Video Transcript
In this video, we will learn how to
describe the structure and function of the gonads, which are otherwise known as the
sex organs, in the male and female human bodies. We’ll be learning about their
endocrine function and releasing hormones and also how the gonads produce the
haploid gametes in males and females, which allow humans to be capable of sexual
reproduction.
Did you know that the female and
male sex organs, otherwise known as the gonads, are responsible for producing one of
the largest and smallest cells in the human body, respectively? These cells are called gametes, and
which one you produce depends on whether you’re a biological male or a biological
female. The female sex organs produce egg
cells, otherwise known as ova, or a singular ovum, which are in theory large enough
to be just visible to the naked eye, measuring about 0.1 millimeters in
diameter. The male sex organs produce
billions of microscopic sperm cells throughout their lifetime, which are the
smallest cells in the male body.
These gametes are referred to as
haploid as they contain half the genetic information of each of the parents and are
responsible for passing this genetic information on to their offspring upon the
fertilization of an egg cell by a sperm cell, which produces a diploid zygote, which
has a full set of genetic information. Production of gametes is not the
only function of the gonads, however. The gonads also function as
endocrine glands. You may recall that an endocrine
gland is a group of specialized cells that secrete hormones directly into the
bloodstream. Hormones are chemical messengers
that are released from these cells in an endocrine gland into the bloodstream. The blood carries these hormones
throughout the body to the target cells in which they are required.
As the gonads are responsible for
producing and secreting a wide range of different sex hormones into the bloodstream
to have different effects on various target organs in the body, they can be referred
to as endocrine glands. Let’s investigate the structure of
the gonads first before we look in more detail of the sex hormones that they produce
and secrete, starting with the female reproductive system. The female gonads are called the
ovaries or, as singular, ovary. You can see from this diagram that
females typically have two ovaries, which are each about the size of a large
grape. The ovaries are primarily
responsible for producing egg cells and female sex hormones. Each ovary is attached to one of
two Fallopian tubes, which are sometimes called oviducts.
The Fallopian tubes lead into the
uterus, which is often used interchangeably with the word womb. However, a womb more accurately
describes the uterus of a pregnant female. The uterus is located in the pelvic
region of the body as you can see in this diagram on the left, so named because of
the pelvis, which is a large bone that sits in the same area and protects the organs
of the reproductive system. The uterus sits above a muscular
tract called the vagina, which is joined to the uterus by a tissue called the
cervix.
High quantities of sex hormones
will begin to be released when a person reaches puberty. Puberty is a stage in adolescence
when humans reach sexual maturity and are capable of reproduction. Puberty is accompanied by a range
of physical and hormonal changes in the human body. And it’s important to note that
though humans may be capable of reproduction at this stage, they are unlikely to be
emotionally or mentally prepared for it.
Let’s take a look at the sex
hormones that begin to be released in large quantities by the female gonads when she
reaches puberty. We can see a magnified view of one
of these gonads, the ovaries, in this diagram on the right. Estrogen is a hormone that is
released from a structure called the Graafian follicle, which is found in the
ovaries and from which a mature egg cell will also be released. Estrogen has many functions, though
one of its main roles is to control the development of female secondary sexual
characteristics. Secondary sexual characteristics,
sometimes simply known as secondary sex characteristics, are physically observable
traits that develop during puberty but are not directly involved in
reproduction.
Secondary sex characteristics can
be similar in males and females, for example, growth of pubic hair or armpit
hair. But they often physically
distinguish the two biological sexes; for example, females tend to develop breasts
and often their hips widen slightly. The beginning of the menstrual
cycle, which we’ll explore in more detail later on in the video, is also often
referred to as a secondary sexual characteristic. The development of these
characteristics are mainly regulated by the secretion of sex hormones. But it’s important to note that
typically male and typically female sex hormones are actually released in both
biological sexes but usually in different quantities.
Another function of estrogen is to
control ovulation during the menstrual cycle. This is the process we just
observed by which an egg cell is released from the Graafian follicle in the
ovary. From the ovary, the egg cell will
travel into the Fallopian tube, where it may or may not be fertilized by a sperm
cell. Another typically female sex
hormone is progesterone. Once the egg cell has been released
from the Graafian follicle during ovulation, the Graafian follicle degrades into a
structure called the corpus luteum which is responsible for producing and secreting
progesterone. Progesterone is also released from
a structure called the placenta, which develops during pregnancy. The placenta is a temporary
endocrine organ that surrounds a growing fetus in the uterus of a pregnant
female.
Let’s erase some of this
information so we can look at the role of progesterone a bit more closely. The main role of progesterone is to
maintain a thick uterine lining. So let’s see how this works. If an egg cell is successfully
fertilized by a sperm cell in the Fallopian tube, the zygote that’s formed tends to
divide many times on its journey into the uterus and becomes known as an embryo. Progesterone, which has been
released by the corpus luteum in the ovary, maintains a thick uterine lining as you
can see in the diagram. This provides the perfect
environment for the embryo to implant into the uterine wall, which is sometimes
called the endometrium, where the embryo can safely develop into a fetus.
If fertilization does occur,
progesterone concentrations remain high throughout pregnancy to maintain a thick
lining of the uterus and provide suitable conditions for the fetus to grow into a
baby. The levels of progesterone and
estrogen tend to fluctuate during a female’s menstrual cycle when she begins
puberty. The duration of the menstrual cycle
can change, and it differs greatly between different females, but a whole cycle
tends to last about 28 days from the start of one period to the next.
Let’s see how the uterine lining
changes in thickness over a typical menstrual cycle and how the levels of estrogen
and progesterone change by looking at them all on a graph. The typical day on which certain
events occur during the menstrual cycle is shown on the 𝑥-axis. And the section in red shows how
the thickness of the uterine lining changes over this approximately 28-day
cycle.
In this diagram, we can see what it
might look like at day zero. Let’s add in a section on this
graph above the changes in the uterine lining to show how the levels of the ovarian
hormones, progesterone and estrogen, change during the menstrual cycle. We can show the levels of estrogen
with an orange line and the level of progesterone with a pink line. From approximately day zero to
seven, the uterine lining sheds, and so its thickness decreases via a process called
menstruation or sometimes a period, which marks the start of the menstrual
cycle. As the lining of the uterus has a
rich blood supply, when a female menstruates, the uterine lining exits the vagina
with blood. Though in the graph it is shown to
be about seven days in duration, the length of the period, or menstruation, will
vary from female to female.
It’s accompanied by low levels of
progesterone, which remain low until approximately day 14. At this point, ovulation occurs,
which you might remember means the Graafian follicle has just released a mature egg
cell. The levels of estrogen are also
lower at the start of the menstrual cycle, but they increase steadily up until the
ovulation date. This is because, as you might
recall, the Graafian follicle is responsible for releasing estrogen. As the Graafian follicle degrades
shortly after ovulation, it can no longer release estrogen, so estrogen levels drop
suddenly.
Following ovulation, the uterine
lining begins to build up again and the Graafian follicle degrades into the corpus
luteum, which, as you might recall, secretes progesterone. So progesterone levels will
increase after ovulation, and these high levels of progesterone will maintain a nice
and thick uterine lining prepared for if an embryo implants. The levels of estrogen will also
increase somewhat during this time period. And the levels of estrogen and
progesterone remain high until approximately day 28 when the levels of both hormones
drop and the menstrual cycle begins again. Relaxin is a hormone that, like
progesterone, is released primarily from the corpus luteum in the ovaries and also
from the placenta and the uterine lining during pregnancy.
Though it is a female sex hormone,
relaxin is also released in small quantities from the male prostate gland, so can
sometimes be detected in male semen. Relaxin, as its name suggests,
plays an important role in relaxing parts of the female reproductive system at
different times. For example, relaxin has been shown
to help the walls of the uterus relax at certain points during the menstrual cycle
when an egg is likely to implant, preparing the body for pregnancy by aiding the
implantation process. If implantation does occur, relaxin
is usually released in higher concentrations during pregnancy. Relaxin has been shown to play a
role in the growth of the placenta and is especially important to prepare a female’s
body for childbirth by helping to open the cervix, soften the surrounding tissue,
and relaxing the pelvic ligaments to aid the delivery of the child.
Let’s take a look at the main
structures in the male reproductive system next. The male gonads are called the
testes, or a singular testis, one of which you can see in this diagram showing a
side view of the male reproductive system. Males typically have two testes
which are primarily responsible for producing sperm cells and male sex hormones. Let’s take a quick look at the
route that these sperm cells take through the male reproductive system after they’ve
been produced by the testes. One sperm has been produced and it
is stored in a region called the epididymis, which is just outside the testes. This green arrow shows the
direction of sperms travel. Sperm is stored in the epididymis
until ejaculation is anticipated, for example, during sexual arousal.
Each testis is attached to a tube
called the vas deferens. The vas deferens carry sperm cells
from the epididymis to another tube called the urethra. On this journey, the sperm cells
travel through a gland called the prostate gland, which has been labeled here in
blue and sits just behind the bladder. The other blue structures are also
glands such as the seminal vesicles and the Cowper’s gland. The pink arrows show how fluids
which are produced in each of these glands are added to the sperm cells as they
travel past them through the urethra. The fluids added to sperm by the
prostate gland, seminal vesicles, and Cowper’s gland all help to make up semen. The semen which contains these
sperm cells then continues to travel along the urethra and into the penis from which
it can be ejaculated from the male’s body.
Let’s take a look at the endocrine
functions of the testes next and their role in secreting male sex hormones. Male sex hormones are sometimes
called androgens. The word androgen contains the
suffix andro-, which means man. In spite of this, androgens are
released in both males and females. The main androgen which is released
from the testes is called testosterone. The role of testosterone is to
stimulate the growth and development of the male sex organs, which is sometimes
called their primary sexual characteristics. And testosterone also develops the
secondary sexual characteristics.
Androsterone is another androgen
secreted by the testes and is formed by the breakdown of testosterone. Androsterone has similar effects to
testosterone, but they’re slightly weaker effects. For this reason, testosterone is
sometimes referred to as more potent than androsterone. As though they both have similar
effects, testosterone’s effects will be stronger. Let’s take a look at the secondary
sexual characteristics that can develop as a result of testosterone and androsterone
being secreted in high quantities when a male reaches puberty. Males tend to develop more and
thicker facial and body hair during puberty than females do as a result of these
high androgen concentrations. Males also tend to develop more
skeletal muscle mass than females do and their voices break and become considerably
deeper.
It’s interesting to come back to
the fact that though they are called male and female sex hormones, both of the
biological sexes will be releasing all of these hormones just in different
quantities. For example, males still produce
small volumes of estrogen in their testes and from their adrenal glands which sit on
top of their kidneys. And as we’ve mentioned, females
will also produce some androgens in their ovaries and adrenal glands. A fine balance of male and female
sex hormones are important for both sexes to go through sexual growth and
development. Let’s see how much we’ve learned
about the gonads by having a go at a practice question.
At what stage in life do the
majority of secondary sexual characteristics most commonly appear? During infancy, during adulthood,
during puberty, or during old age.
Let’s approach this question by
first defining the key term secondary sexual characteristic so we can figure out
when they will appear in humans. Secondary sexual characteristics
are physically observable traits that develop during a period in adolescence called
puberty and are not directly involved in reproduction. Secondary sexual characteristics
can be similar in both biological males and females, for example, growth of pubic
hair, but they often physically distinguish males from females. For example, usually only females
develop breasts that would be capable of producing milk after childbirth to feed a
newborn baby.
The release of high quantities of
sex hormones triggers the development of these secondary sexual characteristics. And different concentrations of
typically male and typically female sex hormones will determine the different
characteristics that appear in the two biological sexes. The release of high quantities of
these sex hormones begins to occur during a stage called puberty. Puberty is a stage in adolescence
when humans reach sexual maturity and become capable of sexual reproduction even if
they might not be emotionally or mentally ready for it yet. Therefore, the life stage at which
the majority of secondary sexual characteristics most commonly appear is during
puberty.
Now it’s time for us to review some
of the key points that we’ve covered in this video. The gonads are responsible for
producing sex hormones and gametes, which are otherwise known as sex cells. The female gonads are called the
ovaries, and the female gametes they release are called egg cells. The ovaries also release sex
hormones such as estrogen, progesterone, and relaxin. While estrogen is mainly
responsible for controlling the development of female primary and secondary sexual
characteristics, progesterone helps to prepare the female body for pregnancy by
maintaining a nice and thick uterine lining. And finally, relaxin helps to
prepare the female body for implantation of a fertilized egg, pregnancy, and
childbirth.
The gonads in males are called the
testes, and the gametes they release are called sperm cells. The testes also release male sex
hormones which are called androgens and include testosterone and the weaker version
androsterone. Androgens are responsible for
controlling the growth of the male reproductive glands or their primary sexual
characteristics and also the development of their secondary sexual
characteristics.