Now, why should we care? What does the practical significance of this have to do with anything?
Essentially, you wanna remember that these blood vessels are not developing in isolation,
we have nerves and bones and viscera, different organs all developing at the same time.
And one thing that this explains is why the recurrent laryngeal nerve takes
such a bizarre course on both sides of the body?
Initially, the recurrent laryngeal nerve,
which innervates structures from the 6th pharyngeal arch
is gonna be going from the vagus nerve into the developing muscles of the larynx
and its looping underneath the 6th aortic arch in order to reach the 6th pharyngeal arch.
As the body elongates, that nerve gets stretched out
and we're left with the vagus nerve coming down into the thorax giving off other branches,
but having its recurrent branch looped back up to our larynx
and have a very extended U as it does so; but on the right,
we find it looping around our subclavian artery and on the left, looping around the aortic arch.
The reason for that is on the right side the 6th aortic arch
losses its connection to the dorsal aorta and as the body grows,
that nerve gets pulled further up and the next thing it hits is the 4th aortic arch
and that remains as part of the subclavian artery,
which is why we have a different course
on the right than on the left for the recurrent laryngeal nerve,
because on the left it winds up going around the arch of the aorta
and gets tethered there, and in particular it's gonna be found
just beside the ductus arteriosus which after birth becomes fibrous
and is known as the ligamentum arteriosum.
So that's why we have two different courses for the same nerve
on the right side versus the left side,
it's reflecting the lack of symmetry between the right and left vascular development.
Now other things that can go wrong are gonna be called vascular slings
where the vasculature actually constricts another organ.
In this case, we can have the esophagus and the trachea get compressed by the vessels.
The most common form of this is gonna be either a double aortic arch
or a subclavian artery that wraps around the back of the esophagus to get to the right upper limb.
What happens in this case is we have the heart pumping blood to the aortic arches
and we have the two dorsal aorta, in this picture,
making a nicely memetic heart-shaped loop as they go to the dorsal aorta.
Normally, on the right side, the segment of the dorsal aorta
between the 7th intersegmental artery and the fused dorsal aorta,
would rescind in the subclavian artery on the right would detach,
move up, and be connected to the upper limb only if that portion of the aorta fails to rescind,
we wind up with a tether and that tether, as the body elongates,
is gonna move up and constrict the esophagus and the trachea.
I'm gonna back-up just a moment,
and you can see that if the esophagus and the trachea are passing posterior to the heart
but anterior to the dorsal aorta, as that dorsal aorta shortens
it's gonna wind up effectively zipping up and then constricting those organs
causing difficulty swallowing and possibly difficulty breathing as those two organs get constricted.
Another form of this is called a subclavian sling
and it's due to the right dorsal aorta rescinding but doing it in the wrong place.
Instead of rescinding between the 7th inter-segmental artery
and the fused dorsal aorta, it will occasionally rescind above that point
and we'll have that right dorsal aorta disappear
but superior to the 7th inter-segmental artery.
Now, what I want you to remember is that the 7th inter-segmental artery
is tethered to the upper limb. It can't just move around it,
can't just detach and find another more efficient route to where it wants to go.
It's stuck right where it is.
So as the body elongates and that vasculature gets more and more tight
we wind up with it passing posterior to the esophagus and the trachea.
So in this case we have our right subclavian artery coming off most distally
from the arch of the aorta because it has no other choice,
it came from much further down and had to take that posterior course simply to get where it was going.
Again, this can cause difficulty swallowing
and even constriction of the airway if it's very pronounced.
Last and certainly not least are gonna be coarctations of the aorta.
As development happens, normally, the aorta should stretch, lengthen,
and stay open along its entire length.
But occasionally there can be constrictions and these are called coarctations.
The most common one is called the post-ductal coarctation
and that simply denotes that the ductus arteriosus leaves the pulmonary trunk
for the aortic arch and the coarctation happens after that.
Now our coarctation is gonna narrow this massive artery
meaning there's gonna be a lot of blood pressure above that constriction
but much less blood pressure below, and our lower limbs,
organs are gonna be deprived of blood if there's no way for the blood to reroute pass that blockage.
So in these cases, the ductus arteriosus is going to close
because there's a lot of pressure in the aorta,
there's a lot of oxygen that signals it to close
and we wind up with the ductus arteriosus closing, yet we have impeded blood flow.
What happens in this case is that arteries that bypass the aorta
will enlarge tremendously to get blood to the organs
and lower limbs so blood will back up into the subclavian arteries
go through the internal thoracic arteries on either side of our sternum
and then back-fill the intercostal arteries into the thoracic aorta
and those intercostal arteries will enlarge
to the point that it actually leave pretty pronounced groves on the underside of the ribs
and then the blood will fill into the aorta,
move down to the organs in the lower limb.
One way to diagnose this at least early is to look for differences
in the pulse in the upper limb and the lower limb.
A very strong bounding pulse in the two upper limbs
but absent or very, very less pronounced pulses in the lower limb
can be an early sign of coarctation of the aorta.
Now, sometimes these are subtle and aren't diagnosed until adulthood
so I want you to note that it's not always gonna be obvious,
these are not constrictions that are always so severe
that they result in immediate distress but can be brought on by exercise
and they're sometimes associated with the bicuspid aortic valve
rather than the normal tricuspid set of semilunar valves leaving the aorta.
Now much less common, maybe between 2% to 5% of coarctations
occur proximal to the ductus arteriosus
and these, not surprisingly, are called pre- ductal coarctations of the aorta.
In this case, we've got a constriction of the aorta but distal to that we have the ductus arteriosus.
Because the pressure in that distal portion of the aorta is so low,
the ductus arteriosus stays open
and it's gonna take poorly oxygenated blood from the pulmonary circuit
that's trying to get to the lungs and spill into the descending thoracic aorta.
Because of this there may be cyanosis or blue tinting of structures in the lower limb
but not in the upper limb, that's going to be getting fresh blood from the aortic arch.
Patients with the pre-ductal coarctation of the aorta
are gonna show cyanosis in any vessel that is distal to the coarctation
and that's important because the coarctation
doesn't always leave all the subclavian and carotid arteries unaffected.
If I have coarctation that occurs between my left common carotid and left subclavian artery,
I'm gonna have good reddish perfusion of my head and neck,
but I'll have cyanosis from my left arm but not my right.
If that coarctation happens between the right brachiocephalic trunk
and the left common carotid artery, I may have one-sided cyanosis of the head and neck,
cyanosis of the left upper limb but unaffected right upper limb
and relatively decent reddish-pinking appearance of the mucus membranes on my right side.
So remember that the position of the coarctation can determine
which vessels are getting cyanotic blood and which once are not,
so it's typically evident right after birth if these are present,
that cyanosis will be noted right away as opposed to a post-ductal coarctation
which may only become notable as the patient gets older and becomes more active.
The previously mentioned congenital defects
are usually related to impaired remodeling of the aortic arches.
The processes of remodeling of the aortic arches
is responsible for the development of the mature arteries
such as the carotid arteries and the differentiation
between the left and right arterial circulation.
Again, this is usually related to a differential expression of the HOX gene family, particularly HOX3.
Mutations in this gene are implicated in the pathophysiology of impaired development
of the carotid arteries from the 3rd aortic arch and the presence of a double-arched aorta.
Alright, thank you very much for your attention and I'll see you in our next talk.