Hello, and welcome to the slideshow in which
we’ll look at embryo folding and development
of the embryonic cavities and the placenta.
Now, I should say at the beginning that this
is quite difficult to understand. It probably
took me several years to get my head around
it, and you may have to watch it several times
before it makes complete sense for you.
What we’re going to do is to look at, first of
all, the folding of the embryo, and then how
the extra-embryonic cavities and membranes
develop. And then finally, we’ll look at
the development and the function of the placenta.
But we’ll have to begin right back at the
very beginning, at the point of implantation.
In this diagram, we can just see the pre-embryo
beginning to implant under the uterine wall.
The outer cells, the trophoblast, as we described
in an earlier lecture, actually begins to
form syncytiotrophoblast, which is lost at
cellular boundaries, and you have nuclei in
a space surrounded by cytoplasm, and this
is eating its way into the uterine wall. We
can see that the primitive ectoderm here marked
in red, and primitive endoderm marked in yellow
have already formed from the inner cell mass.
The uterine wall has been prepared to receive
this, otherwise, as we said previously, this
would be quite dangerous as it implanted into
the surface. Two days later, it has almost
completely disappeared under the surface,
and the primitive ectoderm marked in red here
has formed a cavity, and that is going to
be the amniotic cavity. That kind of the thing
to focus on if you follow the amniotic cavity
all the way through, that’s probably the
most important. The other ones tend to come
and go, but the amniotic cavity is going to
be really important later on. So at this
point, it sunk completely under
the surface. Blood vessels are being engulfed
by the syncytiotrophoblast and maternal blood
is in direct contact with the syncytiotrophoblast.
So nutrients are beginning to make their way
to the developing pre-embryo. As we said,
the inner cell mass has given rise to the
amniotic cavity, and just where the embryo
sunk under the surface, there’s a little
plug or scar which marks where it is. It's quite hard
to see even if you’re looking inside the
uterus at this point. Little later still, 11
days after fertilization,
now sunk completely under the surface, the
amniotic cavity is still visible.
The yolk sac is formed from the primitive endoderm,
and we could begin to see little villi, little
projections from the trophoblast marked in dark brown
here, projecting into the syncytiotrophoblast.
These villi are increasing the surface area
of the developing conceptus so that nutrients
can flow from the mother. So growth, from now
on, can be relatively rapid.
The primitive endoderm has formed that primitive yolk
sac and the primary stem villi are forming to
increase the absorptive area, so rapid growth
ensuing as a result of
nutrients coming from the mother.
In this image, we can see the yolk sac separated
from the rest of the embryo with the amniotic
cavity shown in green up above it. That’s
the amniotic cavity. As I said, that’s the
key part. The body stalk is what’s going
to be the umbilical cord later on.
The embryo itself is going to develop in the bilaminar
disc. That’s the part made up of ectoderm
and endoderm in between the amniotic cavity
and the yolk sac.
As the cavity is expanding, this begins to
push its way back into the cavity of the uterus,
the uterine cavity overall. To remind you
that when we talked about gastrulation, we
saw the primitive streak forming in the floor
of the amniotic activity, and that defines
the main body axis. It also gives rise to
the mesoderm of the embryo. So that two layers
of the ectoderm and the endoderm between the
amniotic cavity and the yolk sac becomes three
layers, the mesoderm as the meat in the sandwich
as it were.