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Where we left off after the
meiosis videos is that we had

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two gametes.

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We had a sperm and an egg.

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Let me draw the sperm.

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So you had the sperm and
then you had an egg.

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Maybe I'll do the egg in
a different color.

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That's the egg, and we all
know how this story goes.

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The sperm fertilizes the egg.

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And a whole cascade of events
start occurring.

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The walls of the egg then become
impervious to other

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sperm so that only one sperm can
get in, but that's not the

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focus of this video.

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The focus of this video is how
this fertilized egg develops

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once it has become a zygote.

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So after it's fertilized, you
remember from the meiosis

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videos that each of these were
haploid, or that they had--

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oh, I added an extra i there--
that they had half the

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contingency of the DNA.

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As soon as the sperm fertilizes
this egg, now, all

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of a sudden, you have
a diploid zygote.

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Let me do that.

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So now let me pick
a nice color.

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So now you're going to have a
diploid zygote that's going to

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have a 2N complement of the DNA
material or kind of the

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full complement of what a normal
cell in our human body

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would have. So this is diploid,
and it's a zygote,

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which is just a fancy way of
saying the fertilized egg.

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And it's now ready
to essentially

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turn into an organism.

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So immediately after
fertilization, this zygote

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starts experiencing cleavage.

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It's experiencing mitosis,
that's the mechanism, but it

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doesn't increase
a lot in size.

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So this one right here will then
turn into-- it'll just

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split up via mitosis
into two like that.

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And, of course, these are each
2N, and then those are going

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to split into four like that.

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And each of these have the same
exact genetic complement

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as that first zygote, and
it keeps splitting.

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And this mass of cells, we can
start calling it, this right

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here, this is referred
to as the morula.

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And actually, it comes from the
word for mulberry because

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it looks like a mulberry.

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So actually, let me just kind
of simplify things a little

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bit because we don't
have to start here.

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So we start with a zygote.

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This is a fertilized egg.

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It just starts duplicating via
mitosis, and you end up with a

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ball of cells.

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It's often going to be a power
of two, because these cells,

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at least in the initial stages
are all duplicating all at

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once, and then you
have this morula.

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Now, once the morula gets to
about 16 cells or so-- and

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we're talking about
four or five days.

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This isn't an exact process--
they started differentiating a

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little bit, where the outer
cells-- and this kind of turns

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into a sphere.

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Let me make it a little
bit more sphere like.

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So it starts differentiating
between-- let me make some

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outer cells.

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This would be a cross-section
of it.

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It's really going to look
more like a sphere.

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That's the outer cells and then
you have your inner cells

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on the inside.

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These outer cells are called
the trophoblasts.

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Let me do it in a
different color.

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Let me scroll over.

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I don't want to go there.

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And then the inner cells, and
this is kind of the crux of

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what this video is all
about-- let me scroll

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down a little bit.

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The inner cells-- pick
a suitable color.

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The inner cells right there are
called the embryoblast.

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And then what's going to happen
is some fluid's going

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to start filling in some
of this gap between the

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embryoblast and the trophoblast,
so you're going

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to start having some fluid that
comes in there, and so

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the morula will eventually
look like this, where the

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trophoblast, or the outer
membrane, is kind of this huge

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sphere of cells.

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And this is all happening as
they keep replicating.

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Mitosis is the mechanism, so now
my trophoblast is going to

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look like that, and then
my embryoblast is going

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to look like this.

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Sometimes the embryoblast-- so
this is the embryoblast.

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Sometimes it's also called the
inner cell mass, so let me

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write that.

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And this is what's going to
turn into the organism.

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And so, just so you know a
couple of the labels that are

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involved here, if we're dealing
with a mammalian

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organism, and we are mammals,
we call this thing that the

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morula turned into is a zygote,
then a morula, then

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the cells of the morula started
to differentiate into

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the trophoblast, or kind of the
outside cells, and then

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the embryoblast. And then you
have this space that forms

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here, and this is just fluid,
and it's called the

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blastocoel.

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A very non-intuitive spelling
of the coel part of

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blastocoel.

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But once this is formed, this is
called a blastocyst. That's

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the entire thing right here.

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Let me scroll down
a little bit.

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This whole thing is called the
blastocyst, and this is the

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case in humans.

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Now, it can be a very confusing
topic, because a lot

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of times in a lot of books on
biology, you'll say, hey, you

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go from the morula to
the blastula or the

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blastosphere stage.

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Let me write those words down.

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So sometimes you'll say morula,

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and you go to blastula.

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Sometimes it's called
the blastosphere.

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And I want to make it very
clear that these are

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essentially the same stages
in development.

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These are just for-- you know,
in a lot of books, they'll

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start talking about frogs or
tadpoles or things like that,

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and this applies to them.

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While we're talking about
mammals, especially the ones

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that are closely related
to us, the stage is the

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blastocyst stage, and the real
differentiator is when people

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talk about just blastula
and blastospheres.

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There isn't necessarily this
differentiation between these

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outermost cells and these
embryonic, or this

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embryoblast, or this inner
cell mass here.

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But since the focus of this
video is humans, and really

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that's where I wanted to start
from, because that's what we

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are and that's what's
interesting, we're going to

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focus on the blastocyst.

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Now, everything I've talked
about in this video, it was

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really to get to this point,
because what we have here,

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these little green cells that
I drew right here in the

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blastocysts, this inner cell
mass, this is what will turn

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into the organism.

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And you say, OK, Sal, if that's
the organism, what's

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all of these purple
cells out here?

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This trophoblast out there?

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That is going to turn into the
placenta, and I'll do a future

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video where in a human, it'll
turn into a placenta.

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So let me write that down.

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It'll turn into the placenta.

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And I'll do a whole future video
about I guess how babies

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are born, and I actually learned
a ton about that this

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past year because a baby
was born in our house.

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But the placenta is really
kind of what the embryo

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develops inside of, and it's the
interface, especially in

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humans and in mammals, between
the developing fetus and its

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mother, so it kind of is the
exchange mechanism that

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separates their two systems,
but allows the necessary

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functions to go on
between them.

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But that's not the focus
of this video.

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The focus of this video is the
fact that these cells, which

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at this point are-- they've
differentiated themselves away

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from the placenta cells, but
they still haven't decided

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what they're going to become.

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Maybe this cell and its
descendants eventually start

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becoming part of the nervous
system, while these cells

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right here might become muscle
tissue, while these cells

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right here might become
the liver.

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These cells right here are
called embryonic stem cells,

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and probably the first time in
this video you're hearing a

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term that you might recognize.

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So if I were to just take one of
these cells, and actually,

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just to introduce you to another
term, you know, we

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have this zygote.

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As soon as it starts dividing,
each of these cells are called

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a blastomere.

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And you're probably wondering,
Sal, why does this word blast

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keep appearing in this kind
of embryology video, these

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development videos?

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And that comes from the Greek
for spore: blastos.

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So the organism is beginning
to spore out or grow.

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I won't go into the word origins
of it, but that's

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where it comes from and that's
why everything has

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this blast in it.

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So these are blastomeres.

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So when I talk what embryonic
stem cells, I'm talking about

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the individual blastomeres
inside of this embryoblast or

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inside of this inner
cell mass.

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These words are actually
unusually fun to say.

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So each of these is an
embryonic stem cell.

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Let me write this down
in a vibrant color.

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So each of these right here are
embryonic stem cells, and

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I wanted to get to this.

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And the reason why these are
interesting, and I think you

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already know, is that there's
a huge debate around these.

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One, these have the potential
to turn into anything, that

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they have this plasticity.

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That's another word that
you might hear.

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Let me write that down,
too: plasticity.

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And the word essentially comes
from, you know, like a plastic

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can turn into anything else.

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When we say that something has
plasticity, we're talking

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about its potential
to turn into a lot

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of different things.

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So the theory is, and there's
already some trials that seem

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to substantiate this, especially
in some lower

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organisms, that, look, if you
have some damage at some point

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in your body-- let me
draw a nerve cell.

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Let me say I have a-- I won't
go into the actual mechanics

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of a nerve cell, but let's say
that we have some damage at

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some point on a nerve cell right
there, and because of

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that, someone is paralyzed
or there's some nerve

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dysfunction.

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We're dealing with multiple
sclerosis or who knows what.

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The idea is, look, we have these
cell here that could

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turn into anything, and we're
just really understanding how

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it knows what to turn into.

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It really has to look at its
environment and say, hey, what

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are the guys around me doing,
and maybe that's what helps

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dictate what it does.

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But the idea is you take these
things that could turn to

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anything and you put them where
the damage is, you layer

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them where the damage is, and
then they can turn into the

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cell that they need
to turn into.

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So in this case, they would
turn into nerve cells.

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They would turn to nerve cells
and repair the damage and

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maybe cure the paralysis
for that individual.

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So it's a huge, exciting area
of research, and you could

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even, in theory, grow
new organs.

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If someone needs a kidney
transplant or a heart

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transplant, maybe in the future,
we could take a colony

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of these embryonic stem cells.

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Maybe we can put them in some
type of other creature, or who

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knows what, and we can turn it
into a replacement heart or a

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replacement kidney.

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So there's a huge amount
of excitement about

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what these can do.

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I mean, they could cure a lot of
formerly uncurable diseases

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or provide hope for a
lot of patients who

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might otherwise die.

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But obviously, there's
a debate here.

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And the debate all revolves
around the issue of if you

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were to go in here and try to
extract one of these cells,

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you're going to kill
this embryo.

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You're going to kill this
developing embryo, and that

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developing embryo had
the potential to

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become a human being.

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It's a potential that obviously
has to be in the

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right environment, and it has
to have a willing mother and

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all of the rest, but it does
have the potential.

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And so for those, especially, I
think, in the pro-life camp,

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who say, hey, anything that has
a potential to be a human

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being, that is life and it
should not be killed.

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So people on that side of the
camp, they're against the

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destroying of this embryo.

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I'm not making this video to
take either side to that

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argument, but it's a potential
to turn to a human being.

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It's a potential, right?

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So obviously, there's a huge
amount of debate, but now, now

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you know in this video what
people are talking about when

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they say embryonic stem cells.

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And obviously, the next question
is, hey, well, why

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don't they just call them stem
cells as opposed to embryonic

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stem cells?

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And that's because in all of our
bodies, you do have what

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are called somatic stem cells.

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Let me write that down.

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Somatic or adults stem cells.

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And we all have them.

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They're in our bone marrow to
help produce red blood cells,

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other parts of our body, but the
problem with somatic stem

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cells is they're not as plastic,
which means that they

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can't form any type of cell
in the human body.

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There's an area of research
where people are actually

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maybe trying to make them more
plastic, and if they are able

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to take these somatic stem
cells and make them more

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plastic, it might maybe kill
the need to have these

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embryonic stem cells, although
maybe if they do this too

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good, maybe these will have
the potential to turn into

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human beings as well,
so that could

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become a debatable issue.

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But right now, this isn't an
area of debate because, left

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to their own devices, a somatic
stem cell or an adult

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stem cell won't turn into
a human being, while an

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embryonic one, if it is
implanted in a willing mother,

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then, of course, it will turn
into a human being.

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And I want to make one side
note here, because I don't

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want to take any sides on the
debate of-- well, I mean,

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facts are facts.

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This does have the potential
to turn into a human being,

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but it also has the potential
to save millions of lives.

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Both of those statements are
facts, and then you can decide

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on your own which side of that
argument you'd like to or what

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side of that balance you
would like to kind of

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put your own opinion.

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But there's one thing I want
to talk about that in the

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public debate is never
brought up.

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So you have this notion of when
you-- to get an embryonic

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stem cell line, and when I say
a stem cell line, I mean you

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take a couple of stem cells, or
let's say you take one stem

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cell, and then you put it in a
Petri dish, and then you allow

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it to just duplicate.

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So this one turns into two,
those two turn to four.

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Then someone could take one of
these and then put it in their

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own Petri dish.

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These are a stem cell line.

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They all came from one unique
embryonic stem cell or what

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initially was a blastomere.

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So that's what they call
a stem cell line.

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So the debate obviously is when
you start an embryonic

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stem cell line, you are
destroying an embryo.

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But I want to make the point
here that embryos are being

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destroyed in other processes,
and namely, in-vitro

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fertilization.

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And maybe this'll be my next
video: fertilization.

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And this is just the notion that
they take a set of eggs

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out of a mother.

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It's usually a couple that's
having trouble having a child,

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and they take a bunch of
eggs out of the mother.

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So let's say they take
maybe 10 to 30

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eggs out of the mother.

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They actually perform a surgery,
take them out of the

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ovaries of the mother, and then
they fertilize them with

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semen, either it might come
from the father or a sperm

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donor, so then all of these
becomes zygotes once they're

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fertilized with semen.

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So these all become zygotes,
and then they allow them to

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develop, and they usually allow
them to develop to the

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blastocyst stage.

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So eventually all of these
turn into blastocysts.

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They have a blastocoel in
the center, which is

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this area of fluid.

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They have, of course, the
embryo, the inner cell mass in

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them, and what they do is they
look at the ones that they

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deem are healthier or maybe
the ones that are at least

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just not unhealthy, and they'll
take a couple of these

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and they'll implant these into
the mother, so all of this is

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occurring in a Petri dish.

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So maybe these four look good,
so they're going to take these

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four, and they're going to
implant these into a mother,

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and if all goes well, maybe one
of these will turn into--

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will give the couple a child.

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So this one will develop and
maybe the other ones won't.

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But if you've seen John & Kate
Plus 8, you know that many

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times they implant a lot of
them in there, just to

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00:17:12,349 --> 00:17:14,520
increase the probability that
you get at least one child.

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But every now and then, they
implant seven or eight, and

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then you end up with
eight kids.

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And that's why in-vitro
fertilization often results in

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00:17:21,150 --> 00:17:25,359
kind of these multiple
births, or reality

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00:17:25,359 --> 00:17:27,270
television shows on cable.

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But what do they do with all
of these other perfectly--

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well, I won't say perfectly
viable, but these are embryos.

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They may or may not be perfectly
viable, but you have

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these embryos that have the
potential, just like this one

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right here.

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These all have the potential
to turn into a human being.

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But most fertility clinics,
roughly half of them, they

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either throw these away,
they destroy them, they

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allow them to die.

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A lot of these are frozen, but
just the process of freezing

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them kills them and then bonding
them kills them again,

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so most of these, the process of
in-vitro fertilization, for

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00:18:04,290 --> 00:18:08,740
every one child that has the
potential to develop into a

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00:18:08,740 --> 00:18:10,500
full-fledged human being, you're
actually destroying

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00:18:10,500 --> 00:18:14,730
tens of very viable embryos.

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00:18:14,730 --> 00:18:18,550
So at least my take on it is
if you're against-- and I

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00:18:18,550 --> 00:18:21,750
generally don't want to take a
side on this, but if you are

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against research that involves
embryonic stem cells because

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00:18:33,320 --> 00:18:36,580
of the destruction of embryos,
on that same, I guess,

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00:18:36,580 --> 00:18:40,630
philosophical ground, you
should also be against

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00:18:40,630 --> 00:18:47,360
in-vitro fertilization because
both of these involve the

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00:18:47,360 --> 00:18:49,540
destruction of zygotes.

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00:18:49,540 --> 00:18:52,520
I think-- well, I won't talk
more about this, because I

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00:18:52,520 --> 00:18:54,320
really don't want to take sides,
but I want to show that

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00:18:54,320 --> 00:18:56,850
there is kind of an equivalence
here that's

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00:18:56,850 --> 00:19:00,755
completely lost in this debate
on whether embryonic stem

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00:19:00,755 --> 00:19:02,920
cells should be used because
they have a destruction of

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00:19:02,920 --> 00:19:04,980
embryos, because you're
destroying just as many

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00:19:04,980 --> 00:19:08,000
embryos in this-- well, I won't
say just as many, but

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you are destroying embryos.

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00:19:09,580 --> 00:19:12,720
There's hundreds of thousands of
embryos that get destroyed

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00:19:12,720 --> 00:19:16,210
and get frozen and obviously
destroyed in that process as

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well through this in-vitro
fertilization process.

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00:19:19,120 --> 00:19:22,010
So anyway, now hopefully you
have the tools to kind of

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00:19:22,010 --> 00:19:25,140
engage in the debate around stem
cells, and you see that

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00:19:25,140 --> 00:19:29,870
it all comes from what we
learned about meiosis.

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They produce these gametes.

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00:19:33,200 --> 00:19:35,790
The male gamete fertilizes
a female gamete.

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00:19:35,790 --> 00:19:39,510
The zygote happens or gets
created and starts splitting

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00:19:39,510 --> 00:19:42,490
up the morula, and then it
keeps splitting and it

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00:19:42,490 --> 00:19:45,730
differentiates into the
blastocyst, and then this is

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00:19:45,730 --> 00:19:47,230
where the stem cells are.

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00:19:47,230 --> 00:19:50,320
So you already know enough
science to engage in kind of a

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very heated debate.