# Human Physiology/The male reproductive system

## Introduction

In simple terms, **reproduction** is the process by which organisms
create descendants. This miracle is a characteristic that all living
things have in common and sets them apart from nonliving things. But
even though the reproductive system is essential to keeping a species
alive, it is not essential to keeping an individual alive.

In human reproduction, two kinds of sex cells or gametes are involved.
Sperm, the male gamete, and a secondary oocyte (along with first polar
body and corona radiata), the male gamete must meet in the female
reproductive system to create a new individual. For reproduction to
occur, both the female and male reproductive systems are essential. It
is a common misnomer to refer to a woman\'s gametic cell as an **egg or
ovum**, but this is impossible. A secondary oocyte must be fertilized by
the male gamete before it becomes an \"ovum\" or \"egg\".

While both the female and male reproductive systems are involved with
producing, nourishing and transporting either the **oocyte or sperm**,
they are different in shape and structure. The male has reproductive
organs, or genitals, that are both inside and outside the pelvis, while
the female has reproductive organs entirely within the pelvis.

The male reproductive system consists of the testes and a series of
ducts and glands. Sperm are produced in the testes and are transported
through the reproductive ducts. These ducts include the epididymis, vas
deferens, ejaculatory duct and urethra. The reproductive glands produce
secretions that become part of semen, the fluid that is ejaculated from
the urethra. These glands include the seminal vesicles, prostate gland,
and bulbourethral glands.

## Structure

!The human male reproductive
system{width="440"}

### Testes

The testes (singular, testis) are located in the scrotum (a sac of skin
between the upper thighs). In the male fetus, the testes develop near
the kidneys, then descend into the scrotum just before birth. Each
testis is about 1 1/2 inches long by 1 inch wide. Testosterone is
produced in the testes which stimulates the production of sperm as well
as give secondary sex characteristics beginning at puberty.

**Scrotum**

The two testicles are each held in a fleshy sac called the scrotum. The
major function of the scrotal sac is to keep the testes cooler than
thirty-seven degrees Celsius (ninety-eight point six degrees
Fahrenheit). The external appearance of the scrotum varies at different
times in the same individual depending upon temperature and the
subsequent contraction or relaxation of two muscles. These two muscles
contract involuntarily when it is cold to move the testes closer to the
heat of the body in the pelvic region. This causes the scrotum to appear
tightly wrinkled. On the contrary, they relax in warm temperatures
causing the testes to lower and the scrotum to become flaccid. The
temperature of the testes is maintained at about thirty-five degrees
Celsius (ninety-five degrees Fahrenheit), which is below normal body
temperature. Temperature has to be lower than normal in order for
*spermatogenesis* (sperm production) to take place. !The scrotum is in
a relaxed
state.!Penile
shrinkage due to low temperatures. The scrotum is in a tense state to
regulate testicular
temperatures.

The two muscles that regulate the temperature of the testes are the
dartos and cremaster muscles:

-   *Dartos Muscle*

The dartos muscle is a layer of smooth muscle fibers in the subcutaneous
tissue of the scrotum (surrounding the scrotum). This muscle is
responsible for wrinkling up the scrotum, in conditions of cold weather,
in order to maintain the correct temperature for spermatogenesis.

-   *Cremaster Muscle*

The cremaster muscle is a thin strand of skeletal muscle associated with
the testes and spermatic cord. This muscle is a continuation of the
internal oblique muscle of the abdominal wall, from which it is derived.
**Seminiferous Tubules**

Each testis contains over 100 yards of tightly packed seminiferous
tubules. Around 90% of the weight of each testes consists of
seminiferous tubules. The seminiferous tubules are the functional units
of the testis, where spermatogenesis takes place. Once the sperm are
produced, they moved from the seminiferous tubules into the rete testis
for further maturation.

**Interstitial Cells (Cells of Leydig)**

In between the seminiferous tubules within the testes, are instititial
cells, or, *Cells of Leydig*. They are responsible for secreting the
male sex hormones (i.e., testosterone).

**Sertoli Cells**

A Sertoli cell (a kind of sustentacular cell) is a \'nurse\' cell of the
testes which is part of a seminiferous tubule.

It is activated by follicle-stimulating hormone, and has FSH-receptor on
its membranes.

Its main function is to nurture the developing sperm cells through the
stages of spermatogenesis. Because of this, it has also been called the
\"mother cell.\" It provides both secretory and structural support.

Other functions During the Maturation phase of spermiogenesis, the
Sertoli cells consume the unneeded portions of the spermatazoa.

**Efferent ductules**

The sperm are transported out of the testis and into the epididymis
through a series of efferent ductules.

**Blood Supply**

The testes receive blood through the testicular arteries (gonadal
artery). Venous blood is drained by the testicular veins. The right
testicular vein drains directly into the inferior vena cava. The left
testicular vein drains into the left renal vein.

### Epididymis

!Anatomy of the testisThe
seminiferous tubules join together to become the epididymis. The
epididymis is a tube that is about 2 inches that is coiled on the
posterior surface of each testis. Within the epididymis the sperm
incomplete their maturation (full maturation occur in female genital
tract) and their flagella become mobile. This is also a site to store
sperm, nourishing them until the next ejaculation. Smooth muscle in the
wall of the epididymis propels the sperm into the ductus deferens. Vasa
efferentia from the rete testis open into the epididymis which is a
highly coiled tubule. The epididymis has three parts- 1)head or caput
epididymis- it is the proximal part of the epididymis. It carries the
sperms from the testis. 2)body or corpus epididymis- it the highly
convoluted middle part of the epididymis 3)tail or cauda epididymis- it
is the last part that takes part in carrying the sperms to the vas
deferens. The cauda epididymis continues to form less convoluted vas
deferens.

### Ductus Deferens

The ductus (vas) deferens, also called sperm duct, or, spermatic
deferens, extends from the epididymis in the scrotum on its own side
into the abdominal cavity through the inguinal canal. The inguinal canal
is an opening in the abdominal wall for the spermatic cord (a connective
tissue sheath that contains the ductus deferens, testicular blood
vessels, and nerves. The smooth muscle layer of the ductus deferens
contracts in waves of peristalsis during ejaculation.

### Seminal Vesicles

The pair of seminal vesicles are posterior to the urinary bladder. They
secrete fructose to provide an energy source for sperm and alkalinity to
enhance sperm mobility. The duct of each seminal vesicle joins the
ductus deferens on that side to form the ejaculatory duct.

### Ejaculatory Ducts

There are two ejaculatory ducts. Each receives sperm from the ductus
deferens and the secretions of the seminal vesicle on its own side. Both
ejaculatory ducts empty into the single urethra.

### Prostate Gland

The prostate gland is a muscular gland that surrounds the first inch of
the urethra as it emerges from the bladder. The smooth muscle of the
prostate gland contracts during ejaculation to contribute to the
expulsion of semen from the urethra.

### Bulbourethral Glands

!Pre ejaculate produced by the bulbourethral
glandsThe
bulbourethral glands also called Cowper\'s glands are located below the
prostate gland and empty into the urethra. The alkalinity of seminal
fluid helps neutralize the acidic vaginal pH and permits sperm mobility
in what might otherwise be an unfavorable environment.

### Penis

The penis is an external genital organ. The distal end of the penis is
called the glans penis and is covered with a fold of skin called the
prepuce or foreskin. Within the penis are masses of erectile tissue.
Each consists of a framework of smooth muscle and connective tissue that
contains blood sinuses, which are large, irregular vascular channels.

### Urethra

The urethra, which is the last part of the urinary tract, traverses the
corpus spongiosum and its opening, known as the meatus, lies on the tip
of the glans penis. It is both a passage for urine and for the
ejaculation of semen.

### Overview of Male Reproductive System Structure and Function

  STRUCTURE                                        LOCATION & DESCRIPTION                                                                                                                                                                                   FUNCTION
  ------------------------------------------------ -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Bulbourethral glands (2)                         Pea sized organs posterior to the prostate on either side of the urethra.                                                                                                                                Secretion of gelatinous seminal fluid called pre-ejaculate. This fluid helps to lubricate the urethra for spermatozoa to pass through, and to help flush out any residual urine or foreign matter. (\< 1% of semen)
  Cells of Leydig (Interstitial cells of Leydig)   Adjacent to the seminiferous tubules in the testicle.                                                                                                                                                    Responsible for production of testosterone. Closely related to nerves.
  Cremaster muscle                                 Covers the testes.                                                                                                                                                                                       Raises and lowers scrotum to help regulate temperature and promote spermatogenesis. Voluntary and involuntary contraction.
  Dartos muscle                                    Layer of smooth muscular fiber outside the external spermatic fascia but below the skin                                                                                                                  Contraction by wrinkling to decrease surface area available for heat loss to testicles, or expansion to increase surface area available to promote heat loss; also helps raise and lower scrotum to help regulate temperature
  Efferent ductules                                Part of the testes and connect the rete testis with the epididymis                                                                                                                                       Ducts for sperm to get to epididymis
  Ejaculatory ducts (2)                            Begins at the vas deferens, passes through the prostate, and empties into the urethra at the Colliculus seminalis.                                                                                       Causes reflex for ejaculation. During ejaculation, semen passes through the ducts and exits the body via the penis.
  Epididymis                                       Tightly coiled duct lying just outside each testis connecting efferent ducts to vas deferens.                                                                                                            Storage and maturation of sperm.
  Penis                                            Three columns of erectile tissue: two corpora cavernosa and one corpus spongiosum. Urethra passes through penis.                                                                                         Male reproductive organ and also male organ of urination.
  Prostate gland                                   Surrounds the urethra just below the urinary bladder and can be felt during a rectal exam.                                                                                                               Stores and secretes a clear, slightly alkaline fluid constituting up to one-third of the volume of semen. Raise vaginal pH.(25-30% of semen)
  Scrotum                                          Pouch of skin and muscle that holds testicles.                                                                                                                                                           Regulates temperature at slightly below body temperature.
  Semen                                            Usually white but can be yellow, gray or pink (blood stained). After ejaculation, semen first goes through a clotting process and then becomes more liquid.                                              Components are sperm, and \"seminal plasma\". Seminal plasma is produced by contributions from the seminal vesicle, prostate, and bulbourethral glands.
  Seminal vesicles (2)                             Convoluted structure attached to vas deferens near the base of the urinary bladder.                                                                                                                      About 65-75% of the seminal fluid in humans originates from the seminal vesicles. Contain proteins, enzymes, fructose, mucus, vitamin C, flavins, phosphorylcholine and prostaglandins. High fructose concentrations provide nutrient energy for the spermatozoa as they travel through the female reproductive system.
  Seminiferous tubules (2)                         Long coiled structure contained in the chambers of the testis; joins with vas deferens.                                                                                                                  Meiosis takes place here, creation of gametes (sperm).
  Sertoli cells                                    Junctions of the Sertoli cells form the blood-testis barrier, a structure that partitions the interstitial blood compartment of the testis from the abdominal compartment of the seminiferous tubules.   Cells responsible for nurturing and development of sperm cells , provides both secretory and structural support; activated by FSH. Also called \"mother cells\" or \"nurse cells\".
  Testes                                           Inside scrotum, outside of body.                                                                                                                                                                         Gonads that produce sperm and male sex hormones.Production of testosterone by cells of Leydig in the testicles.
  Testicular arteries (Gonadal arteries)           Branch of the abdominal aorta. It is a paired artery. Each passes obliquely downward and laterally behind the peritoneum.                                                                                Supplies blood to the testes.
  Urethra                                          Connects bladder to outside body, about 8 inches long.                                                                                                                                                   Tubular structure that receives urine from bladder and carries it to outside of the body. Also passage for sperm.
  Vas deferens                                     Muscular tubes connecting the left and right epididymis to the ejaculatory ducts to move sperm. Each tube is about 30 cm long.                                                                           During ejaculation the smooth muscle in the vas deferens wall contracts, propelling sperm forward. Sperm are transferred from the vas deferens into the urethra, collecting fluids from accessory sex glands on route

### Composition of human semen

The components of semen come from two sources: sperm, and \"seminal
plasma\". Seminal plasma, in turn, is produced by contributions from the
seminal vesicle, prostate, and bulbourethral glands.

Seminal plasma of humans contains a complex range of organic and
inorganic constituents.

The seminal plasma provides a nutritive and protective medium for the
spermatozoa during their journey through the female reproductive tract.
The normal environment of the vagina is a hostile one for sperm cells,
as it is very acidic (from the native microflora producing lactic acid),
viscous, and patrolled by immune cells. The components in the seminal
plasma attempt to compensate for this hostile environment. Basic amines
such as putrescine, spermine, spermidine and cadaverine are responsible
for the smell and flavor of semen. These alkaline bases counteract the
acidic environment of the vaginal canal, and protect DNA inside the
sperm from acidic denaturation.

**The components and contributions of semen are as follows:**

  **GLAND**              **APPROXIMATE %**   **DESCRIPTION**
  ---------------------- ------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  testes                 2-5%                Approximately 200- to 500-million spermatozoa (also called sperm or spermatozoans), produced in the testes, are released per ejaculation
  seminal vesicle        65-75%              amino acids, citrate, enzymes, flavins, fructose (the main energy source of sperm cells, which rely entirely on sugars from the seminal plasma for energy), phosphorylcholine, prostaglandins (involved in suppressing an immune response by the female against the foreign semen), proteins, vitamin C
  prostate               25-30%              acid phosphatase, citric acid, fibrinolysin, prostate specific antigen, proteolytic enzymes, zinc (serves to help to stabilize the DNA-containing chromatin in the sperm cells. A zinc deficiency may result in lowered fertility because of increased sperm fragility. Zinc deficiency can also adversely affect spermatogenesis.)
  bulbourethral glands   \< 1%               galactose, mucus (serve to increase the mobility of sperm cells in the vagina and cervix by creating a less viscous channel for the sperm cells to swim through, and preventing their diffusion out of the semen. Contributes to the cohesive jelly-like texture of semen.), pre-ejaculate, sialic acid

A 1992 World Health Organization report described normal human semen as
having a volume of 2 ml or greater, pH of 7.2 to 8.0, sperm
concentration of 20x106 spermatozoa/ml or more, sperm count of 40x106
spermatozoa per ejaculate or more and motility of 50% or more with
forward progression (categories a and b) of 25% or more with rapid
progression (category a) within 60 minutes of ejaculation.[^1]

## Functions

![](Male_reproductive_hormone_chart.gif "Male_reproductive_hormone_chart.gif"){width="550"}

### Hormone Regulation

Hormones which control reproduction in males are:

Gonadotropin-Releasing Hormone (GnRH):

-   The hypothalamus secretes this hormone into the pituitary gland in
    the brain.
-   There are two gonadotropic hormones, FSH and LH.

Luteinizing Hormone (LH):

-   The pituitary gland secretes this hormone after receiving a GnRH
    signal from the hypothalamus.
-   LH stimulates Leydig cells, in the testes, telling them to produce
    testosterone.

Follicle-Stimulating Hormone (FSH):

-   The pituitary gland also secretes this hormone.
-   Testosterone helps FSH run through the bloodstream to make Sertoli
    cells, located in the seminiferous tubules of the testes, to make
    immature sperm to mature sperm.

Testosterone:

-   Also known as \"the male hormone\" and \"androgen\".
-   Testosterone is vital for the production of sperm.

!150 px\|The constituent cavernous cylinders of the
penis.

### Erection

!250 px\|Transverse section of the
penis. The
erection of the penis is its enlarged and firm state. It depends on a
complex interaction of psychological, neural, vascular and endocrine
factors. The term is also applied to the process that leads to this
state.

A penile erection occurs when two tubular structures that run the length
of the penis, the corpora cavernosa, become engorged with venous blood.
This is a result of parasympathetic nerve induced vasodilation. This may
result from any of various physiological stimuli. The corpus spongiosum
is a single tubular structure located just below the corpora cavernosa,
which contains the urethra, through which urine and semen pass during
urination and ejaculation, respectively. This may also become slightly
engorged with blood, but less so than the corpora cavernosa.

Penile erection usually results from sexual stimulation and/or arousal,
but can also occur by such causes as a full urinary bladder or
spontaneously during the course of a day or at night, often during
erotic or wet dreams. An erection results in swelling and enlargement of
the penis. Erection enables sexual intercourse and other sexual
activities (sexual functions), though it is not essential for all sexual
activities.

### Ejaculation

Emission is the term used when sperm moves into the urethra. Ejaculation
is the term used when sperm is forced out of the urethra and the penis.
These are both stimulated by sympathetic nerves.

### Sperm Production

A spermatozoon or spermatozoan (pl. spermatozoa), from the ancient Greek
σπέρμα (seed) and ζῷον (living being) and more commonly known as a sperm
cell, is the haploid cell that is the male gamete. !A mature human
Spermatozoon

Spermatagonia divides several times during the process of sperm
development. The entire process of sperm formation and maturation takes
about 9-10 weeks. The separate divisions that take place and what
happens in each are as follows:

-   **First division**: The first division is done by mitosis, and
    ensures a constant supply of *spermatocytes*, each with the diploid
    number of chromosomes.

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-   **Second division**: Spermatocytes then undergo a series of two cell
    divisions during meiosis to become *secondary spermatocytes*.

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-   **Third division**: Secondary Spermatocytes finally become
    *spermatids*. Spermatids, which are haploid cells, mature slowly to
    become the male gametes, or *sperm*.

The sperm is the main reproductive cell in males. The sperms differ in
that each carry a set of chromosomes dividing each into either a male,
or female sperm. The females differ in that they carry a X gene, while
the male sperm carry a Y gene. The female sperm also differ
phenotypically in that they have a larger head in comparison to the male
sperms. This contributes to the male sperm being lighter, and therefore
faster and stronger swimmers than their female counterparts (although
statistically there is still a 50% chance of an either XY or XX embryo
forming.

Spermatozoan stream lines are straight and parallel. The tail
flagellates, which we now know propels the sperm cell (at about 1-3
mm/minute in humans) by rotating like a propeller, in a circular motion,
not side to side like a whip. The cell is characterized by a minimum of
cytoplasm. During fertilization, the sperm\'s mitochondria gets
destroyed by the egg cell, and this means only the mother is able to
provide the baby\'s mitochondria and mitochondrial DNA, which has an
important application in tracing maternal ancestry. However it has been
recently discovered that mitochondrial DNA can be recombinant.

Spermatozoa are produced in the seminiferous tubules of the testes in a
process called spermatogenesis. Round cells called spermatogonia divide
and differentiate eventually to become spermatozoa. During copulation
the vagina is inseminated, the spermatozoa move through *chemotaxis*
(see glossary) to the ovum inside a Fallopian tube or the uterus.

### Sperm Pathway

Spermatogenesis takes place inside a male's testes, specifically in the
walls of the seminiferous tubules. The epididymis is a tortuously coiled
structure topping the testis, it receives immature sperm from the testis
and stores it for several days. When ejaculation occurs, sperm is
forcefully expelled from the tail of the epididymis into the ductus
deferens. Sperm travels through the ductus deferens and up the spermatic
cord into the pelvic cavity, over the ureter to the prostate behind the
bladder. Here, the vas deferens joins with the seminal vesicle to form
the ejaculatory duct, which passes through the prostate and empties into
the urethra. Upon the sperm\'s exit from the testes, into the vas
deferens, muscular movements take over. When ejaculation occurs,
rhythmic muscle movements of *peristalsis* propel the sperm forward.
This continues throughout the remainder of the sperm\'s journey through
the male reproductive system.

Sperm cells become even more active when they begin to interact with the
*fertilizing layer* of an egg cell. They swim faster and their tail
movements become more forceful and erratic. This behavior is called
\"hyper activation.\"

A recent discovery links hyper activation to a sudden influx of calcium
ions into the tails. The whip-like tail (flagellum) of the sperm is
studded with ion channels formed by proteins called CatSper. These
channels are selective, allowing only calcium ion to pass. The opening
of CatSper channels is responsible for the influx of calcium. The sudden
rise in calcium levels causes the flagellum to form deeper bends,
propelling the sperm more forcefully through the viscous environment.
!Acrosome reaction on a Sea Urchin
cell{width="440"}

The sperm use their tails to push themselves into the epididymis, where
they complete their development. It takes sperm about 4 to 6 weeks to
travel through the epididymis. The sperm then move to the vas deferens,
or sperm duct. The seminal vesicles and prostate gland produce a whitish
fluid called seminal fluid, which mixes with sperm to form semen when a
male is sexually stimulated.

The penis, which usually hangs limp, becomes hard when a male is
sexually excited. Tissues in the penis fill with blood and it becomes
stiff and erect (an erection). The rigidity of the erect penis makes it
easier to insert into the female\'s vagina during sexual intercourse,
and the extended length allows it to reach deeper into the female\'s
*oviduct*, the passage from the ovaries to the outside of the body
(allowing a shorter travel distance for the spermatozoa).

When the erect penis is stimulated to orgasm, muscles around the
reproductive organs contract and force the semen through the duct system
and urethra. Semen is pushed out of the male\'s body through his
urethra - ejaculation. The speed of the semen is about 70 mph when
ejaculation comes and it can contain 100 to 600 million sperm cells.
When the male ejaculates during intercourse, semen is deposited into the
fornix at the base of the female\'s vagina, near the cervix. From the
fornix, the sperm make their way up through the cervix and move through
the uterus with help from uterine contractions.

Sperm hyperactivity is necessary for breaking through two physical
barriers that protect the egg from fertilization. The first barrier to
sperm is made up of so-called cumulus cells embedded in a gel-like
substance made primarily of hyaluronic acid. The cumulus cells develop
in the ovary with the egg and support it as it grows.

The second barrier coating the oocyte is a thick shell formed by
glycoproteins called the zona pellucida. One of the proteins that make
up the zona pellucida binds to a partner molecule on the sperm. This
lock-and-key type mechanism is species-specific and prevents the sperm
and egg of different species from fusing. There is some evidence that
this binding is what triggers the acrosome to release the enzymes that
allow the sperm to fuse with the egg.

When a sperm cell reaches the egg the acrosome releases its enzymes.
These enzymes weaken the shell, allowing the sperm cell to penetrate it
and reach the plasma membrane of the egg. Part of the sperm\'s cell
membrane then fuses with the egg cell\'s membrane, and the sperm cell
sinks into the egg (at which point the sperm tail falls off).

Upon penetration, the egg cell membrane undergoes a change and becomes
impenetrable, preventing further fertilization.

The binding of the sperm to an ovum is called a zygote. A zygote is a
single cell, with a complete set of chromosomes, that normally develops
into an embryo.

## Puberty

!Male pubertyIn addition to producing
sperm, the male reproductive system also produces sex hormones, which
help a boy develop into a sexually mature man during puberty. When a
baby boy is born, he has all the parts of his reproductive system in
place, but it isn\'t until puberty that his reproductive organs mature
and become fully functional. As an newborn FSH and LH levels are high
and after a few weeks levels drop to extremely low. When puberty begins,
usually between the ages of 10 and 14, the pituitary gland - which is
located in the brain - secretes hormones that stimulate the testicles to
produce testosterone. The production of testosterone brings about many
physical changes. Although the timing of these changes is different for
each individual male, the stages of puberty generally follow a set
sequence.

-   First stage: the scrotum and testes grow larger, the *apocrine
    glands* develop (see explanation of apocrine glands in glossary).

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-   Second stage: the penis becomes longer, and the seminal vesicles and
    prostate gland grow. Hair begins to grow in the pubic region.
    Reproductive capacity has usually developed by this stage.

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-   Third stage: hair begins to appear on the face and underarms. During
    this time, a male\'s voice also deepens. Fertility continues to
    increase.

### Testicular size, function, and fertility

In boys, testicular enlargement is the first physical manifestation of
puberty (and is termed gonadarche). Testes in prepubertal boys change
little in size from about 1 year of age to the onset of puberty,
averaging about 2--3 cc in volume and about 1.5-2 cm in length.
Testicular size continues to increase throughout puberty, reaching
maximal adult size about 6 years later. While 18-20 cc is reportedly an
average adult size, there is wide variation in the normal population.

The testes have two primary functions: to produce hormones and to
produce sperm. The Leydig cells produce testosterone (as described
below), which in turn produces most of the changes of male puberty.
However, most of the increasing bulk of testicular tissue is
spermatogenic tissue (primarily Sertoli and interstitial cells). The
development of sperm production and fertility in males is not as well
researched. Sperm can be detected in the morning urine of most boys
after the first year of pubertal changes (and occasionally earlier).

### Genitalia

A boy\'s penis grows little from the fourth year of life until puberty.
Average prepubertal penile length is 4 cm. The prepubertal genitalia are
described as stage 1. Within months after growth of the testes begins,
rising levels of testosterone promote growth of the penis and scrotum.
This earliest discernible beginning of pubertal growth of the genitalia
is referred to as stage 2. The penis continues to grow until about 18
years of age, reaching an average adult size of about 10-16 cm.

Although erections and orgasm can occur in prepubertal boys, they become
much more common during puberty, accompanied by development of *libido*
(sexual desire). Ejaculation becomes possible early in puberty; prior to
this boys may experience dry orgasms. Emission of seminal fluid may
occur due to masturbation or spontaneously during sleep (commonly termed
a *wet dream*, and more clinically called a *nocturnal emission*). The
ability to ejaculate is a fairly early event in puberty compared to the
other characteristics, and can occur even before reproductive capacity
itself. In parallel to the irregularity of the first few periods of a
girl, for the first one or two years after a boy\'s first ejaculation,
his seminal fluid may contain few active sperm.

If the foreskin of a boy does not become retractable during childhood,
it normally begins to retract during puberty. This occurs as a result of
the increased production of testosterone and other hormones in the body.

!Distribution of androgenic hair on female and male
body{width="150"}

### Genital Erection

The penis contains two chambers called the corpora cavernosa, which run
the length of the organ. A spongy tissue, full of muscle, veins,
arteries, etc. fills these chambers. The corpora cavernosa are
surrounded by a membrane, called the tunica albuginea.

Erection begins with sensory or mental stimulation, or both. Impulses
from the brain and local nerves cause the muscles of the corpora
cavernosa to relax, allowing blood to flow in and fill the spaces. The
blood creates pressure in the corpora cavernosa, making the penis
expand. The tunica albuginea helps trap the blood in the corpora
cavernosa, thereby sustaining erection. When muscles in the penis
contract to stop the inflow of blood and open outflow channels, erection
is reversed.

### Pubic hair in boys

Pubic hair often appears on a boy shortly after the genitalia begin to
grow. As in girls, the first appearance of pubic hair is termed pubarche
and the pubic hairs are usually first visible at the dorsal (abdominal)
base of the penis. The first few hairs are described as stage 2. Stage 3
is usually reached within another 6 to 12 months, when the hairs are too
numerous to count. By stage 4, the pubic hairs densely fill the \"pubic
triangle.\" Stage 5 refers to spread of pubic hair to the inner thighs
and upward towards the umbilicus as part of the developing abdominal
hair.

## Sexual Homology

!The human male reproductive
system{width="440"}

!Cross-sectional diagram of the female reproductive
organs.{width="440"}
In short, this is a known list of sex organs that evolve from the same
tissue in a human life.

  Indifferent           Male                                Female
  --------------------- ----------------------------------- ---------------------------------
  Gonad                 Testis                              Ovary
  Mullerian duct        Appendix testis                     Fallopian tubes
  Mullerian duct        Prostatic utricle                   Uterus, proximal vagina
  Wolffian duct         Rete testis                         Rete ovarii
  Mesonephric tubules   Efferent ducts                      Epoophoron
  Wolffian duct         Epididymis                          Gartner\'s duct
  Wolffian duct         Vas deferens                        
  Wolffian duct         Seminal vesicle                     
  Wolffian duct         Prostate                            Skene\'s glands
  Urogenital sinus      Urinary bladder\|Bladder, urethra   Bladder, urethra, distal vagina
  Urogenital sinus      Bulbourethral gland                 Bartholin\'s gland
  Genital swelling      Scrotum                             Labia majora
  Urogenital folds      Distal urethra                      Labia minora
  Genital tubercle      Penis                               Clitoris
  Prepuce               Foreskin                            Clitoral hood
                        Bulb of penis                       Vestibular bulbs
                        Glans penis                         Clitoral glans
                        Crus of penis                       Clitoral crura

## Aging

For most men, testosterone secretion continues throughout life, as does
sperm production, though both diminish with advancing age. Probably the
most common reproductive problem for older men is prostatic hypertrophy,
enlargement of the prostate gland. This causes the urethra to compress
and urination becomes difficult. Residual urine in the bladder increases
the chance of urinary tract infections. Prostate hypertrophy is usually
benign, but cancer of the prostate is one of the more common cancers in
elderly men. A TURP is commonly used to correct this problem if the
symptoms do not improve in response to home treatment and medication.

Erectile dysfunction (ED) is another common problem seen in aging males.
In older men, ED usually has a physical cause, such as disease, injury,
or side effects of drugs. Any disorder that impairs blood flow in the
penis or causes injury to the nerves has the potential to cause ED.
Although it is not an inevitable part of aging, incidences increases
with age: About 5 percent of 40-year-old men and between 15 and 25
percent of 65-year-old men experience ED. As discouraging as Erectile
dysfunction may be, it is treatable at any age, and awareness of this
fact has been growing. More men have been seeking help and returning to
normal sexual activity because of improved, successful treatments for
ED.

## Things that can go wrong with the male reproductive system

Boys may sometimes experience reproductive system problems. Below are
some examples of disorders that affect the male reproductive system
(Disorders of the Scrotum, Testicles, or Epididymis). Conditions
affecting the scrotal contents may involve the testicles, epididymis, or
the scrotum itself.

-   **Testicular trauma.** Even a mild injury to the testicles can cause
    severe pain, bruising, or swelling. Most testicular injuries occur
    when the testicles are struck, hit, kicked, or crushed, usually
    during sports or due to other trauma. Testicular torsion, when 1 of
    the testicles twists around, cutting off the blood supply, is also a
    problem that some teen males experience - although it\'s not common.
    Surgery is needed to untwist the cord and save the testicle.

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-   **Varicocele.** This is a varicose vein
    (an abnormally swollen vein) in the network of veins that run from
    the testicles. Varicoceles commonly develop while a boy is going
    through puberty. A varicocele is usually not harmful, although in
    some people it may damage the testicle or decrease sperm production,
    so it helps for you to take your child to see his doctor if he is
    concerned about changes in his testicles.

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-   **Testicular cancer.** This is one
    of the most common cancers in men younger than 40. It occurs when
    cells in the testicle divide abnormally and form a tumor. Testicular
    cancer can spread to other parts of the body, but if it\'s detected
    early, the cure rate is excellent. Teen boys should be encouraged to
    learn to perform testicular self-examinations.

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-   **Epididymitis** is inflammation of the
    epididymis, the coiled tubes that connect the testes with the vas
    deferens. It is usually caused by an infection, such as the sexually
    transmitted disease chlamydia, and results in pain and swelling next
    to 1 of the testicles.

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-   **Hydrocele.** A hydrocele occurs when
    fluid collects in the membranes surrounding the testes. Hydroceles
    may cause swelling of the testicle but are generally painless. In
    some cases, surgery may be needed to correct the condition.

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-   **Inguinal hernia.** When a portion
    of the intestines pushes through an abnormal opening or weakening of
    the abdominal wall and into the groin or scrotum, it is known as an
    inguinal hernia. The hernia may look like a bulge or swelling in the
    groin area. It can be corrected with surgery.

### Disorders of Penis

Disorders of the Penis Disorders affecting the penis include the
following:

-   **Inflammation of the penis.** Symptoms of penile inflammation
    include redness, itching, swelling, and pain. Balanitis occurs when
    the glans (the head of the penis) becomes inflamed. Posthitis is
    foreskin inflammation, which is usually due to a yeast or bacterial
    infection.

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-   **Hypospadias.** This is a disorder in which the urethra opens on
    the underside of the penis, not at the tip.

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-   **Phimosis.** This is a tightness of the foreskin of the penis and
    is common in newborns and young children. It usually resolves itself
    without treatment. If it interferes with urination, circumcision
    (removal of the foreskin) may be recommended.

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-   **Paraphimosis.** This may develop when a boy\'s uncircumcised penis
    is retracted but doesn\'t return to the unretracted position. As a
    result, blood flow to the penis may be impaired, and your child may
    experience pain and swelling. A doctor may try to use lubricant to
    make a small incision so the foreskin can be pulled forward. If that
    doesn\'t work, circumcision may be recommended.

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-   **Ambiguous genitalia.** This occurs when a child is born with
    genitals that aren\'t clearly male or female. In most boys born with
    this disorder, the penis may be very small or nonexistent, but
    testicular tissue is present. In a small number of cases, the child
    may have both testicular and ovarian tissue.

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-   **Micro penis.** This is a disorder in which the penis, although
    normally formed, is well below the average size, as determined by
    standard measurements.

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-   **Sexually transmitted diseases.** Sexually transmitted diseases
    (STDs) that can affect boys include human immunodeficiency
    virus/acquired immunodeficiency syndrome (HIV/AIDS), human
    papillomavirus (HPV, or genital warts), syphilis, chlamydia,
    gonorrhea, genital herpes, and hepatitis B. They are spread from one
    person to another mainly through sexual intercourse.

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-   **Erectile dysfunction.** E.D. is the inability to get or keep an
    erection firm enough for sexual intercourse. This can also called
    impotence. The word \"impotence\" may also be used to describe other
    problems that can interfere with sexual intercourse and
    reproduction, such as problems with ejaculation or orgasm and lack
    of sexual desire. Using the term erectile dysfunction clarifies that
    those other problems are not involved.

### Contraceptive for Men

Vasectomy: In the procedure the vas deferens of each testes is cut and
tied off to prevent the passage of sperm. Sperm is still produced and
stored in crypt sites causing inflammation. Because of this inflammatory
response the immune system acts on them destroying them and then having
antisperm antibodies. This causes a lower possibility if the vasectomy
is reversed to becoming fertile again.

Condoms: A device, usually made of latex, or more recently polyurethane,
that is used during sexual intercourse. It is put on a man\'s penis and
physically blocks ejaculated semen from entering the body of a sexual
partner. Condoms are used to prevent pregnancy, transmission of sexually
transmitted diseases (STDs - such as gonorrhea, syphilis, and HIV), or
both.

## Review Questions

Answers for these questions can be found here

1\. This is needed to make immature sperm mature

:   A\) FHS
:   B\) LH
:   C\) FSH
:   D\) HL

2\. These become engorged with blood in an erection

:   A\) corpora cavernosa
:   B\) fibrous envelope
:   C\) septum pectiniforme
:   D\) integument
:   E\) dorsal veins

3\. The difference between male and female sperm

:   A\) female sperm have a larger head
:   B\) male sperm are lighter
:   C\) female sperm are faster
:   D\) male sperm are weaker
:   E\) A and B
:   F\) C and D

4\. The entire process of sperm formation takes about

:   A\) 5-6 weeks
:   B\) 7-8 weeks
:   C\) 3-4 weeks
:   D\) 9-10 weeks

5\. Hyper Activation occurs when

:   A\) the sperm are introduced into the urethra
:   B\) the sperm are ejaculated into the vaginal canal
:   C\) the sperm begin to interact with the fertilizing layer of an egg
    cell
:   D\) the sperm reach the cervix

6\. It takes sperm \_\_\_\_\_\_\_\_\_\_\_ weeks to travel through the
epididymis

:   A\) 6-8
:   B\) 1-3
:   C\) 2-4
:   D\) 4-6

7\. While singing in the choir, Ben suddenly notices his voice is
constantly cracking. This is caused by

:   A\) androgens
:   B\) LH
:   C\) FSH
:   D\) Ben's inability to sing

8\. In sexual homology, the glans penis in the male is equal to
\_\_\_\_\_\_\_\_\_\_\_\_\_ in the female

:   A\) clitoral hood
:   B\) clitoris
:   C\) clitoral glans
:   D\) clitoral crura

9\. In sexual homology, the \_\_\_\_\_\_\_\_\_\_\_ in the male is equal
to the fallopian tubes in the female

:   A\) testis
:   B\) appendix testis
:   C\) vas deferens
:   D\) seminal vesicle
:   E\) efferent ducts

10\. Joe has a bulge in the groin area that seems to get worse when he
lifts things. This most likely is

:   A\) epididymitis
:   B\) testicular cancer
:   C\) varicocele
:   D\) hydrocele
:   E\) inguinal hernia

## Glossary

**Androgen**: The generic term for any natural or synthetic compound,
usually a steroid hormone, that stimulates or controls the development
and maintenance of masculine characteristics in vertebrates by binding
to androgen receptors. This includes the activity of the accessory male
sex organs and development of male secondary sex characteristics. They
are also the precursor of all estrogens, the female sex hormones. The
primary and most well-known androgen is testosterone.

**Apocrine Glands**: Apocrine sweat glands develop during the early to
mid puberty ages approximately around the age of 15 and release more
than normal amounts of sweat for approximately a month and subsequently
regulate and release normal amounts of sweat after a certain period of
time. They are located wherever there is body hair. These glands produce
sweat that contains fatty materials. Mainly present in the armpits and
around the genital area, their activity is the main cause of sweat odor,
due to the bacteria that break down the organic compounds in the sweat.

**Bulbourethral Glands**: male accessory sex glands that secrete mucus
for lubrication

**Chemotaxis**: Chemotaxis is a kind of taxis, in which bodily cells,
bacteria, and other single-cell or multicellular organisms direct their
movements according to certain chemicals in their environment. This is
important for bacteria to find food (for example, glucose) by swimming
towards the highest concentration of food molecules, or to flee from
poisons (for example, phenol). In multicellular organisms, chemotaxis is
critical to development as well as normal function. In addition, it has
been recognized that mechanisms that allow chemotaxis in animals can be
subverted during cancer metastasis.

**Corpora Cavernosa**: one of a pair of a sponge-like regions of
erectile tissue which contain most of the blood in the male penis during
erection

**Ductus Deferens**: epididymal ducts from each testis converge to form
a large, thick walled, muscular duct

**Ejaculatory Ducts**: two ducts, receive sperm from the ductus deferens
and secretions from the seminal vesicle; the ducts then empty into the
urethra

**Epididymis**: comma shaped and loosely attached to the rear surface of
each testis

**Erectile Tissue**: smooth muscle and connective tissue inside the
penis that contain blood sinuses; large, irregular vascular channels

**Erection**: the penis at its enlarged and firm state; occurs when the
corpora cavernosa become engorged with venous blood

**Flagellum**: the whip-like tail of a sperm, propels the sperm towards
the egg in hopes of achieving fertilization

**Follicle-Stimulating Hormone (FSH)**: hormone that stimulates
production of sertoli cells, to make immature sperm to mature sperm

**Glans Penis**: distal end of the penis, covered with the foreskin

**Gonadotropin-Releasing Hormone (GnRH)**: hormone secreted by the
hypothalamus into the pituitary gland; two types, FSH and LH

**Libido**: In its common usage, it means sexual desire; however, more
technical definitions, such as those found in the work of Carl Jung, are
more general, referring to libido as the free creative---or
psychic---energy an individual has to put toward personal development,
or individuation.

**Luteinizing Hormone (LH)**: hormone that stimulates Leydig cells in
the testes to produce testosterone

**Oviduct**: the passage in females from the ovaries to the outside of
the body.

**Penis**: external genital organ of the male

**Prostate Gland**: male accessory sex gland that secretes an alkaline
fluid, which neutralizes acidic vaginal secretions

**Puberty**: the period of maturation and arousal of the dormant and
nonfunctional reproductive system; usually occurs in males between the
ages of 10 and 15

**Scrotum**: skin covered sac that houses the male testicles; keeps the
testicles away from the body so that they can stay a few degrees cooler
than the body, for better sperm production

**Seminal Vesicle**: male accessory sex glands that supply fructose to
ejaculated sperm and secrete prostaglandins

**Seminiferous Tubules**: highly coiled tubules within the testes that
produce spermatozoa

**Sertoli Cell**: A Sertoli cell (a kind of sustentacular cell) is a
\'nurse\' cell of the testes which is part of a seminiferous tubule.

It is activated by follicle-stimulating hormone, and has FSH-receptor on
its membranes.

Its main function is to nurture the developing sperm cells through the
stages of spermatogenesis. Because of this, it has also been called the
\"mother cell.\" It provides both secretory and structural support.

**Sexual Homology**: sex organs that evolve from the same tissues in
both male and females

**Sperm**: main reproductive cell in males

**Spermatogenesis**: sperm production

**Testes**: located in the scrotum, produces testosterone which
stimulates production of sperm

**Testosterone**: male sex hormone secreted by the leydig cells of the
testes, vital for the production of sperm

**TURP**: transurethral resection of the prostate. During TURP, an
instrument is inserted up the urethra to remove the section of the
prostate that is blocking urine flow. This is most commonly caused by
benign prostatic hyperplasia (BPH). A TURP usually requires
hospitalization and is done using a general or spinal anesthetic. It is
now the most common surgery used to remove part of an enlarged prostate.

**Urethra**: the last part of the urinary tract; in males, it is the
passage for both urine and sperm

**Varicocele**: varicose vein of the testicles, sometimes a cause of
male infertility

**Vasectomy**: most common sterilization procedure in males; small
segment of each ductus deferens is surgically removed after it passes
from the testis

## Summary

Both male and female reproductive systems may seem somewhat isolated
from other body systems in that their purpose is to create new life and
not just to maintain existing life. There are however significant
relationships between the reproductive system and other body systems.
All systems relate in one way or another to help our bodies maintain
homeostasis.

## References

-   \"Essentials of Anatomy and Physiology\" by Valerie C. Scanlon and
    Tina Sanders
-   \"Web MD\": <http://www.webmd.com>
-   Wikibook: Sexual Health

[^1]: *Male Infertility from A to Z: A Concise Encyclopedia*. Google
    books
