Hello, and welcome to this lecture on the
development of the pharyngeal arches and their
associated clefts and pouches, and also the
thyroid gland. What we’re going to do is
to look at the mechanics of face development,
and then the derivatives of the branchial
arches, and we’ll see that there are muscle
cartilage and nerve derivatives in the head
and neck coming from each of the arches and
then we’ll look at the derivatives of the
pharyngeal pouches and arches in the head
and neck. This is quite a complex process,
and it takes places against the background
of growth, which makes it hard to understand
in still images. Previously, with funding
from the Wellcome Trust and working with Helen
Storey, I made a video which illustrates some
of those processes. When the video begins,
we’re looking into the face of a very early
human embryo, perhaps, round about three weeks
after fertilization. As we go through the
video, we’ll see various changes taking place.
Now, I’m not going to talk you through in detail.
Really what I want you to get is
the gestalt feeling from the whole process
of growth, of movement, and of fusion. And
we’ll look at the individual structures
by name later on. So, let’s have a look
at the video. Now we can see the formation
of the nose in the center, wide mouth.
The eyes are beginning to come round from the
side. The ears are visible low down on the neck.
And you can see those gradual fusions taking
place between the components
and it will eventually become recognizable as a human
baby. Of course, growth and development continue
for many years after birth. So we can move
through adult stages where there are consistent
changes in the shape of the face, all the
way through to old age.
So that’s to give you an idea of the nature of what’s happening
and you can always go back and look at that
video again. So let’s look at some of the
details here. Here, we begin with a very early
human embryo, perhaps three weeks after fertilization.
In the diagram, we have numbered the branchial
arches. So, there are three visible on the
surface and one deep under the surface and
these correspond to the gill arches in evolutionary
developmental stages. The image on the left
shows scanning electron micrograph of a real
human embryo, and you can see that these arches
are really quite visible and well presented.
So three visible in the surface, a fourth
one lies deep and it’s less easy to spot
from the surface. But you can tell from sections
that this is present. The first arch actually
folds in two. So the maxillary process is
the upper part of the first arch, and the
mandibular arch is the lower part of it.
Sometimes it can give the impression of the big and
extra arch there, but this is misleading this
is just the first arch folded over. The frontonasal
prominence is the large part projecting towards
the heart. So this lies quite far forward.
Bear this in mind when we’re looking at
some of the images because you have to imagine
that frontonasal prominence coming towards
you. In this image, for instance, that frontonasal
prominence is projecting towards you out of
the plane of the screen. Marked in green,
we have some thickenings or placodes and this
indicates where the nostrils are going to
form on that frontonasal prominence.
Gradually, this becomes deeper and the walls surrounding
them become higher. We can imagine there are
an aspect towards the midline, the medial
aspect, and an aspect towards the sides, the
lateral aspect. You can see these thickenings
indicating where the nostrils will form, and
deepening as structures continue. But you
can also see the maxillary and mandibular
processes. You’ll recall that these are
derived from the first arch, and these are
growing towards the midline. From being paired
one in either side, now they’re beginning
to grow towards each other, and we can begin
to see them out forming during this process.
The walls that surround the developing nostrils
are not complete. They open towards the mouth,
and this will be significant when we come
to look at abnormalities in this region during
the course of development. You’ll note that
the mouth is still quite wide at this point
compared to the status in the adult. So the
maxillary process is still separated by grooves
which separate it from the nasal walls and also
from the mandible. Now, the ear is beginning
to become evident, but it has a series of
swellings, and these are relatively far down
on the neck of the embryo at this point. So
they’re going to have to shift quite a lot
in position. But we can just begin to see
the eyes coming around as the head enlarges.
So they shift their relative position from
the sides of the head towards the front of
the head. And of course, as we saw in the
video, the whole process takes place against
the background of growth, which is not well
reflected in still images. So the ears begin
to become a little more recognizable and they’re
still low down on the neck. And again, that
may be significant for some abnormalities
as we can examine in another lecture.
The eyes are continuing to move towards the front
and eyelids are beginning to develop during
this process. Now, if we look at the nasal
part of the frontonasal prominences, as they
meet in the midline, so they begin to fuse, and
they will also fuse with the maxillary process.
And the maxillary process is beginning
to fuse with the mandibular process.
So the mouth from being very wide begins to move
towards these normal proportions in the development
of the head and neck. It’s instructive to
look at the same process, the same sequence,
but this time seen from the side. In particular,
what we can see is that the eyes are shifting
towards the front, and the ears are moving
up from being relatively low down on the neck.
So if something was to happen to disturb this
process, then we might find that the ears
remained in an abnormally low position. Now,
let’s look at the branchial arches.