- We're gonna talk about
early embryogenesis. Say you're an egg cell, and
you have this nice, thick outer glycoprotein coat
called the zona pellucida, and you've got your plasma
membrane just inside of that. And a sperm has made its way
through the zone pellucida and managed to get in
through your plasma membrane and merged its genetic
material with yours. You're now called a
zygote, and you'd like to go on to form an embryo. But not much is gonna
happen when you're stuck as a single cell, and so what you've gotta do is divide into multiple cells,
and you've gotta do it fast. In fact, you've gotta do it so fast that you don't have time to grow. So you actually just split into two cells, and this process of
splitting without growth is called cleavage. And you do this a number of times, dividing from two cells to four, and from four cells to eight, all the while staying
within the zona pellucida. So you've gone from 16 cells to 32 cells, and at this point you
look different enough that somebody decides
to give you a new name. So instead of being called a zygote, you're called a morula. And morula is just a Greek
word that means mulberry. And you actually do look a
little bit like a mulberry. So here's a picture of a mulberry from my front yard to prove it. This one's not quite ripe. So you're finished with the cleavage stage of early embryogenesis. You've gone from one
cell and just divided, without growing at all,
into two, four, eight, sixteen and then thirty-two cells. And now more interesting
things start to happen. So you're still stuck
within that zona pellucida. So we'll draw it in here. And I'm not gonna keep track
of the number of cells anymore. I'm just gonna draw them in. But what you'll notice
is I'm drawing them in quite compacted. I'm drawing 'em all right in the middle of the structure here. And they do look like they're a little bit tighter together, and this is a process that is called compaction. The different cells within
the morula start to get closer and closer together. And in fact, the cells start to get a little bit different
from each other, too. You notice that these cells on the outside are a little bit different. I'll draw them in here a
slightly different color. And we can tell them
apart, and this process of being able to tell
cells apart as they become different things is
called differentiation. So here we have two separate
populations of cells. This one on the outside,
we'll call them trophoblasts. And this mass of cells on the inside we'll call embryoblasts. And so you're gonna
continue with your process. We'll move on to the next
stage, and we'll draw our zona pellucida here, and
we'll draw all of our trophoblasts along the outside here. Another interesting thing is happening to those cells in the middle. They're starting to clump even more. In fact, they clump so
much that they all cluster at one end, leaving a little
cavity on the other end. So here you can see your
trophoblasts and your embryoblasts, this mass
of cells in the middle. Some people actually call
that the inner cell mass, and it's left you with this cavity. And that cavity is called a blastocoel. Now again, you're starting
to look a little bit more different than you have in the past, so we're gonna give you a new name. This structure that you've turned into, with a blastocoel and an outer cell ring of trophoblasts is called a blastocyst. In fact, we named this
whole process after you, and after cleavage the process
is called blastulation. So also about this stage in blastulation, your zone pellucida starts
to disintegrate away. Here I'm gonna draw little
bites being taken out of it here as it falls away and disintegrates. And that's gonna be important later on because you can't be stuck
in this thing forever. And the next step in blastulation, you actually just lose it completely. So now we'll draw our
trophoblasts completely naked without a zone pellucida. But more interesting things
are happening to you. Your inner cell mass of
embryoblasts is starting to look a little bit different. So you still have this
rim on the outside here up at the end of your blastocyst. And you still have this
mass here in the middle, but you've developed another cavity, and this cavity is called
the amniotic cavity. And also your inner cell
mass of embryoblasts has started to differentiate more. Now it's got this layer
on the bottom of it here, and the cells in this layer
are called hypoblasts, while the cells in the layer just above it are called epiblasts. Now at this stage, we're
pretty much completely free of our zone pellucida,
and that's gonna be really important for implantation. But that's a discussion for another time. We're gonna focus on
this ball of cells here. And I really want to stress
that this is a ball of cells. It's not flat like we've got it drawn here on the computer screen. It's three-dimensional,
it's spherical in nature, a spherical melon. So think of it like a melon. Here I'm gonna draw a spherical melon. So now I want you to
picture taking a big knife or a machete or something
and just slicing the top off the melon like this. Here we'll erase the top
part of our melon here, and we're left with a flat surface. So now that you've got a
melon with a flat surface, picture yourself taking a
pancake and just putting it right on top of this
flat surface like that. And that's basically what we've got here with our blastocyst and this forming layer of epiblasts and hypoblasts. It's really a pancake. It's not two-dimensional here. It's a three-dimensional disk of tissue. So we're gonna draw this
again, but this time without the outer sphere of trophoblasts. But we're gonna draw our pancake of epiblasts and hypoblasts. And what you can see here is
our pancake has got two layers. So in fact, this is a
very important structure in embryology called the bilaminar disk. And we're gonna get another
look at the bilaminar disk here. So I'll show you, we're
actually gonna draw a little plane through the disk here. And if we take a cut
view through this plane that I've drawn, it looks
a little bit like this. So this is still our
bilaminar disk, but now we're looking at a slice
through the bilaminar disk, as opposed to here, we're
looking at the entire bilaminar disk from the outside. So we're looking at our pancake of our bilaminar disk here, and we notice, we start to see something
forming on the edge of it here. And that something looks
like a little bit of streakiness that kind of splits
our pancake right in two. You can think of it as pouring
a little streak of syrup along the top of the pancake
and dividing it in two halves. So here I'm gonna draw it on
the surface of the pancake, and then over here on
our cut view, you can see that this little streak
happens right about here in the middle as it reaches
the plane of our image. And this streak of syrup on
our pancake has got a name. It's actually called the primitive streak, and the formation of the
primitive streak marks the beginning of the next
stage of early embryogenesis, and that's called gastrulation. Now what that primitive streak actually is is just the site where the
cells in this epiblast layer of our bilaminar disk start to migrate. Here I'll draw the paths
of the migrating cells. They heap themselves up right
at the primitive streak, and then they start
burrowing their way down into this bilaminar disk. And they go out into the hypoblast layer, and they just kind of go
all over the place here. But it all happens from
that primitive streak. So as the cells migrate out
from the primitive streak, we can see that our structure
has changed a little bit. Now instead of having two
layers, one layer of epiblasts and one layer of hypoblasts,
all of those migrating cells have now differentiated even
further, and we're left with a layer of cells on the
top, a middle layer of cells as they go out into the
body of the bilaminar disk, and we still have a lower
layer of cells down here. And so now you can see
instead of the bilaminar disk that we have, we actually
have a trilaminar disk. So we have three layers,
one, two and three. And these are actually
called our germ layers. So we have a single layer on top. And instead of epiblasts,
this layer is now known as ectoderm, and our
middle layer is mesoderm, and our lower layer is endoderm. And cells from these three layers go off to do very important
things, and each layer forms its own specific structures. So that process of the
formation of the three layers is called gastrulation. Once our three layers are
formed, we can move on to the next step in embryogenesis, and the next step is called neurolation. So we'll draw our three germ layers in. We have a layer of ectoderm on the top, in the middle we have a layer of mesoderm, and then on the bottom
we have our endoderm. So this final stage in early embryogenesis is called neurolation. And as you might expect
with "neuro" in the name, we're gonna see some neural
elements formed here. And now we have our trilaminar disk with our three germ layers,
and in the middle of the mesoderm, the central
layer here, we start to have further differentiation of cells. And right in the middle we get this core that starts forming. And I'm gonna draw it as
a little purple dot here, but what that really
is is underneath where the primitive streak
was, mesoderm cells are differentiating into a chord structure, and that's called a notochord. Now in humans, the notochord doesn't go on to do a whole lot. It does form part of the
intervertebral disks, and very rarely it will cause
a tumor called a chordoma. But in general, its main
goal and its main purpose is here in neurolation. Now this differentiated
bit of mesoderm actually induces a change in the ectoderm above it. So here we get a change
within the ectoderm. I'll draw that in as kind of a thickening of the ectoderm right here. And that thickening in
the ectoderm has a name. Since it's kind of
plate-like when we look at it in section here, this is
called the neural plate. Now we have our notochord
formed, and we have our neural plate formed. The next thing that happens... So we'll redraw our three layers here, our ectoderm, our
mesoderm and our endoderm. And here at the bottom
we have our notochord forming within our mesoderm. And right above it
those neural plate cells actually start to dive into the mesoderm. And as those neural plate cells dive in, they start to form a ring structure. And actually, since this is
a three-dimensional thing, this ring is more like a tube. So you can picture this
tube going off into the rest of this pancake
that we've drawn here. And as this neural plate
zips up and dives down into the mesoderm, it becomes
known as the neural tube. Now this isn't a perfect
process, and as the neural tube is zipping up from one end
of our pancake to the other, little cells are breaking
off from the ectoderm and going out into the mesoderm. Now these cells actually have
a very important role as well, and will go off into differentiate into their own special tissues, and these are called neural crest cells. So once we have the
formation of our neural tube and our neural crest cells are
diving off into the tissues and differentiating, and
we have a pretty good idea of what the ectoderm,
mesoderm and endoderm are, our early embryogenesis is complete. So just to recap, we
started off as a zygote, a single cell that was
fertilized by a sperm. Cleavage happened. We didn't grow, but we split
into a whole bunch of cells and developed into a morula. The morula cells started to differentiate, and we developed
trophoblasts on the outside and embryoblasts in the middle. We formed a little cavity and
became known as a blastocyst. After the blastocyst cavity
formed, or the blastocoel, a second cavity called the
amniotic cavity formed, and a pancake of cells across
the sphere became apparent. Those cells continued to differentiate, and we formed a primitive streak. The epiblast cells dove
into the primitive streak and started to differentiate
into our three layers that became our ectoderm,
mesoderm and endoderm. And in the final stage
of neurolation, we had development of a notochord
which induced a neural plate to form, and the neural
plate dove into the mesoderm and formed the neural tube. Neural crest cells associated
with the neural plate also went off into the tissues
to further differentiate.
