Histology of the cardiovascular system + Anki flashcards

[00:00] Okay, so now I'm going to deal with the histology of the cardiovascular system. Let's start with the basic structure.

[00:20] of the circulatory system. The circulatory system basically is a tube, like blood vessels. Even the heart is a tube that is folded on itself during embryonic development. So this tube has multiple layers and each one of them has a function. So these three, actually there are three layers. We call them tunics.

[00:40] coats. So there is the tunica intima, there is the tunica mediae and the tunica adventetiae. The tunica intima is also called the tunica interne because it is the inner layer. Here is the inner layer here and it is formed of a single layer of flattened epithelial cells. These epithelial cells

[01:00] They are called endothelial cells because they originate from the endoderm. Some other epithelial cells originate from the mesoderm. So they are called mesothelial cells. And other epithelium is derived from the ectoderm. But these are derived from endoderm. So they are called endothelial cells. And they lie.

[01:20] on a basement membrane as well as subendothelial connective tissue. This is a connective tissue here lying just beneath the endothelial cell. So all these constitute what we call the tourniquet intima and then we have the tourniquet media which is as the name indicates the intermediate

[01:40] layer and this is predominantly made of muscle fibers, smooth muscle fibers. In some types of arteries there is a predominance of elastic fibers but in most of the other arteries and even in the veins there are smooth muscle fibers with collagen.

[02:00] and elastic fibers are present but there is a small amount of elastic fibers and in the heart the middle layer of the heart is the myocardium and so it's also formed of muscle fibers but they are not smooth muscle fibers they are cardiac muscle fibers.

[02:20] Then the tourniquet adventitia, which is the outer layer, tourniquet externa, and is formed of connective tissue. And in certain arteries, especially larger arteries, there are small arteries as well supplying the wall. So these are arteries of the arteries.

[02:40] arteries or vazals of the blood vessels and they have a special name. We call them the Vazavasuram. And the reason is that the blood vessels are thick and so the cells that are present in the wall cannot rely on oxygen that is present in the blood.

[03:00] blood to reach them and that's why they are supplemented by blood vessels in the wall. And these blood vessels, this vasovazorum in the heart is represented as coronary circulation, coronary arteries supplying the wall of the heart, the same idea. Now let's start with the first histology slide.

[03:20] slide here. This is a slide of a muscular or distributing artery and you can see that this is a special stain that stains elastic fibers. So you don't see cells, you don't see nuclei. This will be the tunica intima which is lined by a simple squamous epithelium like

[03:40] on subendothelial connective tissue and you can see that there is a layer of elastic fibers called internal elastic laminar. This internal elastic laminar, some textbook considered as part of the Tonicar intima, some textbooks considered as part of the Tonicar media, but it doesn't matter it is located

[04:00] located at the border between Tonica Intima and Tonica Media and it contains and is formed of elastic fibers mainly so that's why it is prominent in the stain. Then we have the Tonica Media which is mainly formed of smooth muscle fibers because this is a muscular artery.

[04:20] But still we can see some evidence of elastic fibers and then there's the external elastic laminar, another layer of elastic fibers that separate tunica media from the tunica adventitia, which is formed of connective tissue. This is another stain of an artery. Look at how the artery

[04:40] is uniformly shaped, circular shape, and you can see the three layers. You can see the tonica intima. The cells, the epithelial cells, are not clearly seen. You can see some nuclei here, but not clearly seen because of it looks cringy.

[05:00] holds because of the presence of the internal elastic lamina. The internal elastic lamina is so obvious here and then you have multiple layers of smooth muscle fibers. Look at the nuclei, only small amount of elastic tissue but there are collagen fibers and then you find another layer.

[05:20] or another collection of elastic fibers here, which is the external elastic laminar, separating the tunica media from the tunica adventitiae, where connective tissue is formed. You can see a nerve accompanying the blood vessel. Here is another nerve. So these are distributing artery.

[05:40] arteries because they have a predominance of smooth muscle and their wall. That's why they are called muscular arteries. But why do we call them distributing arteries? Because these smooth muscle fibers, they will control the caliber of the artery and therefore they will help in distributing the blood to the arteries.

[06:00] to organs depending on the situation. So for example, let's say in situations where there's sympathetic overstimulation like fight or flight, these smooth muscle fibers and the blood vessels of the heart and the blood vessels of skeletal muscles are going to relax so that

[06:20] there will be more blood coming to the heart, more blood coming to the skeletal muscles. But at the same time, the smooth muscle fibers in the wall of the blood vessels supplying the gut will contract and so that the blood will be diverted from the gut to go to more needed

[06:40] organs, the heart, the skeletal muscles at this time. That's why they are called distributing arteries because they control the distribution of the blood and they are muscular because this control over distribution is due to the fact that they have thick tunica media made of multiple layers.

[07:00] of smooth muscle fiber. This is another type of artery which is called elastic or conducting artery. These are the large arteries that are connected to the heart. Like for example, this section is from the aorta and again it is stained with a special stain that shows the elastic fibers only. And because these are...

[07:20] big arteries, you cannot see the entire artery in one section. So this is a section in part of the artery and this is the luminal side and here is the where the endothelial cells are located and they lie on a subendothelial connective tissue of the tinnica intima and then you can see that the

[07:40] media is all formed of multiple laminar of elastic fibers. So you cannot distinguish an internal elastic laminar and external elastic laminar. They are all elastic laminar throughout the whole thickness of the tunica media and then there is the tunica adventitia. So that's why

[08:00] These are called elastic arteries because they have multiple layers, multiple laminar of elastic fibers and at the same time they are conducting arteries because they conduct blood from the heart and in a moment I will give more details about the use of these multiple

[08:20] elastic fibers in the wall of these arteries. Then we have the other vessel here are the arteries. These are the small arteries. They are less than 0.5 millimeter in diameter. They have the same structure of the artery, tonica, intima, and endothelial cells, tonica media with a smooth

[08:40] muscle fibers, but the smooth muscle fibers that are present in the wall of the arterial, they are only limited to one to five layers of smooth muscle fibers because the arterial is very small. So up to five layers of smooth muscle fibers are present in this wall.

[09:00] There is an internal elastic lamina, but there is no way that there is an external elastic lamina and there is a layer of adventetia. You can see this arterial is accompanied by a vein, a small vein, which is called a venule, and you can from here recognize that the venule has a very thin wall in comparison to this

[09:20] size of the lumen and that's why it is easily compressed. So it is not like uniformly rounded like in the arterial because the arterial has a thicker wall than the venule and in both of them you can see that the lumen has red blood cells. So they are definitely they are blood vessels.

[09:40] And this is a vascular bundle, which is also accompanied by a nerve and it is surrounded here by collagen fibers. This is a dense, irregular connective tissue. So you can see collagen fibers in different orientation. Here's a longitudinal. This is an oblique.

[10:00] orientation and the hall is called a neurovascular bundle because it's a nerve and blood vessels, artery, arterioles and venules and the nerve you can see that it has doesn't have a lumen that's why here I am saying that this is a nerve and

[10:20] And the cells here, the nuclei of the cells, make sure that these are not of neurons because a nerve, what's a nerve? It's a bundle of axons. So the neuron, the cell body, is not present in the nerve. It's present somewhere else, either in a ganglion or it's present in the

[10:40] gray matter of the brain or on the spinal cord. But why do we see cell nuclei here? These are the cells, the Schwann cells, that provide the myelin sheath of the peripheral nervous system. This is another slide showing you the muscular artery, but at the same time, it is showing...

[11:00] showing a vein. So if you look at the vein, you will see that the vein has a very thin wall in comparison to the size of its lumen. Compressed because of it has a thin wall, so it is easily compressed. And not only this, you will find that the vein also has the same features. So the principle is the same.

[11:20] is a tonic intima with endothelial cells. They don't look like crinkles because there is no internal elastic laminar. And there is a tonica media here, but the tonica media is very thin in comparison to the tonica adventetia. So the wall of the vein in general is thin because the blood

[11:40] in the vein is under low pressure and the thickest tunic in the vein is not the tonic amnesia like in the artery but it is the tonic adventetia. You can see here again thin tonic amnesia in comparison to the tonic adventetia and again you can see a nerve another small artery here

[12:00] not an arterial, it's an artery and another vein. In comparison to the artery, you see that it is, has a thin, very thin wall in comparison to the size of its lumen. So this is how to, in summary, to differentiate between an artery and vein.

[12:20] the shape is less deformed than the artery as you compare it with the vein which is flattened because it has a thin wall it doesn't need a thick wall because the blood in the vein is under low pressure and the intima is crinkled because of the presence of an internal elastic laminar. Here the intima is smooth because

[12:40] There is no internal elastic laminar. Also, sometimes in sections of the veins, you will find the presence of valves. Very thin layer of valves, like bicuspid valves. If this is a section in the vein, then the valves, they open in the direction of the flow of blood.

[13:00] blood and so they will allow unidirectional flow of blood toward the heart and they will prevent the blood from returning back away from the heart because if the blood tries to return these cusps will come together and will close. This is an important mechanism for venous return.

[13:20] turn in the heart. So when the vein, because they have thin wall, they are easily squeezed. And when they are squeezed, for example, by muscle contraction, like in the peripheral, in the limbs, the muscles are play an important role in the venous

[13:40] return of the blood, they squeeze the veins and they allow unidirectional blood flow. Also, the respiratory movements, increasing intraabdominal pressure, will cause pressure on the veins that the blood has no way to pass a butt to the heart because of the presence

[14:00] of valves in the veins. So you can see that morphology is related to function. Like in the large elastic or conducting arteries, these multiple elastic lamine, they will make sure that during diastole there is a continuous flow of blood.

[14:20] blood vessels. Because during systole, these elastic fibers, they will stretch and so during diastole they will recoil back and they will create a pressure in the wall of the blood vessels that will

[14:40] dampen the pulsatile flow of blood. You can see here this is a section in the aorta. Don't confuse these. These are these folds, dark stained. So these are like artifacts, not really.

[15:00] to any structure. And the endothelium, as everywhere in the cardiovascular system, the endothelium, the lining epithelium, will provide a smooth lining to facilitate the flow and prevent clotting of blood. This is another section showing here an artery.

[15:20] And again, why this is an artery? Because the profile, the section is not deformed like in the vein. I can see an internal elastic lamina and I can see that there is an external elastic lamina and there is a thick layer of, multiple layers, thick tunica.

[15:40] media, multiple layers of smooth muscle fibers and there is a tonica adventetia. This means that the lumen, because of the thick tonica media with smooth muscle fibers, the lumen is controllable in size as I mentioned and these are called distributive.

[16:00] arteries because they direct the blood flow. As I mentioned in the example of directing blood flow from the GI to the heart and to the skeletal muscle fibers at the times of fight or flight. And the elastic tissue will limit the distension of the lumen.

[16:20] is the vein. I can see that it has a distorted shape. The thickness of the wall is thin in comparison to the size of the lumen. The valves are not shown but as I mentioned you might see sometimes you might see the valves which are functionally

[16:40] important to prevent backflow of blood. Erastic fibers are not that much needed, so you can see some elastic fibers, but not in the form of an internal and external elastic lamina. This is to show you the wall of the heart, which is mainly a myocardium.

[17:00] These are cardiac muscle fibers. Cardiac muscle fibers, although they are involuntary, but they are striated. So if you magnify more, you can see the evidence of striation. In addition to that, the cardiac muscle fibers, unlike the skeletal muscle fibers, which are also striated, but unlike

[17:20] them. The cardiac muscle fibers, they have centrally located nucleus, sometimes two nuclei, but usually a single nucleus per cardiac muscle fiber. Unlike the skeletal muscle fiber, they will have the skeletal muscle fibers. They have multiple nuclei and the nuclei are located at the periphery, not centrally located like.

[17:40] in the heart muscle and also these muscle fibers are branched. They are not purely cylindrical, they have branches and so these are features of cardiac muscle fiber. But the other striking feature that you can see here is the presence of...

[18:00] these vertical junctions or dark stained areas. These are called the intercalated discs because they are present in between cardiac muscle fibers and they look dark because the cell membrane here has junctional complexes. So junctions between cells, mainly the junction

[18:20] complexes are in the form of gap junctions that allow communication between the cardiac muscle fibers communication in terms of like movement of electrolytes during contraction. So that's why that's how these muscle fibers they act like

[18:40] network or what we call syncytium because they can communicate with each other through the gap junctions. But it's not only gap junctions that are present in these intercalated discs, but also there are desmosomes, these desmosomes that will prevent separation between the cells.

[19:00] because these cells are contracting, so they tend to separate, and so the desmosomes will keep them together. So the intercalated discs are a unique feature that is present in cardiac muscle fibers. In the wall of the heart, the wall of the heart is formed.

[19:20] of three parts is formed of endocardium, then myocardium, and then we have the epicardium. It's like in the tunic and endometronic media and tunic adventetia, but with slight changes here, slight modifications. So the endocardium, we have

[19:40] endothelial cells so clearly shown here, simple squamous epithelial cells, endothelial cells, and there is a subendothelial connective tissue. You can see the connective tissue, collagen fibers. These nuclei are of fibroblasts nuclei and then the myocardium is obvious here.

[20:00] Now you can see that in the endocardium, there are these cells in some places of the endocardium. You can see these cells. These are the Purkinje fibers, part of the conducting system of the heart. They are modified cardiac muscle fibers and that's why they look like, in one way or another, they look like cardiac muscle fibers.

[20:20] muscle. They are cylindrical. They have a centrally located nucleus. But the cytoplasm, although it looks striated, but it is looks almost empty, especially around the nucleus, because they have less contractile elements.

[20:40] They are not structured. They are not made for contraction. They are made for conduction. They also have intercalated discs. So these are the Purkinje fibers. They are also, they are present in the subendothelial connective tissue of the interferon.

[21:00] endocardium, not in the myocardium but in the endocardium, Purkinje fibers. Again, same features like the myocardial cells but as I said that they like they have intercalated discs but they have they have large

[21:20] amount of glycogen and less amount of contractile elements so they look as if they are empty, lighter stained cytoplasm. So to wrap up, there are three layers in the wall of most of the blood vessels, the arteries, the veins.

[21:40] means the arterioles, the tunica intima, the tunica media and the tunica adventetia. And why do I say most of the blood vessels? Because we have the capillaries. They only have the tunica intima. You can see this is an example of a capillary. It should be also smaller than this. Maybe this one is also a capillary. It's only formed of

[22:00] lining of endothelial cells lying on a basement membrane and the reason for that are the capillaries are for exchange between the blood and the interstitial interstitial fluids. Thank you very much.

[22:20] Thanks for watching.