
In normal men and women, GnRH is secreted in a pulsatile fashion into the
hypophyseal-portal system. GnRH is secreted approximately once every two hours in normal men
(once every hour in castrates) and causes a corresponding pulsatile release of stored FSH and LH
that lasts 30-60 minutes. This intermittent mode of GnRH secretion is absolutely required for the
secretion of LH and FSH by the pituitary. Evidence for the need of pulsatile GnRH secretion is
shown in this study of LH levels in blood. For this study, GnRH was continuously infused and LH
levels assayed. The pulses and levels of LH become much lower after the infusion of GnRH
demonstrating that pulses of GnRH are critical to maintain gonadotropin secretion.
The system regulating testosterone production is a classic negative feedback loop similar
to that regulating estrogen production during the early follicular phase of the menstrual cycle as
described by Dr. Zeleznik. In the male, testosterone is the gonadal feedback signal. The primary
site of testosterone feedback in the human male is at the hypothalamus and the action of
testosterone is to decrease the frequency of the GnRH pulses. Evidence for this mechanism of
negative feedback loop was derived from studies of hypogonadal men that produce low or no
testosterone performed at the University of Pittsburgh by Dr. Steve Winters. On the top of the next
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slide are the pulsatile patterns of LH secretion in the circulation from 3 normal men. On the
bottom is the pattern of LH secrtion from 3 hypogonadal men that have lower levels of
testosterone. The first thing to note is that the scale for the hypogonaldal men has been condensed
and that these men are producing about 500 ng/ml of LH, much higher than the normal subjects. In
addition to the mean levels of LH being higher, the frequency of LH secretion is much greater.
The higher levels of LH secretion and the increased rate of LH pulses correlate with the absence of
testosterone. These studies show that testosterone controls LH secretion by negative feedback.
In another study, treatment of the hypogonadal men with exogenous testosterone
decreased the frequency of LH secretion to rates similar to normal men (1/200 min). This is
additional evidence that testosterone acts at the level of the hypothalamus to regulate GnRH pulse
frequency.
Looking back at the individuals having mutations in their estrogen receptors or aromatase
that resulted in the increased linear growth. These patients exhibit elevated LH levels. This data
has been interpreted as indicating that aromitization of testosterone to estrogen in the
hypothalamus is responsible for the negative feedback action of testosterone.
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Up to now we have focused on the steroidigenic unit, the production of testosterone from
Leydig cells, the effects of testosterone and the regulation of testosterone secretion. Now we will
focus on the spermatogenic unit and the regulation of sperm production.
Spermatogenesis
Seminiferous tubule structure
The processes of germ cell development and sperm production is called spermatogenesis.
Spermatogenesis occurs within the seminiferous tubules that are approximately 0.1 to 0.3
mm in diameter and up to 70 cm in length. The seminiferous tubules are shaped by concentric
layers of peritubular myoid cells that surround the tubule. These cells are responsible for the
contractile activity of the tubules.
Within the seminiferous tubule are the germ cells. Along the basement membrane are
immature germ cells called spermatogonia. Generally, germ cells migrate toward the open center
or lumen of the seminiferous tubules as they mature until the differentiated spermatozoa are
released into the lumen. This process occurs continuously throughout the reproductive lifespan of
the male.
Germ Cell Maturation
Unlike females in which finite numbers of oocytes are held in meiosis I until stimulated by
FSH, in males sperm are continually produced from stem cells. In man, the undifferentiated stem
cells called A pale spermatogonia germ cells undergo mitotic divisions once every 16 days
resulting in another stem cell and an A pale spermatogonia which divides to become a
differentiated type B spermatogonia. The type B spermatogonia divide and detach from the
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basement membrane forming spermatocytes. The spermatocytes progress through the stages of
meiosis to reduce their chromosome number from diploid to haploid. During the prolonged
meiosis stage, spermatocytes are very sensitive to damage and more germ cells die due to normal
apoptosis or toxic insult during the spermatocyte stage than any other. After meiosis, the resulting
haploid cells are called spermatids.
Spermatids undergo extensive differentiation (called spermiogenesis) including
condensation of the nucleus, termination of all gene expression, elimination of most of the
cytoplasm and cell elongation. Also, the flagellum and acrosome develop which are critical for
fertilization. The release of the differentiated spermatids (now spermatozoa) into the tubule lumen
is called spermiation.
These series of mitotic divisions cause a multiplicative increase in the number of germ
cells. From this process in men 8 spermatozoa can be produced from one type A spermatogonia;
however, normally about half are lost during meiosis and other steps.
Because of incomplete cytokinesis during mitosis and mieosis, the resulting germ cells are
joined by intercellular cytoplasmic bridges that persist through the subsequent steps of
spermatogenesis. This means that one cohort of germ cells shares one extended cytoplasm. These
intercellular connections are severed just prior to release of the spermatozoa. This system provides
for a “clonal” type of development and a synchronization of germ cell maturation as all the
co-joined germ cells will mature at the same rate.
This process of spermatogenesis is complex but can be divided into 3 main phases 1) stem
cell renewal 2) proliferation of spermtogonia and spermatocytes and 3) spermiogenesis, the
differentiation of spermatids into spermatozoa.
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The Spermatogenic Cycle and Wave
The time required to complete spermatogenesis from the first cell division of a
renewing germ cell to spermiation varies for different species. In man approximately 70-75 days
are required whereas in rat, spermatogenesis takes about 48 days. The generation of a new cohort
of germ cells from a stem cell occurs precisely every 16 days for humans.
Despite the regimented timing and sequence of cell divisions, stem cell divisions are not
synchronous down the length of the tubule. For some unknown reason stem cells in any one region
of the seminiferous tubule tend to divide at approximately the same time. Flanking stem cells in
the tubule, for example, upstream of the originally mentioned stem cells may divide slightly later
than the originally mentioned stem cells and those downstream slightly earlier. The
unsynchronized stem cell divisions assure the constant production of sperm. In rats, these
episodes of stem cell division lead to the linear progression of the spermatogenesis process down
the length of the tubule in what is called the spermatogenic wave. The situation is slightly different
in man as the spermatogenic wave occurs as a spiral down the tubule.
The cycle of the seminiferous epithelium:
Due to this precise timing of stem cell division and the subsequent germ cell divisions,
germ cells at specific stages of development are always found with other germ cells at other
specific stages of development. The specific germ cell associations are called stages of the
spermatogenic cycle. The continual and cyclical progression of germ cell development gives rise
to the cycle of the seminiferous epithelium.
Regulation of Sperm Output
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Factors supporting germ cell survival
Approximately 800 million sperm are produced each day or about 1 trillion during the
usual reproductive span. Another way to look at sperm production is that 5 million sperm are
made every 10 minutes, so every male in this room has produced over 20 million sperm since the
beginning of this lecture. Germ cell death during spermatogenesis affects ultimate sperm
numbers. The earlier the cell death the greater affect there is on the total number of sperm due to
the multiplicative cell divisions. Toxic chemicals, hormonal insufficiency or particular nutrition
deficits can cause germ cells to undergo apoptosis in early stages (spermatogonia and
spermatocytes). Vitamin A is an example of a nutritional factor required for spermatogenesis.
Vitamin A is essential for the first division of differentiating spermatogonia. In the event of
vitamin A deficiency, all germ cells are absent except for Ap stem cells. In vitamin A deficient
rats, supplementation of the diet with retinol allows for the reinitiation of spermatogenesis. Sertoli
cells secrete retinol binding protein that is required by germ cells to use retinol and Vitamin A.
Vitamin A acts to regulate gene expression by binding to the retinoic acid receptor within the cell.
Mice lacking the retinoic acid receptor are infertile and have testis degeneration.
Another important regulator of sperm output capacity is the number of adjacent Sertoli
cells as Sertoli cells are capable of supporting a fixed number of germ cells.
The Sertoli cell
There is another very important type of cell in the seminiferous tubule. This is a somatic
cell called the Sertoli cell that supports the development of the germ cells. The base of the Sertoli
cell lies on the basement membrane of the tubule and the cytoplasm of the cell extends to the
lumen of the tubule like the branches of a tree encompassing the developing germ cells. Each
Sertoli cell can support a finite number of germ cells. However, germ cells are constantly being
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released and replaced from renewable stem cells. The constant renewal of male germ cells is in
contrast to the finite number of eggs that is determined before birth in the female ovary.
Sertoli cells form specialized tight junction barriers with each other that divides the
seminiferous tubules into two components: the basal or outside region of the tubule and the
adluminal or central compartment of the tubule. The barrier formed is functionally analogous to
the blood-brain barrier. Studies have shown that diffusion of dyes and proteins into the adluminal
compartment are impaired by the barrier. Because there is no blood supply to the adluminal
portion of the seminiferous tubule, Sertoli cells must supply the germ cells beyond the barrier with
required nutrients, minerals, and growth factors (lactate, pyruvate, ABP, transferrin, and SCF, for
example). The barrier causes germ cells to develop in a specialized microenvironment that is
dependent on Sertoli cells secretions. The barrier also does not allow sperm to leak out of the
tubule which is important as sperm antigens elicit an immune response that can severely damage
the testis.
FSH regulation of Sertoli cells
Gonadotropins also regulate sperm output as they regulate testosterone production by
Leydig cells. An example is the comparison of testis tissue from a normal rhesus monkey and a
hypophesectomized monkey. In the testis from the hypophesectomized monkey the size of the
seminiferous tubule are dramatically smaller. There are Sertoli cells but there are no germ cells
beyond the stem cell stage. Studies such as this have shown that gonadotropins derived from the
anterior pituitary are essential for spermatogenesis.
In addition to testosterone, the gonadotropin FSH is a major regulator of Sertoli cell
function. FSH receptors are located on the surface of Sertoli cells and in the testis FSH receptors
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are only present on Sertoli cells. LH receptors are not present on Sertoli cells. The FSH signaling
pathway is similar to that of LH in that G protein linked receptors cause increases in cAMP levels
and activation of cAMP-dependant protein kinases and the phosphorylation of a number of
proteins and the activation of numerous genes. FSH also causes influxes of calcium into Sertoli
cells and can induce calcium-regulated genes. FSH acts indirectly to support germ cell
development by causing Sertoli cells to produce factors required by germ cells. Some of the
FSH-activated genes known to be required for germ cell development are transferrin, androgen
binding protein, pyruvate dehydrogenase, lactate dehydrogenase, androgen receptor, and inhibin.
Inhibin is a peptide hormone consisting of an alpha subunit and a beta subunit. Inhibin
inhibits the production of FSH by the anterior pituitary by inhibiting FSH beta gene expression in
gonadotrophs. This idea is demonstrated by an experiment in which rhesus monkeys were infused
with human inhibin A (dark circles) or vehicle (open circles). The monkeys infused with inhibin A
have reduced serum levels of FSH. Therefore, as is the case for LH, FSH secretion from the
pituitary is controlled by a negative feedback loop. The testicular hormone that inhibits FSH
production is not a steroid hormone like testosterone but is instead the peptide hormone inhibin B.
Both FSH and testosterone act on Sertoli cells and are required for maximal sperm
production. However, the contemporary literature argues that testosterone is the more critical of
the two hormones. This idea is based on the finding that male mice that lack the FSH gene or the
FSH receptor gene can produce offspring. Also, human male patients can remain fertile with
mutations that inactivate the FSH receptor. Nevertheless, in each case of disrupted FSH signaling
there are smaller testes and low sperm counts. There are probably two explanations for the low
sperm count. Firstly, during puberty in man FSH stimulates Sertoli cells to divide which in turn
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determines the number of germ cells that can be supported. Secondly, FSH amplifies the action of
testosterone on the adult testis.
To date, gonadotropin receptors have not been convincingly demonstrated on germ cells.
So how do gonadotropins act to regulate the germ cells???? The answer is that gonadotropins are
thought to stimulate Sertoli cells to produce paracrine factors regulating germ cells. In the case of
LH, its action must be indirect as there are no LH receptors on Sertoli cells. LH acts through the
Leydig cell to cause the production of testosterone that diffuses into the Sertoli cells. In contrast,
FSH acts directly on the Sertoli cells through receptors on the cell surface to activate the same
signaling pathway that we saw for LH.
Summary of testosterone and FSH regulation of spermatogenesis
Testosterone is critical for driving spermatogenesis and some proteins are known to be
induced by testosterone. However, the genes that androgen receptor regulate in Sertoli cells are not
known. FSH is not essential to maintain spermatogenesis but makes the process more efficient.
The actions of FSH in the Sertoli cell are better understood than those of testosterone. FSH acts as
a growth factor that causes Sertoli cells to proliferate prior to puberty. Some of the products FSH
induces that are required by germ cells include transferrin needed to transport iron to germ cells,
and enzymes that produce pyruvate and lactate which are essential energy sources for germ cells.
Paracrine Factors and Nutrition
How do Sertoli cells and germ cells communicate to organize the timing of germ cell
divisions and maturation? It is possible that factors produced by Sertoli cells act in a paracrine
manner to regulate spermatogenesis. For example, the growth factor called stem cell factor (SCF)
or kit ligand produced by Sertoli cells promotes spermatogonia cell division by binding a
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tyrosine-kinase receptor called c-kit on these cells. SCF expression changes in the Sertoli cell
during the spermatogenic cycle with SCF levels being highest in the stages when spermatogonia
undergo their multiplicative divisions. Sertoli cells also produce the hormone activin that also
stimulates DNA synthesis in spermatocytes. Interleukin-1 produced by Sertoli cells also
stimulates spermatogonina DNA synthesis.
The germ cells also produce factors that may regulate Sertoli cell function such as insulin
like growth factor (IGF-1) that stimulates lactate production. Nerve growth factor and the
cytokine TNF- are produced by spermatids and regulate Sertoli cell gene expression. The factors
produced by germ cells may send important signals to Sertoli cells to provide for the needs of the
germ cells during specific developmental stages. The signaling to the Sertoli cell may be one way
that germ cells determine the timing of their development.
Recently, studies have been performed to determine whether Sertoli cells or germ cells are
responsible for controlling the developmental timing of spermatogenesis. It is now possible to
transplant germ cells from one species into the seminiferous tubules of another species. For
example, before being released as spermatozoa, rat germ cells that go through about 4
spermatogenic cycles of 12.9 days each can be transplanted in to the mouse testis that have 4
shorter cycles of 8.6 days. For these studies the testis of the recipient is treated with antimitotic
drugs (busulfan) to delete the recipient tubules of germ cells. After transplantation it is possible to
determine whether the transplanted germ cells progress through the cycle at the rate dictated by the
somatic cells (Sertoli cells) of the recipient mouse or at the rat rate dictated by the germ cells.
Results of these tests showed that the transplanted rat germ cells took 13 days (the rat rate) to
progress through the spermatogenic cycle and thus the timing of the cycle must be determined by
factors intrinsic to the germ cell.
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Transport and Delivery of Spermatozoa
Sertoli cells secrete fluid into the seminiferous tubule lumen that carries the released
spermatozoa. Included in this fluid are nutrients for the sperm and androgen binding protein
(ABP) that carries testosterone. Both ends of the seminiferous tubules open into a branched
resevoir called the rete testis. The blood-testis barrier is not maintained in the rete testis, therefore,
the spermatozoa are exposed to changes in ion and macromoleucule concentrations. From the rete
testis the sperm travel to the efferent ducts at the top pole of the testis and then into the epididymis.
The epididymis is a single highly convoluted duct closely adjacent to the testis. Spermatozoa are
concentrated 100-fold in the epididymus by reabsorption of fluid. The spermatozoa lose Sertoli
secreted products coating their surface but the epididymis coats the surface of the spermatozoa
with new glycoproteins and other factors. In contrast to the spermatozoa that are immotile just
after they leave the seminiferous tubules, by the time sperm reach the epididymis they are able to
swim to some extent.
The actions of the epididymis are controlled by androgens. Without androgen the
epididymis atrophies. Most of the available testosterone is present in the testis fluid bound to
androgen binding protein provided originally by Sertoli cells (30-60 ng/ml). Androgens taken up
by the epididymis are converted by 5- reductase to dihydrotestosterone to yield a more active
androgen.
Sperm transport through the epidiymis is relatively slow taking about 15 days in humans.
From the epididymis the sperm are moved as a very dense mass to the vas or ductus deferens by the
muscular activity of the epididymis and vas deferens. The vas deferens are approximately 25 cm
long in humans. Sperm are stored in the vas deferens. In the absence of ejaculation spermatozoa
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move through the terminal ampulla into the urethra and are removed with the urine. Sperm that are
carried to the female reproductive tract do so in seminal fluid that is produced by the major sex
accessory glands. The sex accessory glands include the seminal vesicles, prostate, ampulla, and
the bulbourethral or Cowper’s gland. The major known function of the accessory glands is to
secrete the constituents of seminal fluid. Seminal fluid contains nutritional factors (fructose,
sorbitol) buffering agents to protect against the acid pH of vaginal fluids and reducing agents to
protect from oxidation. The seminal fluid can also contain immune cells such as leukocytes as
well as free hepatitis B virus and HIV.
The seminal vesicles are paired pouches of 8-9 gm in humans located directly on the
posterior side of the bladder. They were erroneously named because they were thought to store
semen and sperm. They do, however, provide 60% of the seminal fluid.
The Cowper’s gland is a paired pea-sized tubular gland located directly below the prostate
within the urogenital sinus. The prostate lies immediately below the base of the bladder
surrounding the proximal portion of the urethra. It is shaped like a chestnut and is 3.5-5 cm width
and length. The prostate contains a number of individual glands that provide 30% of the seminal
fluid. In contrast to the seminal vesicles, the ampulla at the distal end of the vas deferens does store
sperm.
The seminal vesicles join the ampulla to form the beginning of the ejaculatory duct.
Testosterone, which is remodeled into dihydroxytestosterone by 5 alpha reductase, is required to
maintain the structure and function of the accessory glands.
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Transmission of sperm
Penile erection results from increased blood flow to the penis and reduced blood outflow
from the organ. These alterations in blood flow are caused by the release of nitric oxide from
parasympathetic nerve endings in the walls of precapillary arterioles and sinusoids of the corpora
cavernosa. The released nitric oxide stimulates an increase in cGMP production. cGMP
contributes to muscle relaxation and allows the dilation of arterial smooth muscle and increased
blood flow. The expanded corpora cavernosa press against the inflexible tunica albuginea and
reduce the outflow of venous blood causing an increase in blood pressure in the corpora cavernosa
and penile erection.
With further stimulation, a sequence of contractions of the muscles of the prostate, vas
deferens and seminal vesicle is induced and the components of the seminal plasma together with
the spermatozoa are expelled into the urethra. This process is called emmision. Ejaculation,
whereby semen is expelled from the urethra, is achieved by contraction of the smooth muscles of
the urethra and striated muscles of the ischiocavernosus.
At any one time 10% of men are unable to achieve or maintain errections. The
advent of the medication, sildenafil or Viagra for treatment of male impotence has attracted
widespread attention. Viagra counteracts the actions of a phosphodiesterase called PDE5 that
rapidly breaks down cGMP. The PDE5 phosphodiesterase is particularly abundant in smooth
muscle and limits the response to nitric oxide unless cGMP is continually produced.
In initial clinical trials with Viagra to determine its efficacy in treating angina, men
reported that the drug enhanced the penile erectile response. The reason that Viagra acts more
specifically on penile blood flow and less well on the general circulation, is not clear at this time.
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One possibility is that sexual stimulation, which is necessary for Viagra effectiveness, causes a
rather specific, localized release of nitric oxide in the penis, which would produce a large increase
in cGMP synthesis mainly in this tissue. A PDE5 inhibitor would block cGMP breakdown and
therefore act synergistically with nitric oxide to elevate cGMP and cause penile smooth muscle
relaxation.
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