Two examples are Treacher
Collins and Pierre Robin syndromes.
This is an image of Treacher Collins syndrome, and
here we can see that there is malar hypoplasia
that’s underdevelopment of the bones of
the face, particularly, the zygomatic bones.
There’s also underdevelopment of the jaw,
mandibular hypoplasia. And the palpebral fissures
of the eyes are downward slanting, and the ears
themselves are malformed and in a slightly
abnormal position overall. This represents
an autosomal dominant trait and is present
in something like one in 10,000 live births.
Another condition for which we do not have
an image is Pierre Robin syndrome. Here, we
have hypoplasia of the mandible, and that
can lead to a displacement of the tongue and
may be associated with a bilateral cleft palate.
There will also be defects of the external
eye and ear under these conditions.
And it may be sporadic although it may also have
a genetic basis. The incidence is not entirely
clear, but may be something of the other one
per 8,500 live births. Di George syndrome,
by contrast, is a third and fourth arch phenomenon,
where the development of the third and fourth
pharyngeal pouches has been significantly
affected. As a result, what we’d expect
to see is hypoplasia or even complete absence
of the thymus and malformation or absence
of the parathyroid glands, and that may have
some disturbances to the blood vessel system
as well. Again, we’d expect to see ear abnormalities
and reduction in the size of the jaw.
Because of the reduction or absence of a thymus, the
baby may be more susceptible to infections
since the thymus plays a significant role
in the immune system in early development.
One particular example, we can trace exactly
where the process has gone wrong, is we can
identify a particular deletion on chromosome
22. Incidents would appear to be something
of the order of one in 4,000. But for these
rare and sometimes hard to recognize conditions
is always an element of uncertainty in the
exact frequency. If an abnormality of the
head and neck is observed, there are a number
of things which we can look at to explore
what might have happened. We can list these
structures that are affected, and then we
can look back to the embryology to explore
which arches or pouches or grooves would normally
give rise to those particular structures in
the embryo and looking back in time in this
way might suggest the genes that could be
involved, and that could be something that
could be explored through genetic analysis.
And if they are indeed affected, then this
gives suggestions as to how best this condition
might be treated. So then in summary, we’ve
looked at the appearance of the pharyngeal
arches as we briefly reviewed face development
and since this contain muscle, cartilage,
nerves, and blood vessels, each of these may
be affected by abnormalities. Initially, the
arches are separated by grooves, and the process
of normal face development is one of growth
and fusion of these structures together.
If something happens to interrupt this process,
then complex clefts may arise in ways which
are relatively predictable given the nature
of the underlying processes. Of course, genetic
defects can contribute in significant ways
to all of these kinds of abnormalities.
Thank you very much.