Video: Heart histology

The heart is certainly an incredible organ which actually begins beating just four weeks into the gestation period. This beating continues throughout our entire lives with about one hundred thousand heartbeats occurring each day as the driving force behind oxygen and nutrient transportation around the body. In just one day, your hearts pumps around nine thousand liters of blood. That’s a lot of work! So let’s have a look at the cellular architecture of the heart that makes all this possible. It’s time to study the histology of the heart.

In today’s tutorial, we’ll be looking at the layers and components of the heart at a microscopic level, so let’s see what this resilient organ is really made of.

To begin, we’ll look at the heart on a larger scale to remind us of the gross anatomy of the heart and this will include looking at the four chambers of the heart and the four heart valves. We’ll then dive into the heart on a microscopic scale and look at its histological features. We’ll start with the layers of the heart including the epicardium, the myocardium and the endocardium. We’ll then look at features we can see related to the conductive system of the heart as well as its autonomic innervation.

Next, we’ll look at the valves in more detail and identify the layers that make up those structures, and finally, we’ll talk about a clinical scenario that can be studied using histological slides of heart tissue.

The heart is a muscular organ that is responsible for the pumping and circulation of the blood around the entire body. It is situated in the mediastinum of the thorax, anterior to the thoracic vertebrae and esophagus, and nestled between the lungs. Anterior to the heart is the sternum.

The heart is comprised of four chambers – the right and left atria and the right and left ventricles. Deoxygenated blood first enters the right atrium. From there, blood gets passed into the right ventricle, and when that chamber contracts, blood travels to the lungs to get rid of the carbon dioxide it’s carrying and to pick up oxygen. The oxygenated blood then returns to the heart by entering the left atrium. Finally, it moves into the left ventricle which is the chamber powerful enough to push the oxygen-rich blood into the systemic circulation and it travels throughout the body.

There are also four valves associated with the heart, all of which we can see in this image here. In this image, we’re looking at the heart from a superior view with the right and left atria removed. Anteriorly at the top of the screen is the pulmonary valve which is situated between the right ventricle and the base of the pulmonary trunk. Posterior to it is the aortic valve which is found between the left ventricle and the ascending aorta.

On the left of the screen and on the left side of the heart is the left atrioventricular valve, also known as the mitral or bicuspid valve, and it’s found at the junction between the left atrium and the left ventricle. And finally, on the right, is the right atrioventricular valve, also referred to as the tricuspid valve, which is located between the right atrium and the right ventricle, and we’ll discuss the heart valves in more detail later on in the tutorial.

So we’re now going to start transitioning to the histology of the heart and the images that we’ll look throughout this tutorial will look similar to this one as we identify the various components of the heart as seen on a microscopic scale. And this slide shows us the wall of the left ventricle as well as the mitral valve, and we’ll identify these parts shortly.

The first histological topic we’ll cover are the layers that make up the heart wall. We’ll work our way from external to internal, go through the following layers, and identify what can be observed at the microscopic level in histological slides. We’ll start with the epicardium, and within the epicardium, we’ll look at the mesothelium and subepicardial connective tissue. We’ll then move internally to the myocardium and have a look at the cardiac muscle within it. Next, we’ll look at the innermost layer, the endocardium, which is composed of four layers itself. From external to internal, they are the subendocardial layer, the myoelastic layer, the subendothelial layer, and finally the endocardiac endothelium.

So let’s get started.

The outermost layer of the heart is called the epicardium, and it’s the part of the slide we can see highlighted here in green. The epicardium, also called the visceral layer of serous pericardium, is a protective layer made up mostly of connective tissue. The epicardium has two components within itself which we’ll take a look at next.

The very outermost layer of the epicardium and therefore the outermost layer of the heart itself, the layer of the epicardium that we can see highlighted in green, and this is the mesothelium. The mesothelium is a layer of simple squamous epithelium that lines the epicardium of the heart, and mesothelium can also be found as the outermost layer of tissue lining other serous cavities such as in the pleura and the peritoneum.

Just deep to the mesothelium is another layer of the epicardium called the subepicardial connective tissue, and this is the region we can now see highlighted in green, and this layer, as its name suggests, is made of connective tissue including adipose tissue or fat. And it makes up the bulk of the epicardium as you can see in this image. The thickness of this layer varies around the heart depending on how much fat or adipose tissue is present. The coronary sulcus is an example of where the epicardium is thicker because of the fat deposits found there.

The second or the middle layer of the heart is called the myocardium, part of which we can see highlighted in green in this image. The myocardium is the contractile layer of the heart made up mostly of cardiac muscle, and we won’t be getting into the fine detail of this type of muscle tissue today, but if you would like to know more, be sure to check our dedicated video tutorial which will explain all that you need to know.

Just like the epicardium, the thickness of the myocardium varies a lot as well and in the atria of the heart is where the myocardium is the thinnest. The ventricles especially the left ventricle which has to have the strength to pump blood throughout the whole body has very thick layers of myocardium.

The final layer of the heart wall is the innermost layer and that’s called the endocardium, and this is the layer on the inside of the chambers of the heart and is therefore in contact with the blood pumping through it. Once again, the thickness of this layer varies and it’s thinnest in the ventricles and thickest in the atria, the opposite of what we saw in the myocardium.

The endocardium consists of four parts within it. They are the subendocardial layer, the myoelastic layer, the subendothelial layer, and finally, the endocardiac endothelium, which is the innermost part. What we can see highlighted in green now is the subendocardial layer and this is the layer of endocardium that is just beneath and comes into contact with the myocardium and this layer contains the conducting system of the heart which we’ll look at later on in this video.

The next layer is a layer of varying thickness known as the myoelastic layer. As the name suggests it's composed of irregular dense connective tissue containing elastic fibres, as well as scattered smooth muscles cells.

The next layer is the subendothelial layer which we can now see highlighted in green, and this layer is known as the myoelastic layer and is comprised of connective tissue as well as some smooth muscle cells, and is the middle layer of endocardium situated between the subendocardial layer that we just discussed and the endocardiac endothelium, which we’ll look at next.

And finally, we’ve come to the last layer of the wall of the heart. The innermost layer of all, the endocardiac endothelium is a single cell layer of simple squamous epithelium lining the chambers of the heart and is, therefore, the layer that is actually in contact with the blood itself.

Next up, we’re going to have a look at the conductive system of the heart and the heart generates its own electrical impulses through this conductive system, and remember we said, it is found within the subendocardial tissue, so that’s where we’re heading to next.

Specifically, we’re going to have a look at the Purkinje fibers of the conducting system of the heart and these are the structures that we can see highlighted in green on this image. So Purkinje fibers are modified cardiac cells so they do look similar to cardiac cells themselves, however, there are two main differences. One is that Purkinje fibers are larger than cardiac cells and the second is that they have a pale appearance in the histological slide – and you can see the differences between these Purkinje fibers and the adjacent cardiac cells. It's important to note that even though Purkinje fibres conduct action potential potentials to cause contraction of cardiac muscle, they are not classifed as nerves, or nervous tissue, but rather specialised cardiac muscle cells as I mentioned earlier.

Purkinje fibers are the terminal strands of nervous tissue in the heart and their ends will stimulate the ventricles to contract at the end of the heart’s conduction system part.

So besides the heart’s own conduction system, it’s also innervated by the autonomic nervous system, and it’s through these nerves that the brain can tell the heart to speed up or to slow down, and these nerves are found in the epicardium as well as the regions of the sinoatrial and atrioventricular nodes.

In this image, we can see an autonomic nerve highlighted in green within the epicardium, and this outermost part of the epicardium is the mesothelium, and this area is the subepicardial connective tissue. Within the subepicardial connective tissue, we can see the cross-section of an autonomic nerve.

The heart receives input from both the sympathetic and parasympathetic divisions of the nervous system and parasympathetic stimulation is delivered via the vagus nerve resulting in a decreased heart rate whereas sympathetic innervation input increases the heart rate by accelerating activity of the sinoatrial and atrioventricular nodes.

The final topic we’ll cover in this video is the histology of the heart valves.

We’ll start more generally by looking at a valve cusp as a whole, and this highlighted green projection of the heart wall is a cusp of the aortic valve and this is the valve that is situated between the left ventricle and the ascending aorta. This image on the right shows a superior view of the heart with the entire aortic valve highlighted in green, and remember this valve along with the pulmonary valve is a semilunar valve and consists of three cusps or leaflets.

The valve cusp in this image is a cusp of the mitral valve, also known as the right atrioventricular or bicuspid valve, and this valve is between the left atrium and the left ventricle, and as its name suggests, it is made of two cusps. And as we look at the different parts of the cusp, we’ll be looking at this image of a mitral valve cusp. What we can see highlighted in this image is the annulus fibrosus of the mitral valve and there are four annuli fibrosi in the heart, one for each valve. And these structures are the openings in the cardiac skeleton that support the heart valves themselves. So we see the annulus fibrosus in this area between the valve cusp and the heart wall.

It should be noted that the term cardiac skeleton refers to the fibrous framework of dense connective tissue that passes transversely through the heart including the heart valves, and thereby forming the four annuli fibrosi and providing attachment for the atrial and ventricular myocardium. The cardiac skeleton also functions as an insulator and that it electrically insulates the atria from the ventricles of the heart.

As we look more closely at the valve cusp, we’ll find that it’s got four layers and these are the lamina atrialis, the lamina spongiosa, the lamina fibrosa, and the lamina ventricularis.

The layer on the atrial side of the cusp is called the lamina atrialis and this layer is mostly formed by elastic and collagen fibers and continues with the subendocardial layer and it’s covered by a layer of endothelial cells which are in contact with the blood circulating through the atrium.

The layer of the mitral valve cusp that we can see highlighted here is the lamina spongiosa, and that this layer is made up of loose connective tissue as well as some collagenous fibers. The lamina spongiosa is most developed at the free margin of the cusp, which comes in contact with the opposing cusp of the mitral valve.

The third layer of the mitral valve cusp is the lamina fibrosa, and this layer is closer to the ventricular side of the cusp and is made of collagen connecting to the annulus fibrosus.

The final layer of the mitral valve cusp is the lamina ventricularis and this layer is on the ventricular side of the cusp and is covered by a layer of endothelial cells.

And the very last structure that we’ll identify today is what we can now see highlighted in green, and this is part of the chordae tendineae. Chordae tendineae are attached only to the atrioventricular valves, and the mitral valve is one of them, and these cord-like structures attach the valve cusps to the ventricular walls specifically to the papillary muscles to prevent the valve from prolapsing when the ventricle contracts.

So now we’re going to take a look at a clinical scenario that you could identify on a histological slide, and in the case of a heart attack or a myocardial infarction, the heart tissue is deprived of oxygen. When this happens, the tissue changes and we can see these changes in the histological slides, and in this image we’re looking at some cardiac tissue. Can you see the difference from the normal cardiac tissue?

The main thing we can see in this image is that there are some different-looking structures here, here, here, and more. These darker pink wavy structures are called contraction bands and contraction bands are formed after excessive contraction. This happens in a case of myocardial infarctions when the area of tissue that was deprived of oxygen is then exposed to high levels of calcium, and these high levels of calcium induce the excessive contraction that leads to the formation of contraction bands resulting in contraction band necrosis which is the pattern we observed on the histological slides.

Okay, so now you’re an expert on the histology of the heart. But before I let you go, let’s have a quick run-through of the structures that we looked at today.

So we began by looking at the heart as a whole and reminding ourselves of the four chambers and the four valves of the heart. When we began looking at the histology of the heart, we started with the layers that make up the heart wall and the outermost layer was the epicardium within which we also saw the mesothelium and the subepicardial connective tissue.

We then went a little bit deeper and we looked at the myocardium which is the muscular layer of the heart and the final layer that we looked at and the innermost layer of the heart was the endocardium. Within the endocardium, we identified the subendocardial layer which contains the conduction system of the heart, the subendothelial layer, and finally, the endothelium.

Next, we explored the conduction system of the heart including the Purkinje fibers, which we identified on the subendocardial layer and sticking with nerves, we found an autonomic nerve in the subepicardial connective tissue. The last part of the heart we explored were the valves and what parts they’re made up of, and these parts are the annulus fibrosus – the fibrosus opening of the valve; the lamina atrialis, the lamina spongiosa, the lamina fibrosa, and the lamina ventricularis.

And to finish off, we looked at what you might see in a histological slide of a heart that has had a myocardial infarction and contraction band necrosis.

So thanks very much for joining me. I hope you enjoyed this tutorial on the histology of the heart. Happy studying!