Where we left off after the
meiosis videos is that we had
two gametes.
We had a sperm and an egg.
Let me draw the sperm.
So you had the sperm and
then you had an egg.
Maybe I'll do the egg in
a different color.
That's the egg, and we all
know how this story goes.
The sperm fertilizes the egg.
And a whole cascade of events
start occurring.
The walls of the egg then become
impervious to other
sperm so that only one sperm can
get in, but that's not the
focus of this video.
The focus of this video is how
this fertilized egg develops
once it has become a zygote.
So after it's fertilized, you
remember from the meiosis
videos that each of these were
haploid, or that they had--
oh, I added an extra i there--
that they had half the
contingency of the DNA.
As soon as the sperm fertilizes
this egg, now, all
of a sudden, you have
a diploid zygote.
Let me do that.
So now let me pick
a nice color.
So now you're going to have a
diploid zygote that's going to
have a 2N complement of the DNA
material or kind of the
full complement of what a normal
cell in our human body
would have. So this is diploid,
and it's a zygote,
which is just a fancy way of
saying the fertilized egg.
And it's now ready
to essentially
turn into an organism.
So immediately after
fertilization, this zygote
starts experiencing cleavage.
It's experiencing mitosis,
that's the mechanism, but it
doesn't increase
a lot in size.
So this one right here will then
turn into-- it'll just
split up via mitosis
into two like that.
And, of course, these are each
2N, and then those are going
to split into four like that.
And each of these have the same
exact genetic complement
as that first zygote, and
it keeps splitting.
And this mass of cells, we can
start calling it, this right
here, this is referred
to as the morula.
And actually, it comes from the
word for mulberry because
it looks like a mulberry.
So actually, let me just kind
of simplify things a little
bit because we don't
have to start here.
So we start with a zygote.
This is a fertilized egg.
It just starts duplicating via
mitosis, and you end up with a
ball of cells.
It's often going to be a power
of two, because these cells,
at least in the initial stages
are all duplicating all at
once, and then you
have this morula.
Now, once the morula gets to
about 16 cells or so-- and
we're talking about
four or five days.
This isn't an exact process--
they started differentiating a
little bit, where the outer
cells-- and this kind of turns
into a sphere.
Let me make it a little
bit more sphere like.
So it starts differentiating
between-- let me make some
outer cells.
This would be a cross-section
of it.
It's really going to look
more like a sphere.
That's the outer cells and then
you have your inner cells
on the inside.
These outer cells are called
the trophoblasts.
Let me do it in a
different color.
Let me scroll over.
I don't want to go there.
And then the inner cells, and
this is kind of the crux of
what this video is all
about-- let me scroll
down a little bit.
The inner cells-- pick
a suitable color.
The inner cells right there are
called the embryoblast.
And then what's going to happen
is some fluid's going
to start filling in some
of this gap between the
embryoblast and the trophoblast,
so you're going
to start having some fluid that
comes in there, and so
the morula will eventually
look like this, where the
trophoblast, or the outer
membrane, is kind of this huge
sphere of cells.
And this is all happening as
they keep replicating.
Mitosis is the mechanism, so now
my trophoblast is going to
look like that, and then
my embryoblast is going
to look like this.
Sometimes the embryoblast-- so
this is the embryoblast.
Sometimes it's also called the
inner cell mass, so let me
write that.
And this is what's going to
turn into the organism.
And so, just so you know a
couple of the labels that are
involved here, if we're dealing
with a mammalian
organism, and we are mammals,
we call this thing that the
morula turned into is a zygote,
then a morula, then
the cells of the morula started
to differentiate into
the trophoblast, or kind of the
outside cells, and then
the embryoblast. And then you
have this space that forms
here, and this is just fluid,
and it's called the
blastocoel.
A very non-intuitive spelling
of the coel part of
blastocoel.
But once this is formed, this is
called a blastocyst. That's
the entire thing right here.
Let me scroll down
a little bit.
This whole thing is called the
blastocyst, and this is the
case in humans.
Now, it can be a very confusing
topic, because a lot
of times in a lot of books on
biology, you'll say, hey, you
go from the morula to
the blastula or the
blastosphere stage.
Let me write those words down.
So sometimes you'll say morula,
and you go to blastula.
Sometimes it's called
the blastosphere.
And I want to make it very
clear that these are
essentially the same stages
in development.
These are just for-- you know,
in a lot of books, they'll
start talking about frogs or
tadpoles or things like that,
and this applies to them.
While we're talking about
mammals, especially the ones
that are closely related
to us, the stage is the
blastocyst stage, and the real
differentiator is when people
talk about just blastula
and blastospheres.
There isn't necessarily this
differentiation between these
outermost cells and these
embryonic, or this
embryoblast, or this inner
cell mass here.
But since the focus of this
video is humans, and really
that's where I wanted to start
from, because that's what we
are and that's what's
interesting, we're going to
focus on the blastocyst.
Now, everything I've talked
about in this video, it was
really to get to this point,
because what we have here,
these little green cells that
I drew right here in the
blastocysts, this inner cell
mass, this is what will turn
into the organism.
And you say, OK, Sal, if that's
the organism, what's
all of these purple
cells out here?
This trophoblast out there?
That is going to turn into the
placenta, and I'll do a future
video where in a human, it'll
turn into a placenta.
So let me write that down.
It'll turn into the placenta.
And I'll do a whole future video
about I guess how babies
are born, and I actually learned
a ton about that this
past year because a baby
was born in our house.
But the placenta is really
kind of what the embryo
develops inside of, and it's the
interface, especially in
humans and in mammals, between
the developing fetus and its
mother, so it kind of is the
exchange mechanism that
separates their two systems,
but allows the necessary
functions to go on
between them.
But that's not the focus
of this video.
The focus of this video is the
fact that these cells, which
at this point are-- they've
differentiated themselves away
from the placenta cells, but
they still haven't decided
what they're going to become.
Maybe this cell and its
descendants eventually start
becoming part of the nervous
system, while these cells
right here might become muscle
tissue, while these cells
right here might become
the liver.
These cells right here are
called embryonic stem cells,
and probably the first time in
this video you're hearing a
term that you might recognize.
So if I were to just take one of
these cells, and actually,
just to introduce you to another
term, you know, we
have this zygote.
As soon as it starts dividing,
each of these cells are called
a blastomere.
And you're probably wondering,
Sal, why does this word blast
keep appearing in this kind
of embryology video, these
development videos?
And that comes from the Greek
for spore: blastos.
So the organism is beginning
to spore out or grow.
I won't go into the word origins
of it, but that's
where it comes from and that's
why everything has
this blast in it.
So these are blastomeres.
So when I talk what embryonic
stem cells, I'm talking about
the individual blastomeres
inside of this embryoblast or
inside of this inner
cell mass.
These words are actually
unusually fun to say.
So each of these is an
embryonic stem cell.
Let me write this down
in a vibrant color.
So each of these right here are
embryonic stem cells, and
I wanted to get to this.
And the reason why these are
interesting, and I think you
already know, is that there's
a huge debate around these.
One, these have the potential
to turn into anything, that
they have this plasticity.
That's another word that
you might hear.
Let me write that down,
too: plasticity.
And the word essentially comes
from, you know, like a plastic
can turn into anything else.
When we say that something has
plasticity, we're talking
about its potential
to turn into a lot
of different things.
So the theory is, and there's
already some trials that seem
to substantiate this, especially
in some lower
organisms, that, look, if you
have some damage at some point
in your body-- let me
draw a nerve cell.
Let me say I have a-- I won't
go into the actual mechanics
of a nerve cell, but let's say
that we have some damage at
some point on a nerve cell right
there, and because of
that, someone is paralyzed
or there's some nerve
dysfunction.
We're dealing with multiple
sclerosis or who knows what.
The idea is, look, we have these
cell here that could
turn into anything, and we're
just really understanding how
it knows what to turn into.
It really has to look at its
environment and say, hey, what
are the guys around me doing,
and maybe that's what helps
dictate what it does.
But the idea is you take these
things that could turn to
anything and you put them where
the damage is, you layer
them where the damage is, and
then they can turn into the
cell that they need
to turn into.
So in this case, they would
turn into nerve cells.
They would turn to nerve cells
and repair the damage and
maybe cure the paralysis
for that individual.
So it's a huge, exciting area
of research, and you could
even, in theory, grow
new organs.
If someone needs a kidney
transplant or a heart
transplant, maybe in the future,
we could take a colony
of these embryonic stem cells.
Maybe we can put them in some
type of other creature, or who
knows what, and we can turn it
into a replacement heart or a
replacement kidney.
So there's a huge amount
of excitement about
what these can do.
I mean, they could cure a lot of
formerly uncurable diseases
or provide hope for a
lot of patients who
might otherwise die.
But obviously, there's
a debate here.
And the debate all revolves
around the issue of if you
were to go in here and try to
extract one of these cells,
you're going to kill
this embryo.
You're going to kill this
developing embryo, and that
developing embryo had
the potential to
become a human being.
It's a potential that obviously
has to be in the
right environment, and it has
to have a willing mother and
all of the rest, but it does
have the potential.
And so for those, especially, I
think, in the pro-life camp,
who say, hey, anything that has
a potential to be a human
being, that is life and it
should not be killed.
So people on that side of the
camp, they're against the
destroying of this embryo.
I'm not making this video to
take either side to that
argument, but it's a potential
to turn to a human being.
It's a potential, right?
So obviously, there's a huge
amount of debate, but now, now
you know in this video what
people are talking about when
they say embryonic stem cells.
And obviously, the next question
is, hey, well, why
don't they just call them stem
cells as opposed to embryonic
stem cells?
And that's because in all of our
bodies, you do have what
are called somatic stem cells.
Let me write that down.
Somatic or adults stem cells.
And we all have them.
They're in our bone marrow to
help produce red blood cells,
other parts of our body, but the
problem with somatic stem
cells is they're not as plastic,
which means that they
can't form any type of cell
in the human body.
There's an area of research
where people are actually
maybe trying to make them more
plastic, and if they are able
to take these somatic stem
cells and make them more
plastic, it might maybe kill
the need to have these
embryonic stem cells, although
maybe if they do this too
good, maybe these will have
the potential to turn into
human beings as well,
so that could
become a debatable issue.
But right now, this isn't an
area of debate because, left
to their own devices, a somatic
stem cell or an adult
stem cell won't turn into
a human being, while an
embryonic one, if it is
implanted in a willing mother,
then, of course, it will turn
into a human being.
And I want to make one side
note here, because I don't
want to take any sides on the
debate of-- well, I mean,
facts are facts.
This does have the potential
to turn into a human being,
but it also has the potential
to save millions of lives.
Both of those statements are
facts, and then you can decide
on your own which side of that
argument you'd like to or what
side of that balance you
would like to kind of
put your own opinion.
But there's one thing I want
to talk about that in the
public debate is never
brought up.
So you have this notion of when
you-- to get an embryonic
stem cell line, and when I say
a stem cell line, I mean you
take a couple of stem cells, or
let's say you take one stem
cell, and then you put it in a
Petri dish, and then you allow
it to just duplicate.
So this one turns into two,
those two turn to four.
Then someone could take one of
these and then put it in their
own Petri dish.
These are a stem cell line.
They all came from one unique
embryonic stem cell or what
initially was a blastomere.
So that's what they call
a stem cell line.
So the debate obviously is when
you start an embryonic
stem cell line, you are
destroying an embryo.
But I want to make the point
here that embryos are being
destroyed in other processes,
and namely, in-vitro
fertilization.
And maybe this'll be my next
video: fertilization.
And this is just the notion that
they take a set of eggs
out of a mother.
It's usually a couple that's
having trouble having a child,
and they take a bunch of
eggs out of the mother.
So let's say they take
maybe 10 to 30
eggs out of the mother.
They actually perform a surgery,
take them out of the
ovaries of the mother, and then
they fertilize them with
semen, either it might come
from the father or a sperm
donor, so then all of these
becomes zygotes once they're
fertilized with semen.
So these all become zygotes,
and then they allow them to
develop, and they usually allow
them to develop to the
blastocyst stage.
So eventually all of these
turn into blastocysts.
They have a blastocoel in
the center, which is
this area of fluid.
They have, of course, the
embryo, the inner cell mass in
them, and what they do is they
look at the ones that they
deem are healthier or maybe
the ones that are at least
just not unhealthy, and they'll
take a couple of these
and they'll implant these into
the mother, so all of this is
occurring in a Petri dish.
So maybe these four look good,
so they're going to take these
four, and they're going to
implant these into a mother,
and if all goes well, maybe one
of these will turn into--
will give the couple a child.
So this one will develop and
maybe the other ones won't.
But if you've seen John & Kate
Plus 8, you know that many
times they implant a lot of
them in there, just to
increase the probability that
you get at least one child.
But every now and then, they
implant seven or eight, and
then you end up with
eight kids.
And that's why in-vitro
fertilization often results in
kind of these multiple
births, or reality
television shows on cable.
But what do they do with all
of these other perfectly--
well, I won't say perfectly
viable, but these are embryos.
They may or may not be perfectly
viable, but you have
these embryos that have the
potential, just like this one
right here.
These all have the potential
to turn into a human being.
But most fertility clinics,
roughly half of them, they
either throw these away,
they destroy them, they
allow them to die.
A lot of these are frozen, but
just the process of freezing
them kills them and then bonding
them kills them again,
so most of these, the process of
in-vitro fertilization, for
every one child that has the
potential to develop into a
full-fledged human being, you're
actually destroying
tens of very viable embryos.
So at least my take on it is
if you're against-- and I
generally don't want to take a
side on this, but if you are
against research that involves
embryonic stem cells because
of the destruction of embryos,
on that same, I guess,
philosophical ground, you
should also be against
in-vitro fertilization because
both of these involve the
destruction of zygotes.
I think-- well, I won't talk
more about this, because I
really don't want to take sides,
but I want to show that
there is kind of an equivalence
here that's
completely lost in this debate
on whether embryonic stem
cells should be used because
they have a destruction of
embryos, because you're
destroying just as many
embryos in this-- well, I won't
say just as many, but
you are destroying embryos.
There's hundreds of thousands of
embryos that get destroyed
and get frozen and obviously
destroyed in that process as
well through this in-vitro
fertilization process.
So anyway, now hopefully you
have the tools to kind of
engage in the debate around stem
cells, and you see that
it all comes from what we
learned about meiosis.
They produce these gametes.
The male gamete fertilizes
a female gamete.
The zygote happens or gets
created and starts splitting
up the morula, and then it
keeps splitting and it
differentiates into the
blastocyst, and then this is
where the stem cells are.
So you already know enough
science to engage in kind of a
very heated debate.