embryo is the yolk sac. Now let’s look at
the same image from this side. What we can
see is that the embryo is beginning to fold.
The head end is beginning to bend over as
the brain begins to form, so as the tail.
In this process, it’s associated with the
amniotic cavity sweeping round the embryo
as the embryo rises in to the amniotic cavity,
which is of course filled with amniotic fluid.
So the brain is developing at the fore end,
folding round. At slightly later stage, we
can see that regional structures are beginning
to develop in that brain at the front. At
the hind end, it extends all the way towards
the tail bud. And nervous tissue, at the moment,
is running all the way down towards the tail.
A little later still, we can see that this
process of brain specialization is continuing,
and distinct regions are recognizable in the
brain. We should look at those in a second.
So, we can see that the brain could be roughly
divided into three major areas. The front
part is called the forebrain or prosencephalon.
The midbrain is called the mesencephalon,
and the hindbrain is called the rhombencephalon.
Within each of these regions, additional structures
are beginning to develop. So in the prosencephalon,
we can see a vesicle beginning to grow out.
There’s one on each side of course, growing
out towards the overlying ectoderm.
This is the optic vesicle which is later going to
form the retina. When it reaches the ectoderm,
it will induce the ectoderm there to thicken,
and eventually, sink under the surface to
form the lens of the eye. Then the ectoderm, which
remains overlying it, will become transparent
and form the cornea. In the hindbrain in the
rhombencephalon, there we can see various
structures developing, and these are the cranial
nerves. These are beginning to grow out from
that particular region and that also takes
place down in the spinal cord. A little later
still, the brain is further subdivided. So,
the forebrain, prosencephalon, has given rise
to regions called the telencephalon and the
diencephalon. Mesencephalon continues as midbrain,
as mesencephalon, but the rhombencephalon
has given rise to the metencephalon and the
myelencephalon. Incidentally, you may hear these
words pronounced with a soft C as myelencephalon.
But since it’s a Greek word in origin, I
preferred the harder sound of the two.
But we can also see that the ganglia in the hindbrain
and in the spinal cord are beginning to develop,
and nerve cell is growing out from those towards
various structures in the body. Now, we’ve
described what happens, but we haven’t looked
at why it happens. What’s taking place during
the course of development is that there are
signals which are being sent to the developing
neural tube, particularly in the frontend,
and these signals bring about expression of
particular gene patterns, and these gene patterns
determine the different regions of the brain.
So for instance, in the top image, we can
see that the underlying notochord is signaling
up to the neural tube, and so is an area called
the prechordal plate and the anterior visceral
endoderm signaling up towards the forebrain,
and bringing about particular specializations
in these regions. In the lower image, incidentally,
the lower image is not colour coded to the
image above it. We can see that this specialization
is represented by formation of structures
within the brain. So, various areas of gene
expression can be detected, and these are
influenced by signals from the surrounding
tissue. This is also true if we looked at
the neural tube or spinal cord cut in section.
So it’s well known that there are a variety
of different regions within the spinal cord.
For instance, the lower part, the ventral
part gives rise to motor nerves, and the dorsal
or upper part gives rise to sensory nerves
and this differentiation between the two is
caused by developmental signals again from
neighboring tissues. We can see that this
is a general principle in development that
tissues will signal from one to the other, and
respond to signals from neighboring tissues.
If we’re to summarize this in a diagram, here
we can see the endoderm, which is signaling
towards the ventral, the lower part of the
neural tube in association with the underlying
notochord, and it is this which will induce
this to form into motor nerves.
Conversely, the ectoderm is signaling to the upper
part of the neural tube and inducing that to form
the sensory nerves. If you took a piece of
notochord and endoderm and placed it above
the neural tube and graphed experimentally,
what you’d find is that it would then induce
the motor nerves that are normally found in
the ventral part to appear in the upper part
as well. So you can bring about a change in
what happens as a result of a grafting of
one tissue from its normal position to an
additional position. Of course, the neural
tube itself will be signaling to the somites,
and that helps create the vertebrae, which
will grow around the neural tube as it becomes
the spinal cord and they will protect the
spinal cord within these bony structures,
subsequently. Of course this process may not