Hello and welcome to the talk on how the bulbous cordis
is going to become the two separate outflow tracts
of the heart, the aorta and pulmonary trunk.
Now, the easiest way to conceptualize this is the same way
we conceptualize the atrioventricular canals splitting.
Essentially, we have a common chamber that?s receiving blood
from both the right and left ventricles that?s being pumped,
and to separate it?s going to just pinch together.
And in this case, the portion of the bulbous cordis that?s pinching together
is both the conus cordis, the smooth portion of the outflow,
and the truncus arteriosus,
the part that continues up to the proximal aorta and pulmonary trunk.
So the ridges that close that common chamber are called conotruncal ridges
and the conotruncal ridges will separate
that common tract into an aorta and the pulmonary trunk.
But there?s a problem, and the problem is the left ventricle
is directly connected to the pulmonary trunk
and the right ventricle is directly connected to the aorta
if we just go straight up out to the heart.
So these outflow tracts not only have to separate,
they have to spiral so that the right ventricle winds up next to the pulmonary trunk
and the left ventricle winds up going to the aorta.
So as these two tracts separate into an aorta and a pulmonary trunk
there?s actually gonna be a little bit of a spiral inside as those conotruncal ridges form.
Now as that spiraling happens, the conotruncal ridges enlarge
and have little swellings inside their walls that then hollow out
and those hollowed out spaces are gonna form the semilunar cusps of the aortic valve
and the semilunar valve.
These valve cusps are continuous with the endocardium of the heart
and the lining of the interior and all the blood vessels of the body.
Now, this image is a little bit daunting
so we?re gonna take a moment and walk our way through it.
The conotruncal ridges are shown in green and yellow
and what we can see here is that we have the left ventricle
receiving blood through the mitral orifice,
the right ventricles receiving blood through the tricuspid orifice and at that point,
contraction of the ventricles is going to propel blood up
into both the aorta and the pulmonary trunk.
What we want to have happened is for the blood from the right ventricle
to spiral up into the pulmonary trunk
and blood from left ventricle to spiral up into the aorta.
So the green and yellow conotruncal ridges are growing together in a spiral fashion.
Let?s move to the next image which is a bit simpler
and you can see how those ridges
coming together have formed a continuous septum
and have separated the two outflow tracts.
But there?s a problem,
the problem is the right and left ventricles are still in contact with each other.
This problem was resolved when the endocardial cushions
and the membranous portion of the interior ventricular septum grow together
and actually grow upward to fuse with the conotruncal ridges.
So now, we?re left with something like the middle picture
where we?ve completely separated the right and left ventricles,
we?ve completely separated the aorta from the pulmonary trunk.
And one thing I want you to note is that both the endocardial cushions
and the conotruncal ridges come from neurocrest cells,
and if you have failure of neurocrest cell migration,
a very, very common problem is going to be heart defects
especially defects involving the endocardial cushions and the conotruncal ridges.
So if we take a look at the picture on the lower right,
we see how everything ought to wind up the outflow tracts spiral
connecting the left ventricle to the aorta,
connecting the pulmonary trunk to the right ventricle
and no mixing of blood between the two.