The ovarian cycle | Reproductive system physiology | NCLEX-RN | Khan Academy

Summary

The video script delves into the intricacies of the ovarian cycle, a 28-day process central to a female's reproductive system. It details the maturation of eggs from primordial to secondary oocytes, culminating in ovulation around day 14. Hormones like estrogen and progesterone, secreted by granulosa cells, play pivotal roles. The script also touches on the menstrual cycle, follicle development stages, and the transformation of the dominant follicle post-ovulation into the corpus luteum. It concludes with a discussion on menopause, marking the end of ovulation due to ovarian failure.

Takeaways

• 👩‍⚕️ The ovaries produce eggs and hormones, playing a central role in the female reproductive system.
• 🔄 The ovarian cycle is a 28-day process that includes the maturation of eggs and is linked to the menstrual cycle.
• 🥚 Eggs develop within follicles, starting as primordial follicles and maturing through primary and pre-antral stages to become mature follicles.
• 🌡️ Hormone levels rise as follicles mature, with granulosa cells secreting estrogen, progesterone, and inhibin.
• 🌀 The zona pellucida is a layer that forms between the granulosa cells and the oocyte, allowing nutrient exchange while keeping them separate.
• 🌱 The theca is an outer layer of the follicle that responds to LH, contributing to hormone production.
• 💧 The antrum is a fluid-filled space in the follicle that expands, pushing the follicle to grow and eventually ovulate.
• 🏆 The dominant follicle is the largest and most mature, typically the one that ovulates, while others undergo atresia.
• 🕒 Ovulation occurs around day 14 of the cycle, where the mature follicle releases its egg.
• 🌱 Post-ovulation, the remnants of the follicle transform into the corpus luteum, which produces hormones necessary for potential pregnancy.
• 🔚 Menopause occurs around age 50-51, ending the ovarian cycle due to the depletion of follicles and eggs through atresia.

Q & A

What is the primary function of the ovaries in a female's reproductive system?

The ovaries are responsible for producing eggs and are involved in the maturation process known as the ovarian cycle, which can lead to fertilization and pregnancy.

What is the relationship between the ovarian cycle and the menstrual cycle?

The ovarian cycle is responsible for the menstrual cycle, as it involves the maturation and release of an egg, and if not fertilized, the shedding of the uterine lining.

What is a secondary oocyte, and how does it relate to the ovarian cycle?

A secondary oocyte is an egg that has completed the first meiotic division and is ready to be fertilized by a sperm. It is a product of the ovarian cycle, which prepares the egg for potential fertilization.

How do follicles develop within the ovaries?

Follicles develop from primordial follicles, which contain a primary oocyte surrounded by granulosa cells. As they mature, they go through stages including primary follicles, pre-antral follicles, and antral follicles.

What hormones do granulosa cells secrete during the development of follicles?

Granulosa cells secrete estrogen, a small amount of progesterone, and inhibin, which play roles in the regulation of the ovarian cycle and the nourishment of the developing egg.

What is the significance of the zona pellucida in the development of follicles?

The zona pellucida is a layer that forms between the granulosa cells and the oocyte, allowing for the exchange of nutrients and signals through gap junctions while keeping the egg cell separate from the surrounding cells.

How does the theca layer contribute to hormone production during the ovarian cycle?

The theca layer forms around the follicle and contains cells that produce androstenedione in response to luteinizing hormone (LH). This hormone is then converted to estrogen by the granulosa cells and released into the bloodstream.

What is the role of the antrum in follicle development?

The antrum is a fluid-filled space within the follicle that expands, causing the follicle to grow. This expansion is crucial for the development of the dominant follicle, which is the one that will eventually ovulate.

Why is the dominant follicle significant during the ovarian cycle?

The dominant follicle is significant because it is the one that will ovulate, releasing a mature egg for potential fertilization. Other follicles typically undergo atresia, a process of degeneration and death.

What happens to the follicle after ovulation?

After ovulation, the follicle collapses and transforms into the corpus luteum, which produces hormones like estrogen and progesterone that prepare the uterus for possible implantation if fertilization occurs.

What is menopause, and how does it relate to the ovarian cycle?

Menopause is the cessation of ovulation that occurs around age 50 to 51, marking the end of a female's reproductive years. It is associated with a decrease in ovarian function and the ovaries' inability to respond to gonadotropins, leading to irregular and eventually stopped menstrual cycles.

Outlines

00:00

🌱 Ovarian Cycle and Follicle Development

The paragraph introduces the ovarian cycle, a monthly process in a female's reproductive system involving the maturation of eggs in the ovaries. It explains the role of the ovaries in producing eggs and the creation of a secondary oocyte for potential fertilization. The menstrual cycle is also discussed, detailing the development of primary oocytes in the ovaries, their maturation, and eventual ovulation. The paragraph further describes the structure of a follicle, which houses the developing egg, and the function of granulosa cells that surround the egg and secrete hormones such as estrogen, progesterone, and inhibin. The timeline of the ovarian cycle is outlined, highlighting the transition from primordial to primary follicles and the role of hormones in this process.

05:01

🌐 Follicle Maturation and Ovulation

This section delves into the maturation process of follicles, explaining how they develop from pre-antral to mature follicles. It discusses the formation of the antrum, a fluid-filled space that causes the follicle to expand, and the role of the dominant follicle, which is the largest and most likely to be ovulated. The paragraph also covers the process of atresia, where most developing follicles degenerate and die, leading to the loss of 15 to 25 eggs per menstrual cycle. The transformation of the dominant follicle into the mature follicle and its eventual ovulation is described, including the physical changes in the ovary and the release of the egg. The paragraph also touches on the possibility of multiple ovulations leading to twins or higher multiples and the subsequent formation of the corpus luteum from the collapsed follicle.

10:03

🔄 Corpus Luteum and Menopause

The final paragraph discusses the role of the corpus luteum after ovulation, detailing its transformation and the production of hormones like estrogen and progesterone. It explains how these hormones prepare the uterus for potential implantation if fertilization occurs. The paragraph also addresses the function of the corpus luteum in the absence of fertilization, leading to its degeneration. The discussion concludes with an overview of menopause, a natural biological process that marks the end of a female's reproductive years. It explains that menopause is caused by the ovaries' inability to respond to gonadotropins due to the depletion of follicles and eggs through atresia.

Keywords

💡Ovarian cycle

The ovarian cycle refers to the monthly process in a female's reproductive system where eggs mature and are prepared for potential fertilization. This cycle is central to the video's theme as it sets the stage for understanding the reproductive process. The script describes how the cycle involves the maturation of eggs from primordial follicles to secondary oocytes, which can then be fertilized to result in pregnancy.

💡Ovaries

Ovaries are the female reproductive organs that produce eggs (oocytes) and hormones. In the script, the ovaries are highlighted as the starting point of the ovarian cycle, where eggs develop within follicles. They play a crucial role in the menstrual cycle and fertility, as they are responsible for releasing a mature egg for potential fertilization each month.

💡Follicles

Follicles are the structures within the ovaries where eggs develop. The script explains that follicles start as primordial and progress through stages to maturity, with the granulosa cells surrounding the egg playing a vital role in nourishment and hormone production. Follicles are central to the video's discussion of egg development and the ovarian cycle.

💡Granulosa cells

Granulosa cells are a layer of cells surrounding the egg within the follicle. They are essential in the video's narrative as they secrete hormones like estrogen and progesterone, which are critical for the maturation process and the menstrual cycle. The script describes how these cells increase in number as the follicle matures and how they nourish the egg through gap junctions.

💡Meiosis

Meiosis is the cell division process that reduces the chromosome number by half, preparing the egg for fertilization. The script mentions that primary oocytes complete meiosis I just before ovulation, transforming into a secondary oocyte. This process is vital for genetic diversity and the formation of a viable zygote upon fertilization.

💡Menstrual cycle

The menstrual cycle is the monthly cycle of hormonal and physiological changes leading to the shedding of the uterine lining if fertilization does not occur. The video script relates the menstrual cycle to the ovarian cycle, explaining that the hormonal changes during the ovarian cycle result in the menstrual cycle, which includes ovulation and the preparation for a potential pregnancy.

💡Hormones

Hormones such as estrogen, progesterone, and inhibin are chemical messengers that regulate various physiological processes, including the menstrual cycle and ovulation. The script details how granulosa cells secrete these hormones, which in turn influence the development of follicles and the overall reproductive cycle.

💡Zona pellucida

The zona pellucida is a glycoprotein layer that separates the egg from the granulosa cells within the follicle. As explained in the script, despite the presence of the zona pellucida, granulosa cells can still nourish the egg through gap junctions. This structure is crucial for protecting the egg and facilitating fertilization.

💡Theca

The theca is the outer layer of the follicle that forms during the development of the follicle. The script describes how theca cells have receptors for luteinizing hormone (LH) and produce androstenedione, which is then converted to estrogen by granulosa cells. This process is essential for the maturation of the follicle and the production of sex hormones.

💡Corpus luteum

The corpus luteum is a temporary endocrine structure that forms from the remnants of the follicle after ovulation. If the egg is not fertilized, the corpus luteum degenerates, leading to the onset of menstruation. If fertilization occurs, the corpus luteum persists and produces hormones necessary for maintaining pregnancy. The script explains this transformation as a critical step in the reproductive cycle.

💡Menopause

Menopause is the natural cessation of menstruation and the end of a woman's reproductive years, typically occurring around age 50 to 51. The script discusses menopause as the point when the ovaries can no longer respond to gonadotropins, leading to the end of ovulation and the menstrual cycle. This keyword is important for understanding the lifecycle of the female reproductive system as described in the video.

Highlights

The ovaries produce eggs and hormones, undergoing a monthly maturation process known as the ovarian cycle.

The ovarian cycle is responsible for the menstrual cycle and the potential for pregnancy through ovulation.

Eggs develop in follicles, which are structures within the ovaries, starting as primordial follicles.

Follicles contain a primary oocyte surrounded by granulosa cells that secrete hormones like estrogen.

The ovarian cycle lasts approximately 28 days, with ovulation occurring around day 14.

Granulosa cells increase in number as the follicle matures, affecting blood hormone levels.

The zona pellucida forms between the granulosa cells and the oocyte, allowing nutrient exchange.

Theca cells form around the follicle and produce androstenedione, which is converted to estrogen.

Multiple follicles develop simultaneously, but only the dominant follicle typically ovulates.

Atresia is the process where most follicles degenerate, contributing to the loss of eggs over time.

The dominant follicle enlarges due to the expanding antrum filled with fluid from granulosa cells.

The cumulus oophorus is a mound of granulosa cells that forms part of the dominant follicle's development.

Ovulation occurs when the mature follicle's wall breaks down, releasing the egg.

The follicle transforms into the corpus luteum after ovulation, producing hormones to support potential pregnancy.

If fertilization occurs, the corpus luteum persists, preparing the uterus for implantation.

Menopause marks the end of ovulation, caused by the ovaries' reduced response to gonadotropins.

Transcripts

- We're gonna talk about the ovarian cycle.

The ovaries are two structures in a female's

reproductive system that produce her eggs.

Each month her eggs go through a maturation process

called the ovarian cycle, and that cycle creates

a secondary oocyte than can be then fertilized by a sperm

to result in a pregnancy.

The ovarian cycle is also responsible

for what we commonly know as the menstrual cycle.

Basically, the primary oocytes that are destined

to be ovulated will develop in the ovaries,

complete meiosis one just before ovulation,

and then they'll be ejected out of the ovary

as a secondary oocyte to be picked up by the fimbriae

and swept into the uterine tube to hope for fertilization.

So let's start from the beginning.

Inside the ovaries, eggs develop

in structures called follicles, these purple circles here.

And they start off as primordial follicles.

And so what a follicle is-

I'll just blow that up for you-

It's one primary oocyte, so an egg cell,

surrounded by a layer of cells called granulosa cells.

And the granulosa cells develop and become more numerous

as the follicle matures.

Now the granulosa cells also secrete a few hormones.

Estrogen, a little progesterone and some inhibin,

and we'll talk about the functions of those

a little bit later on.

So let's put a timeline on this.

Now the ovarian cycle lasts 28 days.

This is day zero here at the primordial follicle,

where we're going counter-clockwise.

All the way over here, this is day 13.

Here, where the secondary oocyte gets ejected,

or ovulated, that's day 14.

And then the rest of the time spent getting back to

the primordial follicle stage are days 15 through 28.

So now you have an idea of about how long this all takes.

So you remember when we said that

the granulosa cells produce hormones?

Well, as the follicles develop over the first 13 days,

and you can see the changes between the one here

and the one here.

It's got a lot more purple cells around here.

Those are granulosa cells.

So the number of granulosa cells goes up,

and since they produce hormones, what do you think

happens to the hormone levels in the blood?

They go up.

So that's sort of just a general point.

So keep that in mind, but first we'll jump back to these.

We know these are primordial follicles here.

The next stage of development are these guys here,

and these are called primary follicles.

And in the primary follicles, the layers of granulosa cells

and the oocyte, the egg, start to be separated by

this other layer that starts to form between them.

That's called the zona pellucida,

and I'll draw it here in light blue.

And even though the egg I've drawn in blue,

there's still a layer of zona pellucida,

even though the egg is originally drawn in blue

because I wanted to draw the egg in blue.

There's still a layer of zone pellucida around it.

Now even though the zona pellucida is there

separating the granulosa cells from the actual egg,

the granulosa cells can still nourish the egg

through gap junctions that go through the zona pellucida

and into the egg.

Gap junctions are just little passageways

from one cell to another cell where they can

exchange nutrients or other signals.

And actually, through those gap junctions,

the granulosa cells send through little chemicals

that keep those primary oocytes stuck at that

meiosis one stage, 'cause you remember at this point

all of these primary oocytes are stuck in meiotic arrest.

They're not dividing and reducing their

chromosome copy number.

So as we develop from our primordial to our primary

to our next follicle here, called our pre-antral follicle,

and you'll see why it's called that in a minute,

the granulosa cells are actually starting to divide

and become a lot greater in number.

You can see that there's a pretty big difference

in granulosa cell number from our primary follicles

to our pre-antral follicle here.

And remember the granulosa cells are shaded in

in purple here.

So while the granulosa cells are proliferating,

this wall on the outside of the follicle

called the theca starts to form.

Theca cells have receptors for luteinizing hormone

from the anterior pituitary, and when luteinizing hormone,

or LH, binds these theca cells, they produce a hormone

called androstenedione.

And when the thecas get androstenedione, they give it

to the granulosa cells, who then convert it to estrogen

and release it into the blood.

So the blood estrogen levels start to go up at this point.

And so that's what these red and blue bits

running down the middle of the ovary are,

blood vessels, arteries and veins.

And if they look a little bit weird to you, or unusual,

that's just because they're cut in cross-section as well.

Now you might be wondering what an antral refers to,

like what you see in the pre-antral follicle

and this early antral follicle here.

It actually refers to the antrum,

which will be formed in the next step.

This space here is called an antrum.

And the antrum is just basically fluid

that's being produced by the granulosa cells.

And it's that antrum and the fluid in the antrum

that pushes against the edges of the follicle

and causes it to expand.

Now just so you're aware, during this ovarian cycle,

multiple follicles are actually forming.

It's not just this one pre-antral follicle,

and then this one early antral follicle,

and this one mature follicle.

You're getting a lot of these happening at one time.

But only one of the biggest ones is the one that

eventually gets ovulated, because you only ovulate

one egg every 28 days.

And that one that gets ovulated is called

the dominant follicle.

So let's just say that what we're seeing here

is an example of the dominant follicle's development.

Because the rest of the ones that were developing

along this pathway sort of degenerate and die off

in a process called atresia.

So I'll write that at the bottom here.

And atresia just means to degenerate.

So another note.

In the ones that undergo atresia, both the follicle

and the eggs they contain die off.

And that means that a woman loses anywhere between

15 to 25 eggs per menstrual cycle to atresia,

while only one gets ovulated.

So you can kind of imagine how you go from

two to four million eggs when you were born

to having zero after about 35-ish years of ovulation.

It's not just that one egg you lose by ovulation.

You lose quite a few.

So anyway, back to the development of the dominant follicle.

It enlarges mostly due to the expanding antrum,

as I mentioned earlier.

And granulosa cells actually start to form

this bit of a mound here that protrudes into

the middle of the antrum.

This mound of granulosa cells is called

the cumulus oophorus.

As part of the development of the dominant follicle,

the cumulus oophorus and the egg sort of separate together

from the wall of the follicle and float around

in the middle of the antrum, like a little island.

And the follicle increases in size.

So the actual follicle is increasing in size

as it gets filled with more and more fluid

from the granulosa cells.

And the granulosa cells are just producing fluid

as a by-product of their metabolism

and creation of hormones.

Eventually this dominant follicle, which at this point

is called the mature follicle, it starts to balloon

out the side of the ovary, kind of like this.

Just starts to push out against the edge of the ovary.

And then because the edge of the ovary

and the wall of the mature follicle are in such

close proximity, enzymes within the follicle break down

that common wall between them, and the egg pops out

onto the surface of the ovary,

because now this wall is broken down.

And by the way, an enzyme is a protein that

carries out a specific task.

The task here is to break down the wall

between the mature follicle and the ovary,

and that happens on day 14.

So it takes day zero to 13 of build up to get to this event.

When this happens, some women feel a little bit

of pelvic pain.

And actually sometimes, by chance, two or more follicles

reach maturity, and they all pop out.

And that's how you get twins or triplets

or quadruplets or octuplets, when they all pop out

and get fertilized by different sperm each.

Because they're all subsequently swept up

into the uterine tubes where sperm can fertilize them.

So now you have the egg out here,

but what about the old follicle it was in?

The follicle actually collapses a little

and transforms into a structure called the corpus luteum.

And in this transformation the granulosa cells

get a lot bigger and start to produce more estrogen,

progesterone and that other hormone, inhibin,

that we mentioned before.

Just briefly, inhibin lowers the amount of FSH,

follicle stimulating hormone, that comes from

the anterior pituitary.

And it does that because follicle stimulating hormone

actually propagates this whole process of

follicle maturation, as you can imagine from the name.

So if you didn't know this before, these are the

exact follicles that follicle stimulating hormone refers to.

At least in a female.

Anyway, if the egg doesn't get fertilized,

then the corpus luteum reaches a maximum size

in about 10 days.

So that's about day 25, which it's probably sitting at

in this diagram.

And then it degenerates by apoptosis.

That's a process that cells use to sort of

self-destruct and die off.

And here I'm abbreviating corpus luteum as CL,

just so you know what I mean.

But if the egg is fertilized, i.e., it travels

into the uterine tubes and gets fertilized by a sperm,

then the corpus luteum persists, I mean it keeps living,

because we want it to keep producing estrogen

and progesterone.

That's because estrogen and progesterone prepare

the inner lining of the uterus,

that's called the endometrium,

for implantation, which would be really handy

since we have a fertilized egg now that needs to develop.

And that's where it does it, by implanting

in the endometrium of the uterus.

So just a final note.

Ovulation doesn't happen forever.

At about age 50 to 51, females undergo

something called menopause.

First menstrual cycles become less and less regular.

In other words, they don't happen every 28 days

like they do when you're under the age of 50.

And then ultimately, they stop happening entirely.

And that cessation of ovulation is called menopause.

The main cause of menopause is sometimes referred to

as ovarian failure.

Basically the ovaries lose the ability to respond

to signalling hormones from the brain called gonadotropins.

And we know these as LH and FSH.

And this happens because most, or all of the follicles

and eggs have already gone through that process

that we talked about called atresia.

In other words, they've degenerated.