- [Instructor] What we're
gonna do with this video
is talk about fertilization
and development in human beings
or at least early
development in human beings.
And this right over
here is an actual image
of fertilization about
to happen or happening.
So this right over here is a sperm cell.
One of the many sperm
cells that will come from
a male human being, and
this right over here
is the egg cell that is
inside the female human being
that it is actually fertilizing.
Fertilization is the
sperm cell coming together
with this egg cell and this act,
this is the conception
of what has the potential
to turn into a real organism,
in this case, a human being.
So let's just first
appreciate the scale here.
Just appreciate how
small these things are.
A distance of about that
would be approximately
one-fiftieth of a millimeter.
So this is happening on a
very, very, very small scale.
Now another thing that
you should be appreciate
and something like this occurred
at the beginning of every one us,
is that this is one of maybe
several hundred million sperm
cells are in competition
to be the one that gets to that egg cell
and so you should feel proud of yourself
or at least half of yourself.
You won a pretty big race, think about it,
there's 300 million people
in the United States
that half of you that was from your father
that won a race with the
several hundred million
of the other sperm cells from you father
to get to be that DNA, that combination
of DNA from your father
that will then fuse
with the combination of DNA
that's from your mother in this egg cell.
And so how is that DNA packaged?
Well it's packaged in chromosomes.
Chromosomes are just strips of DNA
and a full set of human
DNA in most of the cells
in your body, you have 46 chromosomes
and it's really 23 pairs of two.
In each pair, you have one from your mom
and one from your dad.
But it turns out that in these sex cells,
and we call these gametes right over here,
let me write this down, these are gametes.
So a sperm cell is the gamete
that comes from your father
and an egg cell, or an ovum, is the gamete
that comes from your mother.
Gametes have half the
number of chromosomes.
Each of these have 23, not 23 pair,
they have 23 chromosomes.
And so when the sperm
and the egg come together
when the egg is fertilized,
then all of a sudden,
you once again have 46
chromosomes or 23 pair.
But what does it look like
as soon as fertilization has occurred?
Well, this right over here
is a picture of a zygote
which is this fertilized,
you could view it
as a fertilized egg,
it's now starting to have
all 46 chromosomes, and
what's interesting here is
you can see the nuclei from the two cells,
they haven't completely fused yet.
These right here, these
are called pronuclei
but they will eventually fuse and that,
when you think about your first day
as a cell, that's when the genetic makeup
that you got form your father
and your mother came together
to be the genetic makeup
that would eventually
give all the information
or most of the information
necessary to construct you.
Now as soon as this happens,
you start having cell division
so the zygote, this zygote right over here
through the process of mitosis,
it will then split into two,
and then split into four,
and you have about a cell
division or two everyday or so.
And so after about two, three days,
you have about eight, then 16 cells,
those 16 cells are called morula,
and then eventually, that keeps splitting
and after about five to nine days,
you have something that looks like this
and you can't really appreciate it
but there's two to three
hundred cells here.
There's two to three hundred set cells
in this sphere and it's
actually hollowed out
on the inside, you can't
fully see it right over here,
but this thing is called a blastocyst.
And then, and notice it's still
roughly on the same scale,
it's a little bit bigger
than that original egg cell
that was fertilized, but
then this will continue
to split and split and split.
And just to appreciate the entire stages
that we're talking about, so when we talk
about fertilization, we're
talking about right over here.
So all these pictures that I'm showing you
are right in this phase,
and then if you go
maybe three, four, five, six, seven days
maybe in that first week, you
get to this blastocyst stage
and so we are right over
here on our timeline.
Now one thing that you
might be intrigued by
is why am I starting this at week two.
Well it turns out that there
is also a gestational age
and gestational age is
something that doctors
and scientists will use as a measure
of how far long this embryo is
and eventually the embryo or the fetus is
and gestational age in particular
is measured from the first day
of the mother's last menstrual cycle
which can be two weeks before
the actual moment of conception
which I am showing right over here.
So that's just a little technical thing
that is interesting and why you see
this roughly two week shift.
But then we get into this phase
where people would
consider it to be an embryo
and near the end of the embryo stage,
you might have the potential human being
to look something like this.
This would be about seven
weeks after conception
and you could see even
here, it is quite small.
This is about one centimeter in length
so about the size of a blueberry.
But you hardly see something
that's starting to look
like a human being or at least a mammal
and one of the fascinating things
about developmental biology
is that if you look at the animal kingdom
especially things that are closely related
to human beings, you will
see even at this stage
things look quite similar
but you already start to see things
that you'd recognize, you
can see arms right over here,
you can see kind of an early ear,
you can see an early eye, nose,
even ribs it looks like, and
so this is already beginning
to resemble a human being
but it is quite small
and if you look at the
development of other mammals
or even things that aren't mammals,
even things like fish, you see things
that look not too different
from this at this stage.
But then once you get into,
you're going into the
10th, 11th, 12th weeks,
then people will call this a fetus.
So right now, this is an
embryo, so this was an egg
being fertilized, zygote,
zygote goes to morula,
goes to blastocyst, you're an embryo,
then from an embryo, you
go into being a fetus.
And so this is an image of a fetus
being connected to the placenta
and the placenta is really the interface
with the mother's body.
And you can see even
here, it is quite small
depending on your screen size,
this might be about the same
scale on my screen right now
this is about five
centimeters in actuality,
so this is about the size of this fetus.
And the boundary between embryo and fetus
isn't super well-defined
that's why you see this
kind of transition point, but
most people would consider it
to be once you get into
about the 12th week,
you're definitely a fetus.
Now some interesting
things right over here
as you see the entire cycle all the way
until you get to the 36th
through the 42nd week
which is when most,
when what is considered
kind of full term when a
baby is ready to come out
but you start to see over
here, this notion of viability
which is roughly a time
where there's a decent chance
that if the baby were
to come out of the womb,
they would survive on their own
and this is just an interesting stat
that there's roughly a 50% survival rate
at around the sixth month.
But the general big idea's here
and it really is mind boggling,
all of us start in this
almost grand race of sperm
between hundreds of
millions of sperm cells
to be the one to fertilize
your mother's egg
and then that one cell, that zygote
that has the genetic information
from both your mother
and father, now that that
egg has been fertilized,
it then keeps replicating until it turns
into a fairly complex
organism that is capable
of making videos and reflecting
about how it developed.
