Video Transcript
In this video, we will learn how to
describe the structures that can be found within the human male testes and their
functions. We will learn specifically how
sperm production occurs within the seminiferous tubules of the testes to produce the
male gametes, sperm, capable of fertilizing an egg cell.
Sperm cells are the male gametes,
which are otherwise known as their reproductive cells or their sex cells. The purpose of a sperm cell is to
fertilize the female gamete, otherwise known as the egg cell in a process known as
sexual reproduction. The male testis is the site of
sperm cell production. Male humans typically have two of
them, which are referred to as plural testes.
Did you know that producing these
sperm cells requires a constant temperature about two to three degrees lower than
the rest of the abdomen? This is proposed by some scientists
to explain why the scrotum, which is the sac of skin that contains these two testes,
is located hanging just outside the abdomen. The cooler temperatures may also be
helpful in preventing mutations in sperm cells. Spermatogenesis is the process by
which a mature sperm cell is produced and developed In these testes. The prefix sperm refers to the
sperm cells themselves that are being produced, and the suffix genesis comes from
the Greek word meaning production, generation, or creation. The testes are also responsible for
producing the most male sex hormones, such as testosterone, of any organ in the
human body. These male sex hormones are
sometimes called androgens. And they’re responsible for
stimulating the development of male sex organs and their secondary sexual
characteristics.
Secondary sexual characteristics
are physical traits that develop when a person reaches sexual maturity during a
period called puberty. In males, some examples might
include deepening of the voice and development of thicker facial and body hair. Let’s take a closer look at the
overall structure of a testis. Seminiferous tubules are structures
that make up the bulk of the testes. These seminiferous tubules are long
coiled tubes that are located throughout the testes and at the site of
spermatogenesis. Once mature sperm cells have been
produced in the seminiferous tubules, they’re transported to this region which is
called the epididymis. The epididymis is responsible for
storing these mature sperm cells until they are needed, at which point they will be
transported into the vas deferens. The vas deferens can then transport
these sperm cells to the urethra, at the end of which sperm is ejaculated out of the
male’s body. Sometimes, this ejaculation might
result in sperm entering a female’s vagina where they might be able to swim towards
an egg cell to fertilize it.
Let’s take a look at the structure
of a seminiferous tubule and the area surrounding it in more detail. This diagram shows the transverse
section of a seminiferous tubule and, in blue, some of the cells surrounding it. Transverse is a word that means
across. And a transverse section is the
view you get when you slice across something like this portion of a seminiferous
tubules at a right angle to its vertical axis. Each seminiferous tubule contains a
channel of space in its center called the lumen. The lumen is used to transport
mature sperm cells like this one to the epididymis for storage. You can see that there are a number
of cells making up the epithelium or the lining of the seminiferous tubules
surrounding this in a lumen. One type of cell that this
epithelium contains are called Sertoli cells.
Sertoli cells provide structural
support and secrete a fluid which functions to nourish and support these sperm cells
during their development. These secretions also provide the
mature sperm cells with a fluid that they can swim through. Sertoli cells also help to regulate
spermatogenesis and might even play a role in the immune defense to help the sperm
cells survive for longer. The rest of the cells in the
seminiferous tubule epithelium tells a story about sperm production. If we observed a transverse section
of a seminiferous tubule under a microscope, we’d be able to see cells at each
different stage of spermatogenesis. The cells produced in the initial
stages of spermatogenesis are located in the outer region of a seminiferous
tubule. And as the stages progress, the
cells move inwards towards the lumen. Spermatogenesis overall converts
diploid primary germ cells into haploid mature sperm cells.
Primary germ cells, which are
sometimes called primordial germ cells, are undifferentiated diploid stem cells. In most organisms, they are present
when the individuals adjust embryos in both the male testes and in the female
ovaries. As this diagram is showing us the
transverse section of a seminiferous tubule in a mature male, we therefore cannot
see any primary germ cells here. Primary germ cells divide and
differentiate to form haploid mature gametes such as sperm cells. The word “diploid” is used to refer
to most body cells that have a full set of chromosomes. In humans, this number is 46
chromosomes, which is sometimes referred to as two n. In vertebrates like humans, the
term haploid is only used to refer to gametes. Gametes have half the number of
chromosomes of a normal body cell, only 23 in humans, and is sometimes referred to
simply as n. Both sperm cells and the female
gamete egg cells are both haploid cells, as when they fuse together in
fertilization, they will form a zygote with a full set of 46 chromosomes.
Like primary germ cells in a
sexually mature male, the outermost cells in the seminiferous tubules, called
spermatogonia, are also diploid. This is because they were formed
through a type of cell division called mitosis. Moving closer towards the lumen, we
can see some haploid spermatids. These cells are haploid, as they
are formed through another type of cell division called meiosis, which halves the
number of chromosomes in a cell. These haploid spermatids can then
differentiate and specialize into haploid mature sperm cells. These mature sperm cells can then
be released into the lumen of the seminiferous tubule to be transported to the
epididymis for storage.
Interstitial cells are any cells
that lie in the spaces between the functional cells of a tissue. So which cells that we can see in
the diagram do you think might be the interstitial cells of the testes? Pause the video and have a
think. If you worked out that it was these
blue cells surrounding the seminiferous tubules, well done! They’re called the Leydig
cells. And they’re the interstitial cells
specific to the testes. As the cells that make up the
seminiferous tubules are the functional cells of the testes, those that surround it
must be the interstitial cells. Can you remember what the other
main role of the testes is? The other main function of the
testes is to secrete androgens like testosterone, and that’s exactly what the Leydig
cells do. As you can see, they’re located in
between seminiferous tubules.
Let’s look at how hormones can
control the process of spermatogenesis by stimulating the cells that are involved
and regulating the process as a whole. Production of mature sperm cells
begins when a male reaches sexual maturity at the onset of a time called puberty and
will then typically occur continually throughout his life. Puberty causes many changes in the
male body that are driven primarily by hormones. Some of these hormones are released
from a gland in the brain called the anterior pituitary gland. One such hormone that is released
in greater quantities at puberty from the anterior pituitary gland is luteinizing
hormone or LH.
The blood then transports LH all
around the body, including to its target tissues in the testes, where LH combine to
receptors on its target cells, the Leydig cells. You might recall that the Leydig
cells are the interstitial cells of the testes responsible for secreting
testosterone. When LH binds to receptors on these
Leydig cells, it stimulates them to carry out this function and release
testosterone. Both testosterone and luteinizing
hormone are essential hormones to the process of spermatogenesis because when either
hormone is absent, spermatogenesis stops.
Meanwhile, in the anterior
pituitary gland, another hormone called follicle stimulating hormone, or FSH for
short, is being released into the bloodstream. FSH has a role in gamete production
in both males and in females. So it’s going to travel to target
tissues in the testes too. In the male reproductive system,
the target cells for FSH are the Sertoli cells. Upon binding to receptors on these
Sertoli cells, FSH can stimulate spermatogenesis. FSH stimulates these Sertoli cells
to produce testosterone receptors, which is sometimes called androgen-binding
proteins. These receptors are presented on
the surface of the Sertoli cells, so the testosterone, which has been produced by
the Leydig cells, combine to them. Both testosterone and FSH help to
regulate spermatogenesis. They do this through their effects
on Sertoli cells by helping the sperm cells to mature and function effectively.
For example, testosterone and FSH
stimulate the secretion of nourishing nutrient filled fluids and the production of
regulatory molecules from the Sertoli cells to aid spermatogenesis. Testosterone also helps the Sertoli
cells support the adjacent developing sperm cells until they are mature enough to
detach from the seminiferous tubule epithelium. A lack of testosterone has been
found to result in sperm cells either detaching when they are immature or not
detaching at all. Testosterone might even be
responsible for completing the process of meiosis, among its various other functions
in spermatogenesis.
Let’s take a look at the stages of
spermatogenesis in more detail and the different cells that each stage produces. There are four main stages to
spermatogenesis: the multiplication phase, the growth phase, the maturation phase,
and the metamorphosis phase. First, let’s look at the
multiplication phase. When the male is still in his
embryonic form, most of the cells in the seminiferous tubules are primary germ
cells, which as we learned earlier are diploid cells. We also know that the male primary
germ cells undergo mitosis. The process of mitosis and
proliferation produces a large number of diploid prospermatogonia. The prefix pro- means before. This can help us remember that
prospermatogonia are produced before birth. Following birth, these
spermatogonia are going to develop into the cells we already learned about earlier,
diploid spermatogonia.
A single one of these cells is
called a spermatogonium. But let’s see what these
spermatogonia might look like in the seminiferous tubules. Spermatogonia will be located in
the outer region of the seminiferous tubule, as they will be the first to develop
from the primary germ cells. As we go through the other stages
in this process, we can add the other types of cell to this diagram.
Next, let’s look at the second
stage of spermatogenesis which is sometimes called the growth phase. During this stage, some
spermatogonia divide by mitosis, and they differentiate into cells called primary
spermatocytes. As you can see, primary
spermatocytes will be located a little bit closer to the lumen of the seminiferous
tubules. And they generally lose contact
with the seminiferous tubules basement membrane.
Can you work out whether these
cells will be diploid or haploid? Pause the video and have a
think. If you said diploid, good job! Mitosis has occurred, so diploid
primary spermatocytes are produced. The third stage of spermatogenesis
is the maturation phase. We’ve already learned that when a
male reaches puberty, there is usually an increase in his testosterone levels. This triggers the primary
spermatocytes which have duplicated their genetic information during the growth
phase to undergo meiosis. You might recall that meiosis
involves two stages of cellular division, meiosis one and meiosis two.
In meiosis one, the primary
spermatocytes becomes secondary spermatocytes. Now, will these cells be diploid or
haploid? Pause the video and have a
think. Secondary spermatocytes are haploid
cells. So these cells have half the number
of chromosomes of primary spermatocytes. In meiosis two, the secondary
spermatocytes divide again to form haploid spermatids. The final stage of spermatogenesis
is the metamorphosis phase. During the metamorphosis phase, the
round haploid spermatids develop tails and other specializations to become mature
haploid sperm cells, otherwise known as spermatozoa or a singular spermatozoon. These mature sperm cells are now
capable of fertilizing a female’s egg cell.
Aside from the formation of a tail,
another specialization that the sperm cells undergo is the formation of a structure
called an acrosome on the sperm’s head, from which hydrolytic enzymes can be
released that might eventually help them to penetrate the egg cell. These mature sperm cells are now
ready to go and can be released into the lumen of the seminiferous tubule to move
towards the epididymis for their final development and storage.
To summarize, spermatogenesis
consists of a multiplication phase where diploid primary germ cells divide many
times by mitosis to form many diploid prospermatogonia which then develop into
diploid spermatogonia after birth. The growth phase then follows,
where spermatogonia divide by mitosis and differentiates to develop into diploid
primary spermatocytes. In the maturation phase, the
primary spermatocytes enter meiosis one which halves their chromosome number to form
haploid secondary spermatocytes. These cells then enter meiosis two
to form haploid spermatids. The spermatids then undergo
metamorphosis and develop into mature haploid sperm cells.
Now it’s time for us to review the
key points that we’ve covered in this video. The human male testes contain
structures called seminiferous tubules, where spermatogenesis or sperm production
occurs. The seminiferous tubules also
contain Sertoli cells that support the developing sperm cells and secrete a fluid
that nourishes them. The testes also contain specific
interstitial cells called Leydig cells that secrete testosterone.
There are four stages to
spermatogenesis: multiplication, growth, maturation, and metamorphosis. Multiplication involves primary
germ cells dividing by mitosis to produce many spermatogonia following the male’s
birth. Growth includes these spermatogonia
dividing by mitosis again and differentiating into primary spermatocytes. Maturation includes primary
spermatocytes dividing by meiosis, first producing haploid secondary spermatocytes
and then haploid spermatids. Metamorphosis converts these
spermatids into mature sperm cells that can be released into the lumen of the
seminiferous tubule.
