Hello. We are now going to discuss development of the face itself.
Way back at the beginning when I started talking about development of the skull,
I teased that we were gonna talk about how the face forms
and gives us our distinctive human appearance
and this time, I´m finally gonna deliver on that promise.
Initially, we have the stomodeum,
the very early mouth flanked by the mandibular prominence on its inferior side,
two maxillary prominences lateral to it
but allowing a gap to exist so the stomodeum is actually open
and above it we have something called the frontonasal prominence.
So the maxillary prominence and mandibular prominences are located on either side,
roughly where we would expect the mandible and the maxilla to form.
But the frontonasal prominence is the truly odd structure that´s involved here.
It is ectoderm covered, exists more or less where our forehead is located
and is gonna grow down as the forebrain expands
and the telencephalon gets larger and larger.
As the frontonasal prominence descends,
it develops two small ectodermal thickenings on either side.
These are called the nasal placodes
and the nasal placodes mark the earliest evidence
we have of the nasal pits and our actual nostrils beginning to form.
Now, the mandibular prominence on the underside of the stomodeum fully fuses
making an unbroken arch that´s gonna become our jaw.
The maxillary prominences stay right where they are
and the nasal placodes are then going to deepen
and become something called the nasal pit
and the nasal pits are gonna burrow farther and farther
into the head as development proceeds.
Now, early on, we actually have the eyes
develop extending off the hypothalamic area, the diencephalon,
and extending out to either side of the head.
Early on, our eyes actually face outward, kind of like a cow,
but as development proceeds and the frontonasal prominence gets more narrow,
our eyes are going to swivel forward to the position that we´re used to seeing them in.
Now, the nasal placodes, as I mentioned, deepen to become nasal pits,
and eventually burrow their way into the skull to form actual nasal cavities
like we have in our own head.
At the point where the frontonasal prominence meets the maxillary prominence,
we have a little divot called the nasolacrimal groove
and that´s gonna mark the place where the eye
and nose are positioned close to each other leading down to the upper lip.
So the nasal pit is going to form the nasal cavities
but on either side of its surface, we have two very important structures,
the medial and lateral nasal prominences.
They´re going to fuse with the maxillary prominence
to create the cheek and the upper lip.
Now, the point where the lateral nasal prominence
meets the maxillary prominence is called the nasolacrimal groove
and it extends more or less from the medial canthus or corner of the eye
down along the developing nose towards the upper lip.
The lateral nasal prominence is gonna stay
in contact with that maxillary prominence as the face develops
and maintain that nasolacrimal groove
and in fact that´s where your nasolacrimal duct will be located as we continue growing.
The medial nasal prominences do something very interesting.
If we imagine that my thumbs are the lateral nasal prominences
and my fingers are the medial nasal prominences,
the medial nasal prominences grow together and fuse on the midline,
but then they grow inferiorly and they stretch inferiorly
and we are gonna see that in the next several slides.
As they do so, they fuse with the maxillary prominence,
and that´s what creates an unbroken upper lip.
So the frontonasal prominence keeps moving inferiorly
and keeps getting relatively smaller as the head enlarges.
We can see here that the eyes have begun to swivel forward
and be directed a little more anteriorly.
The medial nasal prominences
connect to the maxillary prominences on the left and right
and form an unbroken upper lip and as development proceeds,
it stretches all the way down to form that little divot
on the top portion of our upper lip right in the middle called the philtrum.
So the philtrum is derived from the medial nasal prominences,
the lateral side of our nostril is derived from the lateral nasal prominence
and most of the rest of our lip and cheek is derived from the maxillary prominence.
So here´s more or less the mature confirmation
that we´re having present at the end of the embryonic period
moving into the fetal period where the face is more or less human looking.
The mandible is made from the mandibular prominence,
cheeks from the maxillary prominence,
and the forehead and nose and don´t forget the philtrum,
from the frontonasal prominence.
And even though things may stretch,
they retain more or less that same confirmation as development proceeds.
So blue for mandibular prominence, yellow for maxillary, and green for frontonasal.
So what can go wrong in this process?
Well, if we have too much tissue present
because of excessive neural crest cell migration into the area,
overactivity of Sonic Hedgehog
can cause too much tissue in the face causing frontonasal dysplasia.
Sometimes, this manifests a little bit subtly with a slightly broadened nose
and widely spaced eyes called hypertelorism.
But if it´s excessive, we can wind up with
something that we see in this picture where there´s actual separation
and almost duplication of facial features near the midline.
If too much signaling of sonic hedgehog can create too much tissue,
stands to reason that too little signaling
and too little tissue in the area can create the opposite problem,
narrowing of midline structures.
This is going to result in hypotelorism where the eyes are too closely set together.
And one very, very prominent condition that´s associated with this
is called holoprosencephaly.
Now, you may recall that in the nervous system talk,
we discussed that holoprosencephaly
is due to the left and right cranial hemispheres.
So the right and left cortex not separating properly and not becoming wide enough.
Holoprosencephaly is mostly due to narrowing of midline structures
and in the brain, that´s resulting in the two cortexes not separating.
The eyes growing too close together as they migrate outward from the diencephalon
and it´s reflected in facial features as well.
Very mild holoprosencephaly might just result in narrow structures
near the midline of the face.
In fact, there´s some instances where a person will be affected
with very mild holoprosencephaly
and have just a single incisor tooth rather than two in the front.
But more pronounced holoprosencephaly creates very big problems.
Because if there´s not enough space on the face,
the eyes don´t have a chance to grow outward
with enough space between them from the frontonasal prominence to descend.
Now, more pronounced forms of holoprosecephaly occur
because as in the nervous system where the two cerebral cortexes did not separate fully
and the eyes did not grow laterally enough to each other,
we´re going to have the eyes grow too close together anteriorly.
If the eyes are present too close together on the face, it causes cyclopia.
Either fusion of a single eye or very closely fused two eyes
which is what we see in this picture.
Cyclopia is named for the Greek monster,
the cyclops that had one eye in the center of its face
but in this case, the cyclopia is associated with what´s called a proboscis.
That kind of fleshy structure hanging out on the forehead.
That´s actually the nose.
That´s the frontonasal prominence that wants to descend
and reach the upper lip but because the eyes were too closely spaced,
it was unable to do so and remained up on the forehead.
So that is severe holoprosencephaly as you´d expect,
a signaling molecule deficiency of this sort affects multiple organ systems
and is usually incompatible with continued life outside the womb.
Now, less severe problems that can occur during facial development
include the oblique facial cleft.
This occurs when parts of the lateral nasal prominences
fail to fuse completely with the maxillary prominence
and you´re left with a divot running from the medial canthus of the eye
down the nose towards the upper lip or slight variations on that.
Additionally, if you have the medial nasal prominence on one side
fail to fuse with the maxillary prominence, you can wind up with cleft lip.
If this only happens on one side where the philtrum doesn´t connect
with the maxillary prominence, you have a unilateral cleft lip.
If it happens bilaterally, you have no surprise, a bilateral cleft lip.
The philtrum is still present
and it´s still hanging out just underneath the opening of the nose
but it isn´t fused with the upper lips and the maxillary prominence.
So these are relatively easy to fix if they´re occurring in isolation,
they can be surgically repaired
and create a great deal of improved quality of life for the people
who are suffering from them,
but they may be associated as parts of other syndromes
such as fetal alcohol syndrome, Stickler´s syndrome, Treacher-Collins,
Van der Woude, and Pierre-Robin Sequence
all have cleft lips occasionally occurring with them.
And you´ll notice that cleft lips and cleft palates
sometimes are mentioned very, very close together
because palate formation and formation of the lip are very closely tied together
and that will be the topic of our next discussion.
Thank you very much and I´ll see you at the next talk.