Egg, sperm, and fertilization

Visit us (http://www.khanacademy.org/science/healthcare-and-medicine) for health and medicine content or (http://www.khanacademy.org/test-prep/mcat) for MCAT related content.

These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video.

- [Voiceover] Organisms
that reproduce sexually

have got to get their
genes together somehow.

To do this, they package
their genetic material

into specialized cells called sex cells.

This is the sperm cell.

It's the sex cell of the male.

The sole purpose of the sperm,

the entire reason for its existence,

is to transfer the male's genetic material

into the female sex cell or the egg.

So the sperm cell is packed with features

that allow it to fulfill its job.

It's basically a little torpedo.

You can see, just like a torpedo,

it has a pointed head which allows it

to travel in the forward direction.

At the back end it has the tail,

and the tail is just a flagellum,

and as the flagellum spins it
acts like a little propeller.

And then it has a middle section.

Now within that middle section,

wrapped around the base
of the flagellar tail,

are all of these little
organelles called mitochondria.

You can see I'm drawing these mitochondria

wrapped around the flagellar base here.

And mitochondria are organelles

that are responsible for
giving energy to a cell.

And the sperm cell has
a whole bunch packed

right into the base of the flagellum here.

Probably 75 to 100, and they're
quite large mitochondria.

In fact, these mitochondria
are often fused together

to create big organelles.

And the reason is because
in order to propel

the sperm torpedo towards the egg,

it needs a lot of energy.

And that all comes from
these mitochondria.

Now the payload of the
torpedo is here in the head,

and that's the genetic material,
our DNA within the nucleus.

I'll draw it kind of all coiled back here.

And here's the nuclear envelope here.

And just like any regular torpedo,

the sperm torpedo has a
warhead right in the front.

And that warhead is a
little collection of enzymes

called the acrosome.

And the acrosome is gonna be important

later on for fertilization.

But that's pretty much it.

There's a tail at the back
to provide propulsion,

some mitochondria in the middle section

to give energy to the tail,

a head which contains
the nuclear material,

and the acrosome.

This is a pretty bare bones cell.

It's designed to move fast
and to get to the egg.

There are no bells and whistles here.

Now that's in contrast to the egg cell.

Now the first thing you'll notice

is that the egg cell is round,

unlike the torpedo
shape of the sperm cell.

This is not a cell that's
made for active mobility.

The second thing is that
the egg cell is huge

compared to the sperm cell.

It's so big, in fact,
that it's almost visible.

In fact, sometimes it is
visible to the human eye.

Now compared to the sperm
cell, which I'll draw in here,

the egg cell is about
10,000 times more massive.

And similar to the sperm cell,

the egg cell has its
share of genetic material

ready to combine during fertilization.

You can see it here within the nucleus.

And so you may have noticed this

thick outer coating on the egg cell here,

that's a very important structure
called the zona pellucida.

And the zona pellucida is a
thick layer of glycoproteins

that sit on the outside of the egg.

And glycoproteins are basically a protein,

I'll draw a protein here in green,

with a whole bunch of
branching sugar chains

that are coming off of them.

And so what this looks like
is basically a little tree

or a long branching
thing that's growing out

of the edge of the egg cell.

And there's a whole bunch of them,

and they form this very thick
kind of protective layer

that the sperm has to get through.

And the edge of the egg cell
is the actual plasma membrane.

And once the sperm can deliver

its genetic material beyond that,

fertilization has occurred.

Now there's a whole
bunch of other structures

within the egg cytoplasm as well.

And remember this thing is huge.

And I'm gonna draw in a few here in green.

Now what I'm drawing in are
actually more mitochondria.

Now remember the egg cell had
75 to 100 big mitochondria

right at the base of the flagellum

to provide energy to drive locomotion.

Well, the egg cell has mitochondria too.

It's got a lot of other
different organelles as well.

But the egg cell is so large, it's got

somewhere between 100 and 200
thousand mitochondria present.

So keep those mitochondria
in mind, we'll talk

about them a little bit
later in the next section.

So now that you've met the
two major players here,

the sperm and the egg,
or the male and female

sex cells respectively, we can talk

about what happens when they meet.

And that process is called fertilization.

So we'll go ahead and
label our egg down here.

And we'll put a quick
label on this as well.

This is the zona pellucida
that we talked about earlier.

So we'll draw the sperm here
coming in to meet the egg.

We'll draw its tail.

We'll draw its middle section here.

And we'll draw its torpedo-like head here.

Get rid of all this zona pellucida

glycoprotein in the middle here.

Now here are the mitochondria
in the middle section here,

and we have the genetic material payload

of our sperm torpedo here in the back,

and our acrosome here in the front.

Now the first thing that
happens during fertilization

is that the sperm comes into contact

with the zona pellucida.

And the zona pellucida actually binds

to the outside of the sperm,

and that's called sperm binding.

And it's step number one.

Now what happens when the sperm gets bound

to the zona pellucida is
that that sets up a reaction

called the acrosomal reaction.

So step number two is called
the acrosome reaction.

And that little warhead
tip of the sperm torpedo

gets released, and so we have
all of the acrosomal enzymes

that were sitting in the head
that just kind of leak out

into that zona pellucida.

And as those enzymes leak
out, they actually start

to digest away the zona pellucida.

Here you can see I'm kind of eating away

here at those glycoprotein,

and that allows the sperm
head to dive in deeper

towards our plasma membrane.

Now as the sperm gets closer
to the plasma membrane

of the egg, and it comes
in contact right here,

it starts up a process of binding.

The two touch and they come together.

And as they start together,
it causes another reaction,

and that third reaction is
called cortical reaction.

And what I haven't drawn here

is another structure in the egg,

and those structures are right

underneath the plasma membrane,

and they just sit there waiting.

And they wait and their entire job

is to wait for a sperm to bind.

And as soon as a single sperm binds,

they get ejected out into
that zone pellucida as well.

Just like the acrosomal enzymes,

these enzymes that are contained

within the cortical granules also start

eating away at the zona pellucida.

These enzymes eat away, and
they dissolve and chew up

these glycoproteins, but
specifically they chew up

the glycoprotein that
allows sperm to bind.

So at this point we have a
single sperm that's bound,

set off the cortical reaction,

and these cortical granules are released

that chew up all the other places

that more sperm can bind.

So as our other sperm
torpedoes are coming in,

they're just bouncing off.

They hit the glycoproteins,
but the one that they need

to bind to isn't there because
it's all been chewed up

and degraded by these cortical granules.

So that's actually called
a block to polyspermy.

Now that's a very important concept.

Polyspermy is a term that
just means multiple sperm.

And what we don't want is
for more than one sperm

to inject its nuclear material,
its DNA into this egg.

What you'd end up with is an egg

with a single contribution from mom

and then one or two or
three or 100 contributions

or genetic material from dad.

And that would never
work, you'd end up with

all sorts of problems as
the egg started to divide.

Occasionally, that does happen,

and it can result in a zygote that fails.

But for the most part, as
these cortical granules

dissolve away all the
sperm binding glycoproteins

of the zona pellucida, other
sperm just can't get in

and they bounce off as they arrive.

So now we have a sperm that's made its way

all the way to the plasma
membrane of our egg cell.

It's started to bind
to the plasma membrane,

the acrosome is gone,
I'll erase that here.

It's done its job.

The cortical granules have been released,

and they're preventing
other sperm from getting in.

And we start to actually
fuse our plasma membranes

of our sperm cell and our egg cell.

And that allows for this
entire structure to come in.

All of the genetic
material here within the,

all of the genetic
material within the nucleus

of the sperm cell can start to come out

and get released here into the egg.

And once we have fusion
of genetic material,

that is fertilization.

So just to recap, we'll go back to look

at our close-up of our sperm.

We can see that it's a
very mobile structure

evolved basically to get genetic material

from the male to the female egg cell.

It's got a tail that propulses it,

it's got mitochondria that feed it energy,

it's got a head with a
payload of nuclear material

and an acrosome warhead on the tip.

The egg cell is a giant
cell by comparison.

It's got a specialized
layer of glycoproteins

on the outside that have a
bunch of specialized features

and then a bunch of cytoplasm,
including mitochondria.

And then the process of
egg meeting sperm itself

is called fertilization.

Sperm binds to the zona
pellucida, the glycoproteins,

you have an acrosomal reaction,

and then a cortical reaction prevents

more than one sperm getting in.

And then material, the genetic material

of the sperm is transferred.

Now you'll notice here that I drew

the genetic material from
the nucleus coming in.

Now some of you may be wondering,

"Well don't mitochondria have
genetic material as well?"

Well, that's true. Mitochondria
do have mitochondrial DNA.

And potentially, some
of these mitochondria

can get sucked in during that

genetic transfer process as well.

But remember, our egg cell had

100 to 200 thousand mitochondrial copies,

and our sperm cell only had 75 to 100.

Now there's a little bit of debate,

but in the end the male contributes

essentially no mitochondria to the zygote

that's formed after
the egg and sperm fuse.

Now, it could be that
some of those mitochondria

actually do make their way
in and then are degraded.

We're not really sure.

But given just the numbers,

statistically with one
to two hundred thousand

versus only 75 to 100, nearly
all of the genetic material

from the mitochondria is gonna
be from the mother anyway.