Artery and vein histology

The circulatory system consists of two functional parts, the blood circulatory system and the lymphatic system.The blood circulatory system or cardiovascular system consists of a circuit that transports blood to and from tissues throughout the body.The heart pumps the blood through a series of arteries that branch into smaller and smaller blood vessels that supply the tissues with blood.The smallest arteries branch further to become arterioles, which drain the blood into capillaries.The capillaries form a network of tiny blood vessels that perfuse the tissue.The capillaries then drain into venules, before converging to form small veins.The veins continue to join or converge with one another, forming larger and larger veins that eventually drain back into the heart.Arteries and veins have walls that share a common overall structure that consists of three layers: the tunica intima, tunica media, and tunica adventitia.The inner layer or tunica intima is lined with a single layer of a specialized epithelium called the endothelium, which acts as a semipermeable barrier.The subendothelial layer is the layer beneath the endothelium and is still part of the tunica intima.This layer, which consists of loose connective tissue and occasionally smooth muscle as well.In some arteries, the tunica intima will also have a very thin layer of elastic tissue along the outer edge of the subendothelial layer called the internal elastic lamina.The internal elastic lamina has holes throughout the layer, which allow substances from the blood to more easily diffuse through this layer, deeper into the wall.The middle layer or tunica media consists mostly of smooth muscle.Arteries will also contain a lot more elastic fibers in comparison to veins.Both the smooth muscle and elastic fibers are arranged in circular or concentric layers surrounding the lumen of the blood vessel.Similar to the tunica intima, in arteries, the tunica media may also have a layer of elastic tissue that surrounds it, but in this case it’s called the external elastic lamina.The tunica adventitia is the outermost layer of connective tissue that consists mostly of longitudinally arranged collagen and elastic fibers.Generally, arteries will typically have a thicker tunica media and smaller lumen; whereas veins will have a larger lumen and the thickest layer is the tunica adventitia.Large arteries are categorized as either elastic arteries or muscular arteries.Elastic arteries include some of the largest arteries in the body, such as the aorta and carotid arteries.These arteries have an outer diameter that’s typically more than 10 mm.Muscular arteries, such as the femoral and radial arteries, are slightly smaller, with an outer diameter between 1-10 mm.As you might expect, the elastic arteries will have significantly more elastic fibers in the tunica media compared to muscular arteries and the muscular arteries have more layers of smooth muscle.Although, it’s the muscular arteries that actually have a more prominent internal elastic lamina, and larger muscular arteries can also have a prominent external elastic lamina as well.This section of tissue was taken from the wall of the carotid artery and stained with Hematoxylin and Eosin, or H&E for short.The lumen is on the right side of this image, which means the very thin layer next to it is the tunica intima.This layer has small folds that allow arteries to easily expand when blood pressure increases.For example, when the heart contracts during systole, blood pressure within the carotid arteries will increase significantly.The endothelium is composed of simple specialized cells, but don’t have their typical flat appearance because of the folds.Along the outer edge of the intima, we can see portions of the very thin layer of elastic tissue called the internal elastic lamina.Now, let’s move to the thickest layer in arterial walls, the tunica media.This layer is composed of circularly or concentrically arranged elastic fibers that more or less alternate with layers of smooth muscle.With an H&E stain, the smooth muscle is stained a combination of purple and pink; and the elastic fibers are seen as wavy sheets that either stain pink or are unstained.The majority of the elongated nuclei are from the smooth muscle cells, and the rounder nuclei are more likely to be from the fibroblasts.In order to visualize the elastic fibers more easily, a specialized stain such as aldehyde fuchsin can be used.This stain won’t stain smooth muscle, but will stain the elastic fibers dark purple.Individuals that have chronic high blood pressure will actually have an increased number of elastic lamellae or layers in order to compensate for the increased pressure.Surrounding the entire artery is the tunica adventitia, which mainly consists of dense irregular connective tissue, but in this image we can also see some small blood vessels that supply the tunica adventitia and tunica media with oxygen and nutrients.These vessels are called vasa vasorum, because in Latin, vasa vasorum means “the vessels of the vessels.”The round empty spaces seen in this image are adipocytes.Although technically, they’re just the empty spaces where adipocytes used to be present.That’s because adipocytes and other hydrophobic structures are actually dissolved during slide preparation when stained with H&E, so they’re not seen in the final slide.Overall, the tunica adventitia is typically less than half the thickness of the artery’s wall.Alright, let’s now look at a large vein such as the brachiocephalic vein.Large veins have thinner walls when compared to large arteries and the 3 layers of the venous wall are not as well defined.The outer tunica adventitia is the thickest layer, followed by the tunica media, then a very thin tunica intima.Let’s first take a closer look at the adventitia.A significant difference compared to large arteries is the abundance of smooth muscle that’s found in the adventitia of large veins.The connective tissue surrounding the smooth muscle contains a lot of collagen fibers, which are very eosinophilic and stain bright pink or red.On the other hand, smooth muscle has both eosinophilic and basophilic structures, so as a result, the muscle fibers will have a purplish pink color in comparison to the collagen fibers.The majority of the smooth muscle runs longitudinally, in the same direction as the vein.That’s why in this cross section of the vein, the longitudinal muscle fibers have a round appearance.The muscle fibers that have a longer appearance have a spiral arrangement around the vein.Similar to the large arteries, large veins also have small blood vessels or vasa vasorum that supplies blood to the outer portions of the venous wall.Moving to the large vein’s tunica media, this muscular layer is much thinner when compared to the tunica media of elastic arteries and contains only 3 to 8 layers of concentrically arranged smooth muscle.If we zoom in closer to the vein’s tunica intima, the endothelial cells have prominent nuclei, but the flat cell bodies of the endothelial cells can sometimes be more difficult to differentiate from the underlying layer of connective tissue or subendothelial layer.Similar to large arteries, the endothelium will often also have folds that allow for additional flexibility when the blood vessels have increased pressure within their lumen, although these folds are typically smaller than the folds found in arteries.Both medium and large veins with diameters more than 2 mm can also have valves, such as the one in this large vein.The valve is formed by two projections or leaflets of fibroelastic tissue.The leaflets are lined by endothelial cells similar to the ones that normally line the walls of veins.This image was stained with Masson’s trichrome, which will stain the endothelial nuclei dark brown or black; and collagen within the valves will stain blue or purple.The valves point in the direction of blood flow.So in this image, blood would be flowing up within this vein.If blood flows in the opposite direction, the valve will close to prevent further backflow of blood.The second category of arteries are the muscular arteries, such as the mesenteric muscular artery in this image.These arteries have a smaller diameter than elastic arteries and can vary a lot in size.Although, the larger muscular arteries will often have a medium-size companion vein that runs along a similar path as a muscular artery within the body.An example of this can be seen in this low power image.Here, the artery is a mesenteric muscular artery and its companion medium vein.Typically the companion vein will have a larger overall diameter than the artery, but its walls are thinner.If we take a closer look at the wall of a muscular artery, we can see that it’s very similar to the elastic arteries, although the muscular arteries have a more prominent internal elastic lamina, as well as a distinct external elastic lamina that can be found in some of the larger muscular arteries.Similarly, medium veins are also more likely to have a prominent internal elastic lamina compared to larger veins.Although, both large veins and medium veins don’t have an external elastic lamina.Verhoeff stain is a specialized stain that’s useful for identifying elastic fibers by staining them dark-brown or black.When a muscular artery and medium vein are stained with Verhoeff stain, the elastic laminas in both walls can be identified much more easily.We can also see more clearly that the muscular arteries only have a small amount of elastic fibers present in the tunica media.This is in contrast to elastic arteries, which can have so many elastic fibers that the tunica media is almost entirely dark brown from all of the overlapping fibers.

elastic artery and can vary a lot in size. Although the larger muscular arteries.

Will Austin have a medium-sized companion vein that runs along. A similar path is a muscular artery within the body.

An example of this can be seen in this low power in it here. The artery is a mesenteric muscular artery and its companion.

Medium vein. Typically the companion vein will have a larger overall diameter than the artery but its walls are thinner.

If we take a closer look at the wall of a muscular artery. We can see that it's very similar to the elastic arteries.

Although the muscular arteries have a more prominent internal elastic land as well as a distinct external elastic lamina that can be found in some of the larger muscular arteries.

Similarly, medium veins are also more likely to have a prominent internal elastic lamina compared to larger veins. Although both large veins and medium veins, don't have an external elastic lamina.

Their hot stain is a specialized and that's useful for identifying elastic fibers by staining them dark brown or black. If a muscular artery and medium vein were stained with verhoff stain, the elastic limit in both walls can be identified much more easily.

We can also see more clearly that the Tunica Media of muscular arteries, only have a small amount of elastic fibers present.

This is in contrast to elastic arteries that can have so many elastic fibers that the Tunica Media is almost entirely dark brown from all of