We will now continue following how the spaces inside the embryo
will separate to become the pericardial, pleural, and peritoneal cavities.
This is largely accomplished by formation of the diaphragm.
Now, recall early on that we have the intraembryonic coelom
present in a bit of a horseshoe shaped appearance
around the developing embryo.
But as the heart folds forward, it pulls that cavity,
in this case, the early pericardial cavity with it.
But one thing to notice is that as that happens,
it also pulls a large strip of mesoderm called the septum transversum with it.
And the septum transversum moves from a space a bit superior to the forming head
to a spot just inferior to the developing heart
and this is gonna start pinching portions of the intraembryonic coelom.
In particular, it?s going to create small little canals
connecting the pericardial cavity above to the peritoneal cavity below
and for good reason, these are called pericardioperitoneal canals.
Now, there?s gonna be a lot of words starting with the letter P in this talk.
So just do your best to follow along with each of them as they go.
The septum transversum is gonna form the early diaphragm
and pinch that cavity but not completely obliterate it.
To get a complete separation,
we?re gonna have to further compress the pericardioperitoneal canals
and that?s accomplished by in folding of some mesoderm
from the lateral body wall.
We?ll get back to that in a moment
but first, let?s discuss the innervation of the diaphragm.
For those of you who know your anatomy,
you?ll know that the diaphragm is innervated by the phrenic nerve
and the phrenic nerve comes from C3, C4, C5, keep the diaphragm alive.
The phrenic nerve initially innervates the diaphragm
when it?s very high up in the middle cervical region.
So that makes sense.
It?s gonna have its innervation be nearby.
But as the body elongates moving into week eight, the body may elongate
but the diaphragm is more or less stuck in place.
And as it gets pulled away from C3, C4, C5, the nerves have no choice
but to stretch out and maintain their connection
to the septum transversum so that by the time we get to week eight,
that phrenic nerve has taken a very extended course
down the anterior neck alongside the pericardium in the chest,
and finally, to the diaphragm which it innervates
and that?s one reason that the phrenic nerve is so prone to injury
both in trauma to the neck
and any sort of tumor affecting the lungs very medially or the pericardium.
Now, let?s return to the body wall and the cavity within it.
The pericardioperitoneal canals are going to be closed
not by the septum transversum alone
but because some folds of the lateral body wall grow in
and fuse to the septum transversum.
These folds are called pleuroperitoneal folds.
So say this five times fast if you can.
Ready. The pericardioperitoneal canals are closed by the pleuroperitoneal folds
and once that?s accomplished, you functionally separated the thoracic cavity
from the peritoneal cavity.
Now, just a quick hint, we do not yet have pleural cavities up in the chest.
We?ll discuss those a little bit later
but right now, let?s turn our attention to the diaphragm itself.
The diaphragm is now keeping the abdominal organs
separate from the thoracic organs.
And that?s important because the abdominal organs
are under a considerable amount of pressure as they develop.
So the diaphragm continues developing its muscular wall.
Initially, the muscular wall is not present in the septum transversum
or the pleuroperitoneal folds
but grows in as the body expands from the lateral wall.
So the little purple rim of tissue that you see in this image
is actually going to turn into the muscle of the diaphragm.
Note that we have the mesentery containing the foregut
as well as the inferior vena cava
and a bit of the esophagus hanging out from the posterior body wall,
coming in, and surrounded by the developing diaphragm.
We?re now gonna look at the same structure a little bit further
in development and from its inferior side.
So it?s as though we?re in the abdomen looking up at the diaphragm
and we can see the central tendon.
That non-contractile connective tissue portion of the diaphragm,
it?s derived from the septum transversum
and the muscles of the diaphragm that allow it to dome up
and flatten out when it contracts are gonna be coming from the lateral body wall.
Note that it has to allow the inferior vena cava, the esophagus, and the aorta
to pass through it if we?re gonna have blood flow
and movement of food into the stomach.
So the inferior vena cava pierces it at T8, the esophagus at T10,
and the abdominal aorta at T12.
Those are the levels at which they?re gonna pass through the diaphragm.
If the diaphragm were flat, it would all be at one level
but since its domed, there?s a bit of a stretch from T8 to T10, to T12 as that happens.
So things that can go wrong.
If one of the pleuroperitoneal folds fails to completely fuse to the septum transversum,
you have a connection between your abdominal peritoneal cavity
and your thoracic pericardial cavity.
Now, that in it of itself wouldn?t be problematic but the problem arises
because the pressure in the abdomen is very great.
During development, it?s typical for part of our midgut to herniate
out into our umbilical cord
and then, gradually return as more space becomes available
but because the gut is simply responding to pressure,
if I?ve got a hole in my diaphragm, instead of going out the umbilicus,
my gut will herniate up into my thorax and it will not return prior to birth
and as it moves into the thorax, it?s going to compress the lungs and the heart
and that?s gonna cause hypoplastic formation of the lungs.
They will likely be nonviable
and because the lungs are not essential for life up until birth,
these embryos and fetuses can go to term
but suddenly die after delivery as they?re unable to breathe
due to abdominal contents compressing the heart.
This is almost exclusively found on the left side.
In this image, we can see a superior view of the diaphragm, the vertebra, and the back,
the imprint of the heart in its pericardial sac are in front
and we can see then on the left side,
there?s a defect in the diaphragm
that allowed gut to herniate through and compress the lungs.
There may be defects on the right
but they?re largely hidden by the fact that you?ve got a massive liver right there
preventing any tremendous movement of gut through the right side.
So this tends to occur primarily on the left side
and can be detected by ultrasound during healthy screenings
and if you?re very lucky, you may be actually able to go in
and correct this defect with a mesh prior to delivery.
Actual intrauterine surgery can be used to correct these defects,
move the gut contents back into the abdomen,
and allow the lungs to develop normally.
A similar but slightly different condition is called eventration of the diaphragm.
This occurs when a portion of the diaphragm is not properly innervated
and it becomes very floppy.
In this case, the gut contents don?t enter the thoracic cavity
but they push that floppy area of the diaphragm up
and effectively invade the thoracic space.
So a similar outcome even though there?s still a thin layer of diaphragm present
between the gut contents and the lungs.
Thank you very much for your attention and I?ll see you at our next talk.