anatomically unexpected position. Now here,
we have a quick summary of the derivatives
of the arches. So the first arch gives rise
to its major blood vessel and it will be the
maxillary artery. The cranial nerve associated
with the first arch is the trigeminal.
And the major muscles are the muscles of mastication
along with some other more minor muscles.
The skeletal derivatives or key ones are the
malleus and incus, and the cartilage which
will form the base, the core of the lower
jaw. The second arch will give rise to the
hyroid artery and the stapedial artery. This
cranial nerve is VII, the facial nerve, and
there are major muscles or the muscles of
facial expression. The cartilage derivatives
mostly represent the stapes and the styloid
process, but also other parts of the hyoid
bone. The third arch gives rise to the internal
carotid artery. Its nerve is IX glossopharyngeal
and gives rise to sylopharyngeal muscle. And
the skeletal derivatives are part of the hyoid.
The fourth arch gives rise to the right subclavian
artery and to the vagus nerve, and its muscles,
for instance, would be the pharyngeal and
laryngeal musculature. And the cartilage will
give rise to the laryngeal cartilages. So
far I’ve been describing natural history.
This is what happens. But obviously, a crucial
question is, why does this happen? Again,
we’re not exploring this diagram in great
detail, but really, what it represents is
patterns of gene expression, the different
levels of the developing neural tube.
What you can see is that these regions of the head
and neck have particular codings of gene expression,
and these are shown in the coloured illustrations
above. You can actually read this almost as
if it was a barcode so that in one particular
segment, certain genes are switched on, and
in another segment other genes are switched off.
So each segment you can see represented by
lines here has a unique pattern of gene expression.
And because there’s a relatively small number
of genes involved here, we can identify that
these are common universal genes. First identified
largely in fruit flies, but we now know that
these are present throughout the development
of all animals, and indeed in some cases,
plants as well. There’s a kind of universal
gene coding for different regions of the body,
and that has significant consequences.
You can readily imagine, for instance, that a
gene mutation might convert one segment into
the barcode for another segment, and therefore,
that segment is duplicated in the body, and
that obviously would have significant clinical
implication, subsequently. This is the lower
part of the diagram, showing that these barcodes
extend all the way down the body.