Hello, and welcome to this lecture on the
derivatives of the three germ layers.
What we’ll be looking at is the formation of each
of the following: the ectoderm, the mesoderm,
and the endoderm. We will look briefly at tissue
signalling between the different layers
that gives rise to different structures of the body,
and some interactions between tissues
themselves. Finally, we’ll note that there
are some implications for cancer. So let’s
go back to the beginning, to the stage of
gastrulation when the main body axis is
formed. It has been described that it’s
the most important thing that ever happens
to you. In our diagram, in the upper image,
we have the amniotic cavity marked in green,
and beneath that, the yolk sac marked in yellow.
Between the two of them is a bilaminar disc
made up of ectoderm on the top, and endoderm
in the bottom. If you were inside the amniotic
cavity looking down, in the floor, you would
see the primitive streak start to form.
So, you can see that marked A and B in the diagram,
roundabout day 16 after fertilization.
This is a key moment. It’s the moment of individuation.
From now on, you could no longer be twins
in any form. It also determines the main body
axis. So it establishes left and right,
and the head is towards the center of the
disc, and the tail is towards the margin of
the disc. But also, crucially, for our purposes
today, it leads to the formation of a third
germ layer in between the ectoderm and endoderm
and that third layer is the mesoderm.
What will happen is that the ectoderm on the top
surface will begin to migrate towards the
midline. So moving towards the middle, and
if we saw that in the section where we have
the ectoderm or epiblast on the top and the
endoderm in the bottom, as the cells move
towards the midline. As they reach that midline,
they form bottle cells, they change their
shape, and then they detach and fall off
and then they will migrate
laterally out to the sides.
So, forming that third mesodermal layer in
between the ectoderm and endoderm, a bit like
the filling in a sandwich. So, spreading out
from either side to give rise to the mesoderm.
That primitive mesoderm will give rise to
a variety of other tissues subsequently.