hello and welcome to the first of our
discussions on development of the heart
this amazing process is going to give
rise to an incredibly complex pumping
machine hanging out in our thoracic
cavity but how we get there is very
convoluted and you might ask yourself
why should we care
reason is heart malformations and heart
defects are incredibly common and
prevalent some form of cardiac
malformation tends to occur in about 1%
of all birds and is a leading cause of
both infant morbidity and mortality
especially if it's not diagnosed prior
to delivery in the United States alone
between 1999 and 2000 there were roughly
6,000 deaths associated with cardiac
anomalies and malformations so let's go
into the process by which the heart
moves from being a single tube to
becoming the incredibly complex
mechanism that it is so you may recall
from the lecture we had on the Organa
genital layer of lateral plate muse or
folds anteriorly to create the gut tube
it has two tubes within it called
endocardial tubes and these are going to
be the early primordia of the heart as
they're brought together as we can see
on the right side they're going to come
into existence just ventral to the
developing gut tube and they're gonna
pull a small cavity with them that
cavity is part of the intra embryonic
Silom and specifically in this region we
refer to it as the pericardial cavity
although it's slightly different from
the mature pericardial cavity for
reasons we'll go into in a subsequent
talk the endocardial tubes are brought
together fuse and develop some what's
referred to as cardiac jelly around them
and around the cardiac jelly is
developing myocardium or muscle of the
heart this myocardium is going to be
able to be relatively early but it's not
until those tubes fuse that it's going
to be able to do anything with the
chamber that's inside and propel blood
through it
now the heart is suspended in the
pericardial cavity by a dorsal Mesa
artyom which is essentially a mesentery
of the heart connected to the dorsal
side of the embryo as the heart
continues to develop it moves from its
initial location to its final location
and one amazing thing about the heart is
it actually develops anterior to our
face it grows right about in this area
early on and as the embryo enlarges and
the forebrain gets larger and larger it
folds the heart down into the thorax
into its eventual mature position as it
does so it pulls the pericardial cavity
with it along with a strip of music
durum called
the septum transverse 'm that mesa derm
is going to travel with it and take up
residence inferior to the heart and the
septum transverse is the earliest
primordia we have of the diaphragm so
we'll discuss how the septum transverse
'm becomes the diaphragm and separates
the pericardial cavity from the
peritoneal cavity in a subsequent talk
now the heart has more or less found its
normal position in the eventual thorax
and it starts beating about 22 days into
development this is important because at
this point the embryo can't get much
larger without a heart simple diffusion
of gases and nutrients from cells is no
longer sufficient to handle the size of
the embryo we can't just create cavity
after cavity for these things to diffuse
we need to have a circulatory system and
the heart starts pumping at 22 days
because we absolutely have to have it in
order to get any larger so the heart
pumps bringing blood in from inferior
peristaltic ly pumps it out superiorly
and into a paired set of dorsal a aorta
on the posterior body wall so blood
that's d oxygenated comes in in fear lee
gets pumped superiorly and is going to
travel on either side of the gut tube
through what are called aortic arches
and those aortic arches carry the blood
dorsally to a pair of a Horta now at
this point the heart instead of being a
simple cylindrical tube is going to
develop some distinctive areas that
bulge outward the
first and most inferiorly is the part
that receives blood from the body you're
going to be our left and right side of
the sinus venosus and the sinus venosus
pumps blood to the primitive or
primordial atrium from there the atrium
is going to pump blood to the
embryologic ventricle and then to an
area called the bulbous cordis now
thereafter
blood is going to travel through aorta
cartridges to reach the dorsal a or de
and at this point day 21 we only have a
single aortic arch but as we develop
more aortic arches we have a common
chamber called the aortic sac that's
going to receive the blood from the
boldest cordis and distribute it through
aortic arches to the dorsal a order now
as the heart enlarges and gets those
distinctive five segments the dorsal
music Artyom is going to start to break
down and that's going to allow the heart
to essentially sag ventrally in the
developing chest and it's gonna sag into
the pericardial sac that's already
present around it as that happens the
ventricle and Baldus cordis wind up more
anterior the sinus venosus in atria wind
up more posterior and if you think about
how the heart appears in the mature
human the atria are posterior to the
ventricles which are more anterior so
that little fold between them is called
the bow bow ventricular loop and you've
got the primordial ventricle just on the
backside of that bulge and the bulbous
cordis on the anterior side of that
bulge and as development proceeds the
ventricles continue to move a bit more
anteriorly here's an early electron
micrograph of a developing heart
posteriorly
we have the atria which has been
straddled by the outflow tracts it's
more posterior and as we move more
internally we see the ventricle and then
the boldest cordis so we're looking at
this from an anterior view and can see
that the boldest cordis and ventricle
are actually located in front of the
atria which are receiving the blood now
when we discuss formation of the heart
we're going to make a single linear flow
of blood into four separate chambers
and two completely separate circuits
what we have to do is follow the
development of each one of these areas
as they move from the embryo logic
structure to the adult structure the
sinus venosus is going to form the right
atrium along with the superior vena cava
the inferior vena cava and the blood
drainage of the heart itself the
coronary sinus the primitive atrium or
primordial atrium is going to form the
oracles the little dog-eared appendages
that are just hanging out on the side of
each atrium along with quite a bit of
the left atriums wall the primordial
ventricle becomes the left ventricle and
the bulbous cordis is going to
transition to become the muscular
portion of the right ventricle along
with the outflow tracts of both
ventricles
so the initial smooth portion of the
aorta and the pulmonary trunk as they
leave the ventricles and travel up to
become the very proximal pulmonary trunk
and proximal aorta thereafter the aortic
sac is going to divide to become very
proximal but just a little further along
portion of the aorta and pulmonary
artery there after the aortic arches
will contribute most of the large
vessels leaving the heart so at this
point I'd like to take a moment and
follow the flow of blood through the
embryo logic heart on the left side of
your screen we've got a sagittal cut
we've taken the lid off the left side of
the heart and on the right side of your
screen we've taken a coronal cut through
the anterior wall of the heart so as
blood enters it's going to come in the
sinus venosus from there it's going to
enter the atrium the primordial atrium
and then pass into the next chamber
which is the ventricle so atrium to
ventricle from there it passes from the
ventricle to the Baldus cordis and the
boldest cordis has two subdivisions the
conus cordis a smoothened area and the
truncus arteriosus the initial outflow
tract thereafter it's going to move to
the aortic sac and be distributed
through however many a or t'k arches are
present
at that stage of development so here
we're going to take a break and then
move on to the further subdivisions of
the heart and how this single flow of
blood in a relatively linear sense
becomes two separate circuits in a four
chambered heart thank you very much for
your attention