We´re gonna now discuss the development of the skull and thereafter, the face.
Now, during the first five weeks, the embryo is developing relatively quickly
and we get a head of the embryo relatively quickly but it doesn´t really look human.
It´s not until the skull and the face really come into their own
that we have what would appear to be a more human face
and that´s done by the end of the embryologic period
and only gets larger during the fetal period.
So the development of the face is something that we could say really
and truly makes us really appear human
is something that´s happening shortly after five weeks.
So here in this process, some things are gonna happen that are relatively strange.
The skull is going to develop and contain the brain
but a bony skull would actually compress the growing brain and not allow it to expand.
So the body has to get the best of both worlds.
A skull that protects the brain but still allows it to grow.
The face is going to form from a series of folds that develop on the neck.
These are called pharyngeal arches
and we´ll have an entire lecture devoted just to those
but that´s gonna be what creates the maxilla, the mandible, the cheeks, and the nose.
And then, various glands including the calcitonin secreting
and parathyroid secreting, glandular cells
are all gonna be coming from the inside of the early throat or pharynx.
So a lot of different things are going on,
not just morphologic, not just change in shape,
but actual, functional physiologic glands
will be developing at the same time as the face is developing.
Now, the skull itself can be roughly divided into two parts:
the neurocranium which is the base of the skull
and the viscerocranium which is going to be the facial bones
that grow down from the neurocranium.
So neurocranium contains the brain
and the viscerocranium would be the portion of the skull that is basically our face.
At the very base of the skull, we have several cartilages form.
The basioccipital, the basisphenoid, the presphenoid, and the mesethmoid cartilages
and these form a line of cartilaginous precursors
right along the base of the skull where the brain sits.
Now, if you take a look at this picture,
you´ll see there are a large number of other smaller cartilages
but they´re all going to consolidate together
and grow into the image we see on the right
where we have not necessarily something that appears to be a skull
but a group of cartilages that are growing, expanding,
and very gradually turning into bone.
And starting from the front, we have the mesethmoid, the basisphenoid,
then the basioccipital a little bit further back and it will surround the foramen magnum.
The sphenoid bone which is going to contain part of the temporal bone
on its lateral side, the sphenoid bone itself is gonna develop with the orbit a little bit
in front of the basioccipital bone.
So let´s move a little further in and see how that cartilage becomes bone.
If you´ve seen the video on limb development,
you´ve already heard of endochondral ossification.
As a quick review, endochondral ossification denotes
that we have a mesenchyme precursor that turns into cartilage and then, turns into bone.
Hence the name, endochondral, chondral meaning cartilage.
But in the skull and a few other bones in the body that are flat,
we have intramembranous ossification.
So let´s start with the endochondral ossification.
These cartilaginous precursors at the base of the skull
and the roof of the face are going to enlarge.
The great thing about cartilage is that it is continually expanding.
The chondrocytes that are within the cartilage keep on creating daughter cells
and the cartilage gets bigger
and that´s why our face changes shape over time.
The facial bones grow inferiorly and anteriorly as this process occurs
because they´re basically growing down from the base of the skull.
The continued growth takes us from this appearance
that we see on the screen here with a relatively short face
and a huge, huge calvarium containing the brain
to something that´s a little more one to one.
The face grows down and takes up basically
the same amount of vertical height as the brain.
So the face gets longer and longer the older we get.
Now, if we maintain this basically baby-like appearance during life,
that´s going to be problematic and is tied to a couple clinical conditions.
And you´ll notice that not only does the face grow down
but the jaw elongates and allows the chin to protrude just a bit further.
Now, we´ve discussed endochondral ossification,
let´s back up and talk about intramembranous ossification.
This is basically the same process but we cut out the middle step.
So we have a mesenchyme precursor and it transitions directly to bone.
And this happens in the flat bones of the skull
such as the frontal bone, the parietal bone, part of the occipital bone and so forth.
This intramembranous ossification creates little bony spikes that grow outward.
So let´s take a look at this both in development and in its mature state.
Initially, that mesenchyme just creates little trabeculae or struts of bone.
In this image, you can see them at the center
and they´re those kind of purplish red structures
that are going to be at the center of this core of mesenchyme
but as we develop fully, we wind up with that transitioning
to compact bone on the outside of the skull, compact bone on the inside of the skull
and those trabeculae are little struts in between.
To get to this stage, we usually start with little cores of ossification that grow outward
and they just expand like a coral reef growing outward and outward
and very soon, those cores of bone
whether they be parietal, frontal, or occipital will grow close and meet their neighbors.
Now, it would make sense that if that happens, that the bones would fuse.
But that´s not what happens
and in fact, there´s a whole regulatory cascade involving the dura mater
and the periosteum of these bones
that prevent them from actually locking together completely.