Video Transcript
In this video, we will learn how to
identify structures in the female genital system and state their functions. We will also learn how to describe
the process of formation of egg cells in the ovary through oogenesis. An egg cell, which is sometimes
called an ovum or plural ova, are a female’s gametes, sometimes called her
reproductive cells or sex cells. Did you know that at birth a female
human has approximately a million immature egg cells? By the time she reaches puberty,
this number decreases to about 300,000. But over her reproductive lifetime,
only 300 to 400 of these egg cells will be released.
Gametes like female ova or male
sperm cells are both described as haploid cells. This means that they contain only a
single set of chromosomes, which is half the genetic material of a normal body cell,
often represented as 𝑛. This is because in fertilization
when an egg cell and a sperm cell fuse together, they form a diploid zygote with a
full set of genetic information in its nucleus. This zygote is diploid as its
nucleus contains two full sets of chromosomes, often represented as two 𝑛. In humans, both types of gamete
tend to have 23 chromosomes in total, so the zygote will have 46 chromosomes in
total as it receives one set of 23 chromosomes from each parent.
Let’s look at some of the main
structures in the female genital system, which is sometimes called the female
reproductive system, that may help fertilization to occur. The female reproductive system
typically contains two ovaries, which in their singular form are known as an
ovary. The ovaries are responsible for
releasing egg cells, which, as we learned, are sometimes called ova, and also for
releasing sex hormones. Each ovary is connected to a
structure called the Fallopian tube, or sometimes an oviduct. Each Fallopian tube has a
thumb-shaped opening close to the ovary and are responsible for connecting the
ovaries to the uterus.
When ova are released from the
ovary, they travel along the corresponding Fallopian tube towards the uterus. This diagram shows us a magnified
view of some of the cells that lie in the Fallopian tube and an ovum moving past
them. The pink cells here are called
ciliated epithelial cells, and they have projections called cilia on their surfaces
that sway gently to help waft or move the ovum along the Fallopian tube towards the
uterus. Some other cells called goblet
cells, which can be identified here in blue, will also be present in the ciliated
epithelium of the Fallopian tube. These cells secrete mucus that aid
in the movement of the egg cell. The Fallopian tubes are usually
where a sperm cell will fuse with an egg cell.
If this occurs, then the fertilized
egg cell will develop into a zygote, which then develops into an embryo, which will
need to find somewhere to implant in order to safely grow into a new human. This will take place in an organ
called the uterus. The uterus is a hollow organ in a
female’s pelvis. Let’s keep a checklist to some of
the key adaptations of these organs in the female genital system. One of the key adaptations of a
uterus is that its wall contains many blood vessels that provide it with a rich
blood supply. There is also a smooth muscle in
the wall of the uterus that allows it to contract. The main role of the uterus is to
provide a place for the implantation and growth of an embryo.
We already learned that when a
sperm cell fertilizes an egg cell, it initially forms a cell called a zygote. After the zygote has divided, it
becomes known as an embryo. The uterus not only provides a
place for this embryo to implant in its wall, but it also provides it with a home to
develop safely for the next approximately nine months of pregnancy. There are many ligaments in the
female reproductive system that play a role in supporting the uterus and the other
reproductive organs, partly to help to maintain their position during pregnancy. At the base of the uterus is a
region of tissue called the cervix. The cervix connects the uterus to
the vagina. The vagina is a tube with smooth
muscle in its wall. In humans, it’s around seven
centimeters long, though this will vary as most organs in the human body do.
The vagina extends from the cervix
to the female’s external genitalia, and it’s a structure that’s usually responsible
for receiving sperm from a male’s penis during sexual intercourse. The vagina is a moist environment
that aids this process of sexual intercourse. The vagina is lined with a mucous
membrane that does secrete some mucus, but its moisture is mainly maintained by
lubrication from the glands of the cervix and from two small glands called the
Bartholin’s glands, which are located by the vaginal opening and can secrete fluids
to lubricate the vagina, especially during sexual arousal. The vagina has other helpful
adaptations such as an acidic pH, usually between 3.8 and 4.5, and its own
microclimate of healthy bacteria to help protect it against more dangerous
microorganisms.
Once they have entered the vagina,
most sperm cells will attempt to travel up through the cervix and into the
uterus. From there, sperm cells can access
one of the two Fallopian tubes, so they might be able to find an ovum to
fertilize. Let’s look in more detail at how
some of these structures in the female genital system can change over her lifetime
and on a monthly basis. When puberty begins, a female’s
body begins preparing for the possibility of reproduction by going through physical,
emotional, and mental changes. The changes that occur during
puberty are mainly controlled by the release of hormones such as estrogen and
progesterone, primarily from the ovaries but also from other regions of the
body.
One such change that females tend
to experience upon starting puberty is the beginning of the menstrual cycle. The menstrual cycle describes the
approximately monthly rhythmic changes in the secretions of hormones that cause
changes in turn to the female reproductive system. One change that occurs during the
menstrual cycle is the thickness of the lining of the uterus, which we can see
outlined on this graph as this pink line. The menstrual cycle begins with
menstruation, which is sometimes called a period. You can see this occurring from
approximately day zero to day seven on the graph. The lining of the uterus is
sometimes called the endometrium, and it has a very rich blood supply.
During menstruation, the uterine
lining exits the vagina with blood initiated by sometimes-painful contractions in
the smooth muscle in the uterine wall. The duration of menstruation can
vary considerably between different females and also within the same female during
different times in her life. This also applies to the duration
of the whole menstrual cycle, though the average duration is around 28 days. After menstruation, the uterine
lining begins to build up again as you can see happening from approximately day
seven to day 18. This process is also controlled by
sex hormones, which are primarily released from the ovaries.
When the lining of the uterus has
fully built up, it’s ready to receive a fertilized egg cell in a process called
implantation. The lining of the uterus remains
thick until approximately day 28 when the cycle will repeat again. In order for a sperm cell to
fertilize an ovum successfully, an ovum must first be released from the ovary in a
process called ovulation. Ovulation tends to occur around day
14 of the menstrual cycle. This process is also controlled by
hormones, primarily luteinizing hormone or LH, which is secreted from the pituitary
gland in the brain. This occurs approximately monthly
when the uterine lining is thick so ovulation can lead to the implantation of an
embryo.
If an embryo does implant in the
uterine wall, then approximately nine months later, the smooth muscle in the uterine
wall will play another role in contractions to push the baby out of the uterus and
through the vagina in childbirth. The menstrual cycle continues until
a female reaches menopause when menstruation and ovulation stops. Menopause is also often associated
with many hormonal and physical changes in the human body. Let’s look at how a mature ovum
develops. This process, which is called
oogenesis, occurs before a female is even born while she is still a fetus, which is
the stage of development that follows the formation of an embryo. There are three main phases of
oogenesis: multiplication, growth, and maturation.
First, let’s look at the
multiplication phase. When a female is a fetus between
eight to 20 weeks old, a lot of her cells are dividing and multiplying by
mitosis. The immature ova at this stage are
called primary germ cells, primary ova, or primordial germ cells. Primary germ cells are diploid
cells that will eventually give rise to mature gametes in both males and in
females. As primary germ cells are diploid,
they have a full set of 46 chromosomes. But for simplicity’s sake, in this
diagram we’re going to represent these 46 chromosomes as just four chromosomes. The multiplication of primary germ
cells in this first phase of oogenesis produces diploid cells called oogonia, or a
singular oogonium. We can tell these cells are diploid
as they have a two 𝑛 in brackets following them and they’ve divided from the
diploid primary germ cells by mitosis.
The multiplication phase continues
until a few weeks before birth, at which point most of these oogonia die while the
remaining end to the next stage of oogenesis. The second stage, called the growth
phase, begins in the ovaries while the female is still a fetus. The oogonia increase in size and
replicate their chromosomes to be ready for meiosis. They are now known as primary
oocytes. The primary oocytes begin meiosis
one but this is halted or arrested early in the process during prophase. As the primary oocytes have not yet
completed meiosis one and so have a full set of chromosomes, they are referred to as
diploid cells. The primary oocytes remain dormant
in the ovaries until around 12 years later, when puberty begins.
Sometimes, the stages of oogenesis
are classified according to whether they occur before the birth of a fetus, which is
sometimes called prenatal oogenesis, or after the birth of a fetus. The word prenatal comes from Latin
and it literally means before birth. The word postnatal means after
birth, and this refers to the phase of the oogenesis, the maturation phase, that
follows the growth phase. The maturation phase is triggered
by hormones, and it occurs during ovulation in a female who has started her
menstrual cycle. It includes a process called
meiosis, which you might recall consists of two stages of cell division. Remember that this process started
in the prenatal growth phase of oogenesis but was arrested in meiosis one during
prophase.
In the maturation phase, the
primary oocyte completes its first age of meiosis, meiosis one, which splits its
nucleus to produce two haploid daughter cells. Half of the primary oocyte’s
genetic material goes into a cell called a secondary oocyte, and the other half of
the genetic material goes into a much smaller nonfunctional cell called the first
polar body. As they contain half the genetic
material of a normal body cell, both the secondary oocyte and the first polar body
are haploid cells. As you can see, the polar body is a
much smaller cell containing very little cytoplasm, and it is sometimes called
nonfunctional as it will not develop into a mature ovum. Instead, it effectively acts as the
cell into which the excess chromosomes can be discarded.
Meiosis two, the second stage of
cell division in meiosis, will begin after the completion of meiosis one. But it will stop or be arrested in
metaphase before the first polar body or secondary oocyte can split. Before meiosis two can complete,
ovulation must first occur in which both of these cells are released from the
ovary. Meiosis two resumes during the
process of fertilization to form a mature ovum, only if the secondary oocyte is
successfully fertilized by a sperm cell. When this occurs, the secondary
oocyte will divide a second time, forming a fertilized ovum and another small cell
called a second polar body. The first polar body also divides a
second time, producing two more second polar bodies.
As the genetic material from the
haploid sperm cell and the haploid secondary oocyte have joined together, the
fertilized ovum that they produce is a diploid cell which can now be referred to as
a zygote. Therefore, the final products of
meiosis are three second polar bodies and a fertilized ovum. The three polar bodies that are
produced tend to degrade after fertilization has occurred. But interestingly, in some species,
the polar bodies remain and play a role in the future life cycle events of an
organism.
Let’s summarize the stages of
oogenesis and learn a few more facts about how the secondary oocyte is released by
looking at this process occurring within the ovary itself. This diagram shows us a transverse
section of a developing ovary, with some cells right at the start of oogenesis. You can see that at this stage, the
ovary is made up of lots of small, fluid-filled sacs called follicles, each of which
contains an immature ovum. In a female fetus that’s around age
20 weeks old, these cells will be called primary germ cells, and just a few of them
have been shown in this diagram. The primary germ cells are diploid,
and they divide many times by mitosis to produce diploid oogonia. Many of these oogonia die, but the
few oogonia that remain after the fetus is 20 weeks old will enter the growth
phase.
In the growth phase, these oogonia
develop into primary oocytes which are also diploid cells. The follicle that surrounds the
primary oocytes is called the primordial follicle, and it becomes enriched with
nutrients like proteins and hormones. The primary oocytes then remain
dormant in a female’s ovaries until she reaches puberty around 12 years later. At this point, the maturation phase
begins and it will continue to occur approximately once a month unless a pregnancy
occurs until a female reaches menopause. Each month, usually one primary
oocyte undergoes meiosis one to form a haploid secondary oocyte and a haploid first
polar body. Both of these haploid cells are
contained within a follicle called the Graafian follicle at this stage and they
begin meiosis two.
When ovulation occurs, the Graafian
follicle bursts open, and the secondary oocyte and the polar body are released into
the Fallopian tube. If the secondary oocyte is
fertilized by a sperm cell, meiosis two resumes and completes, forming a mature ovum
and subsequently a diploid zygote. Once the secondary oocyte has left
the ovary, the Graafian follicle transforms into a structure called the corpus
luteum. The corpus luteum is useful in
releasing hormones to control early pregnancy. If the secondary oocyte is not
fertilized, in a few days’ time the corpus luteum will break down, causing the
release of hormones to decrease and triggering menstruation to begin.
As we mentioned earlier, meiosis
two will only resume and complete if the sperm cell fertilizes the secondary
oocyte. But how does fertilization
stimulate the completion of the second meiotic division? Let’s find out.
The Graafian follicle which
surrounded the secondary oocyte and the first polar body while they were still in
the ovary contains many tiny follicle cells. When it’s released from the ovary
at ovulation, some of these follicle cells remain attached to the surface of the
secondary oocyte. This forms a structure called the
corona radiata that surrounds the secondary oocyte. The corona radiata is many cell
layers thick. Let’s take a closer look at the
secondary oocyte and the structure surrounding it. There is a glycoprotein matrix
outside the oocyte cell membrane that contains carbohydrates, proteins, and a
substance called hyaluronic acid. This is often referred to as a
jelly coat or more accurately as the zona pellucida when it’s found in mammals such
as humans.
This jellylike layer must be
penetrated by a sperm sell for fertilization to occur. You may be familiar with the fact
that the head of a sperm cell contains a structure called an acrosome. This acrosome contains enzymes
called hyaluronidases. These enzymes are released from the
acrosome when the sperm cell penetrates the corona radiata, and the enzymes break
down hyaluronic acid in the jelly coat. These enzymes help the sperm cell
to make its way through the outer layers surrounding the secondary oocyte. Though many sperm cells may be
attempting to digest through these outer layers, only one sperm cell will usually
enter the secondary oocyte cytoplasm.
This resumes the process of meiosis
two in the secondary oocyte, forming a mature ovum. And now the nuclei of the sperm
cell and the ovum can fuse together in fertilization to form a diploid zygote. And remember, those second polar
bodies are most likely going to degrade.
Let’s review what we’ve learned in
this video by wrapping up with some key points. The main structures in the female
genital system are the ovaries, the Fallopian tubes, the uterus, the cervix, and the
vagina. The ovaries release hormones and,
during ovulation, egg cells. The egg cell can then travel along
the Fallopian tube towards the uterus. If it’s fertilized by a sperm cell,
the egg cell can develop into an embryo. Oogenesis is the process by which a
primary germ cell develops into one mature egg cell and three small polar
bodies. The three stages of oogenesis are
multiplication, growth, and maturation. And they involve both mitosis and
meiosis. Meiosis two only completes if the
egg cell is fertilized.
