Video Transcript
In this video, we will learn to
describe the structure of human sex cells: sperm cells and egg cells. We will also explain how
fertilization of these sex cells can occur to create a new human life.
You may be aware that in humans
sexual reproduction can produce new life in the form of offspring, or children. An important part of sexual
reproduction is fertilization, which involves two gametes, otherwise known as sex
cells or reproductive cells. In humans, the gamete of a
biological male is called a sperm cell, and the gamete of a biological female is
called an egg cell or ovum. Each of these gametes contains half
the genetic material of most other body cells.
You may remember that body cells,
which are also known as somatic cells, typically contain 46 chromosomes in
humans. Can you work out how many
chromosomes will be present in each gamete? If you figured out that as they
have half this number, the human gametes contain only 23 chromosomes, great
work!
Fertilization is the process of
combining a male gamete with a female gamete. The fusion of these two sex cells
produces a cell called a zygote with a full set of genetic material. So, how many chromosomes will be
present in this zygote? If you said 46, well done! This is because 23 chromosomes are
contributed from the sperm cell and 23 from the egg cell. This zygote can develop into an
embryo, which might grow into a new human life. To understand how fertilization can
occur, let’s first take a closer look at how egg cells and sperm cells are produced
in the female and male reproductive systems, respectively.
Let’s begin with the reproductive
system of biological females. Egg cells are produced by the
ovaries of the female reproductive system. Typically, only one egg cell is
released by one of the two ovaries during each menstrual cycle. This event is called ovulation and
involves an egg cell being released into a fallopian tube, sometimes called an
oviduct. The egg cell travels along the
fallopian tube, away from the ovary and toward the uterus. The uterus is where an embryo may
implant if this egg cell is fertilized and where it can develop and grow. The cervix separates the uterus
from a muscular tract called the vagina. The vagina extends from the cervix
to the female’s external genitalia.
Now, let’s learn some more detail
about the egg cell itself, which is a large, vaguely spherical cell. In fact, it is one of the largest
human cells, measuring about 0.1 to 0.2 millimeters in diameter, which is roughly
the same size as a single grain of table salt. Its large size is due to its
cytoplasm, which stores large quantities of nutrients to support development after
fertilization.
Let’s review some of the other
structures of the egg cell. The outermost structure surrounding
the egg cell is the corona radiata. The word corona means crown in
Latin, describing how this part of the egg cell consists of cells that surround the
egg cell, like a crown. The corona radiata provides
physical protection to the egg cell while it develops. Moving inward from the corona
radiata, we find the egg cell’s jelly coat. The jelly coat helps to protect the
egg cell and releases chemical signals that attract and guide sperm toward the egg
cell. Importantly, the jelly coat also
ensures that only one sperm cell fuses with the egg cell in fertilization.
Within the cytoplasm is the egg
cell’s nucleus. The nucleus contains the genetic
material that will be inherited by offspring. Remember, as the egg cell is a
gamete, the nucleus only contains half of the genetic material found in body
cells.
Now that we know more about the
female reproductive system and the egg cell, let’s learn about the male reproductive
system and how it can produce sperm cells. In biological males, the penis and
two testes are key organs in sexual reproduction and hang outside the body. The testes are contained in a sac
called the scrotum, and the penis hangs in front of them. The testes are the male sex organs
responsible for producing sperm cells, the male gametes. Within the testes, sperm are
constantly being produced at a very fast rate, up to 1000 per second.
In contrast to the egg cell, the
sperm cell is one of the smallest cells in the human body. When measured from end to end, a
sperm cell is approximately 55 micrometers long, which is only 0.055
millimeters. The structure of sperm cells is
uniquely adapted to accomplish their one major task: to fertilize an egg cell. The structure of a sperm cell
includes a head, a midpiece, and a long tail. The head of the sperm contains a
small amount of cytoplasm and the nucleus, which is where the genetic material is
stored. A structure in the head of the
sperm cell helps break down the protective layers that surround the egg cell. We’ll learn a bit more about this
later.
The midpiece of the sperm cell
contains many mitochondria. These mitochondria release the
energy the sperm cell needs to move through the female reproductive system toward
the egg cell. The tail of the sperm cell allows
it to move, propelling it through the male and then the female reproductive system
and helping it to burrow through the outer layers of the egg cell.
You might be wondering, how do the
sperm cell and egg cell actually meet so fertilization can occur? Well, sexual intercourse is usually
what leads to fertilization. Sometimes, sperm may also enter the
female’s reproductive system and cause fertilization even if sexual intercourse does
not occur. It’s important to note that
fertilization can also be achieved outside the female’s body in a laboratory
setting. Human sexual intercourse involves a
male penis entering a female vagina. It often results in the penis
releasing sperm cells into the vagina through a process called ejaculation. After ejaculation, the tail of
sperm cells help them travel through the female reproductive system toward the egg
cell.
Let’s see how this happens in more
detail. When sperm are released into the
vagina, they swim up toward the cervix. After the sperm pass through the
cervix, they move into the uterus. From the uterus, the sperm then
travel into the fallopian tubes. It is in the fallopian tubes that
sperm might find the egg cell. Of around 500 million sperm
released during ejaculation, only about 200 are likely to reach the egg cell in the
fallopian tube. In contrast to sperm, which are
able to swim from the vagina into the fallopian tubes, the egg cell is not adapted
for movement and cannot move on its own.
So, how do you think it is able to
make its way into the fallopian tube so it can be fertilized? First, fingerlike projections move
the egg cell from the ovary into the fallopian tube. Then, when the egg cell is inside
the fallopian tube, small, thin, hairlike projections called cilia waft the egg cell
along in a sweeping motion. This moves the egg cell closer to
the uterus and to the approaching sperm cells.
To reach an egg cell, sperm have to
swim around 1000 times their own length. The fastest sperm can make this
journey in about an hour, while the slowest sperm can take up to 12 hours. Most of the sperm do not make it to
the egg cell at all.
Let’s take a closer look at the
process of fertilization itself. Although hundreds of sperm cells
will travel through the female reproductive system to reach the egg cell, only one
will be able to fertilize it. Only this single sperm cell can
make it through the corona radiata and the jelly coat of the egg cell. To help with this process, the head
of the sperm secretes special enzymes that dissolve the corona radiata and the jelly
coat. After one sperm cell passes through
the egg cell membrane, the jelly coat immediately hardens, which prevents the
catastrophic outcomes that can occur if more than one sperm cell tries to fertilize
the egg cell.
Once the head of the sperm cell
passes the jelly coat, its nucleus can find the nucleus of the egg cell and fuse
with it. The fusion of the two nuclei is how
the sperm fertilizes the egg cell. You may recall that when the sperm
and egg cells’ nuclei fuse, a zygote forms. After fertilization, the zygote
will continue moving through the fallopian tube toward the uterus and repeatedly
divides to form an embryo. About six days after fertilization,
the embryo implants into the wall of the uterus. Pregnancy is marked as starting
from the first day of the mother’s last period and lasts for approximately 40
weeks.
After the embryo has implanted in
the wall of the uterus around week three, it will remain here for the remainder of
the pregnancy for about 37 weeks, developing in the safety of the uterus. About eight weeks after
fertilization, almost all the major organ systems have formed. Then, the embryo will develop into
a fetus. The fetus grows in size within the
uterus, and its tissues mature. The transition from fetus to baby
occurs about 37 weeks after fertilization when the fetus exits the uterus through
the process of labor and delivery. Let’s apply what we’ve learned
about fertilization to a practice question.
The diagram shows the basic
structure of a sperm cell. How does the tail help the sperm
cell? (A) It helps the sperm swim through
the female reproductive system. (B) It contains the genetic
material needed for fertilization with an egg cell. (C) It helps the sperm catch the
egg cell as they move past each other in the vagina. Or (D) it contains the hormones
needed to stimulate the development of an embryo after fertilization.
Sperm cells are a biological male’s
reproductive cells, which are sometimes called sex cells or gametes. The main function of a sperm cell
is to fertilize a biological female’s gamete, the egg cell. To understand the key function of
the sperm cell’s tail, we need to take a quick look at a biological female’s
reproductive system. For fertilization to occur, first,
an egg cell is released from one of the female’s ovaries. This egg cell then travels along a
fallopian tube toward the uterus. If fertilization does occur, it
will usually happen in the fallopian tube.
When sperm cells move from a male’s
reproductive system into a female’s, the first place they enter is a muscular tract
called the vagina. Keep in mind that though one large
sperm cell has been displayed here, there are usually millions of sperm cells
released from the male reproductive system. And they are much, much smaller
than this one.
You might notice that the vagina
and the fallopian tube are pretty far away from each other. In fact, to reach an egg cell to
fertilize it, sperm have to swim around 1000 times their own length, up through the
vagina and uterus to the fallopian tube. The tail is what allows the sperm
cell to make this journey. So, the correct answer to this
question asking us to work out how the tail helps the sperm cell is (A). It helps the sperm swim through the
female reproductive system.
Let’s recap some of the key points
we have covered in this video about fertilization. The egg cell is a biological
female’s gamete. It is a fairly large, spherical
cell, which consists of several structures: the corona radiata, the jelly coat, and
the cytoplasm, where the nucleus is located. The sperm cell is a biological
male’s gamete. It is a relatively small cell,
consisting of a long tail, a midpiece, and a head that contains the nucleus.
For fertilization to occur, sperm
cells leave a male’s penis and enter a female’s vagina then travel through the
cervix and the uterus to reach the fallopian tubes, where they can attempt to
fertilize an egg cell. Since the egg cell is not capable
of movement, small hairlike projections known as cilia move the egg cell away from
the ovary and through the fallopian tube toward the uterus. Fertilization is when the sperm and
egg cell nuclei fuse to form a zygote. During pregnancy, the zygote
develops into an embryo then a fetus. The transition from fetus to baby
occurs when the fetus exits the uterus through the process of labor and
delivery.
