Let’s now look again at this diagram and understand
that there are three different components
to the wall of the heart. First of all, the
heart is lined on the outside by a thin capsule
or thin membrane called the epicardium. This
epicardium is actually a visceral layer of
the pericardium because that epicardium reflects
back on itself and creates a pericardial cavity
or sac in which the heart sits and in which
the heart pumps. And that cavity, that serous
cavity is lined by fluid. It's a very thin cavity.
It’s lined by fluid that reduces the friction
during the beating of the heart. So the epicardium
is on the outside. The thickest layer is the
myocardium. The myocardium is really full
of cardiac muscle. That’s the work part
of heart wall. That’s the part of the heart
wall that does the contracting or the pumping,
the myocardium. And then internally, the endocardium
lines the entire inner surface of the heart.
It lines the heart valves as well, and the septum
between the atria and also both ventricles
called the interatrial and interventricular
septums. So now let’s look at those structures
in a bit more detail. Let’s look at the
histology of those structures. First of all,
the epicardium, the covering on the outside
of the heart. As you can see from the diagram,
and also from the histological section, it
contains a lot of fatty tissues or
adipose tissue. And I’m sure those of you who have
been in a dissection lab always find that
rather frustrating when you’re trying to
locate some of the positions of the coronary
arteries. It’s also composed of some connective
tissue, collagen, elastic tissue, all the
sorts of normal connective tissue fibres you’d
expect to find in some loose connective tissue.
And on the outside, it’s lined by a squamous
type epithelium called the mesothelium.
So that mesothelium, underlying connective tissue
and adipose tissue create the outside coating
or covering of the heart, the epicardium.
The myocardium, as I mentioned before, consists
entirely of cardiac muscle, with some connective
tissue skeletal components, and also, blood
vessels. But it’s mostly cardiac muscle, and
as I said before, that’s the pump component
of the heart. It’s lined on top of course
by endocardium. It’s lined on top of the
section here. But remember, it’s lining
the internal chambers of the heart. So have
a look at the epithelium of the heart lining
here, endothelium, and it consists of a squamous
epithelial cell lining as I’ve mentioned
before. And that squamous epithelium sits
on a subendothelial layer of connective tissue.
It’s a very thin layer of supporting connective
tissue similar to a lamina propria in other
organ systems. And sometimes there’s also
a third layer, the subendocardial layer again
of connective tissue. And this separates the
epithelial surface from the underlying cardiac
muscle, and in some way, protects the delicate
endothelial surface of the heart from the
rather harsh vigorous activity of the pumping
cardiac muscle. Well, one of the things you
can’t really appreciate when you look at
histological section of the heart is that
it actually does have a very strong
fibrous skeleton. And that fibrous skeleton, very
dense connective tissue, dense collagen, is
the insertion point for all the cardiac
muscle. So it’s a very important component.
On this slide, you could see a tiny little
section taken through the myocardium of
the atrium. And also, you can see that there’s
a septum of connective tissue between both
atria here, and also between the ventricles.
Here’s a section here showing you a label
of a piece of the myocardium of the ventricle.
Notice though that in between this myocardium
of the atrium and myocardium of the ventricle,
there is this clear component. That clear
component is part of the connective tissue
fibrous skeleton of the heart. It’s a membrane
that separates the atria from the ventricle.
And that’s very important because that then
stops the wave of impulse, and therefore,
the wave of contraction going directly from
the atrium muscle to the ventricle muscle.
So this allows a delay, and of course, that
delay is very important between the filling
of the ventricle and the contraction of
the atrium. So it’s really an electrical isolation
sort of tissue there. It’s very important.