Hello, and welcome to our lecture on gastrulation.
Now, you know, the most important thing that
ever happens to you is not birth, it is not
marriage, it is not even death, it's gastrulation.
And yet, it might be something that you’re
not really familiar with. So obviously, it’s
going to be very interesting to explore just
what it actually means. So, what we’re going
to do is to look at the formation of something
called the primitive streak, and that gives
rise to the main body axis. At the same
time, we’ll go from having two body layers
to three body layers. Just after this process,
we also have neurulation when the neural tube
begins to form and obviously, this is a key
time in the formation of neural tube defects.
Finally, we’ll look briefly at twinning
and the different varieties of twinning that
you can get. So, First of all, gastrulation.
If we look at the embryos at the moment, we
are studying the second week after fertilization
and we have the formation of the body cavities.
So the upper cavity in green is the amniotic
cavity, and the lower cavity in yellow is
the yolk sac. If you could imagine being a
tiny diver, a boat, a millimeter long swimming
in the green amniotic cavity and looking down
at the floor, what you would see round about
the second week is formation of a little streak
growing in from the margin, growing in from
the edge of the amniotic cavity and it’s
growing in the ectoderm in the upper layer.
That little streak is called the primitive
streak, and that is the formation of the main
body axis. So round about 16 days after
fertilization, we’d expect to see that in
the floor of our amniotic cavity. It doesn’t
look very impressive, but this is actually
the moment of individuation. Up till now, you
could have been twins or triplets or quads,
but after this, then you’re an individual
for the first time and therefore, it’s the
really significant moment in the process of
Also, of course, it determines left and right,
and head and tail. In this whole diagram,
the head is towards the center of this area
in the floor of the amniotic cavity, and the
tail is towards the edge. So, all of these
important axes are laid down at this particular
time. It’s a shallow depression. So if we’re
to section through it, it would appear like
a little trough in the floor of the amniotic
cavity, and it actually grows towards the
midline. It grows towards the center of the
amniotic cavity. Shortly, we’ll look at
a section through the part marked with the
red line in the diagram. At that point, we
could imagine cutting through it and we were
to look at it end on. So, let’s just review
with labels what we’ve been looking at.
You can see the amniotic cavity marked in
green. The yolk sac is marked in yellow. And
in between the two are the primitive ectoderm
and the primitive endoderm, together called
the bilaminar disk. The yolk sac is extending
beneath the bilaminar disk and the amniotic
cavity is extending above the bilaminar disk.
The body stalk is what will be the umbilical
cord in time that’s connecting it to the
rest of the developing embryonic cavities.
We’ll look at that again in more detail
in the lecture on the formation of the placenta.
Then we can see the primitive streak, and
at the very tip of it, a structure called
the primitive node. It’s just the kind of
starting point of the primitive streak. And
we’d labeled a cranial towards the head
and caudal towards the tail. These are the
anatomical terms for those directions in the
body. Now, I said that we are cutting a section
through the red line two slides back.
This is what the section looks like. So, up above,
we have the ectoderm, epiblast that is marked
here, and down below, we have the endoderm
and what happens is that the cells are actually
tracking towards the primitive streak. They
are moving towards the primitive streak from
either side, when they reach the midline,
when they reach the streak itself, a little
depression that you can see, and then what
happens is that they dive down and then migrate
out to the sides. They are actually detached
from the upper epiblast, the ectodermal layer
and then they migrate out again laterally,
and that gives rise to a third layer.
That third layer, helpfully, is called mesoderm,
the in-between derm layer. So we then have
three layers, ectoderm on top, mesoderm in
the middle, and endoderm underneath.
The cells are spreading out from the ectoderm to give
rise to the mesoderm. In the later lecture,
we will look at the different kinds of tissue
that come from these three different body
layers. So, that’s our third layer, the
mesoderm between the primitive ectoderm and
the primitive endoderm.