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Muscle & Cardiovascular System
The key objects to concentrate on during this lab are listed below. You need to learn 1) how to recognize each object, 2) understand it's primary functions, and 3) understand how structure is related to function.
In this laboratory you will be looking at slides of muscle and the cardiovascular system. You should recognize the following structures after completing this laboratory:
We will start with slides of the heart. The function of the heart is to pump blood at sufficient pressure and volume to perfuse the body's tissues. It has three layers: an inner endocardium, a middle myocardium, and an outer epicardium (also known as visceral pericardium). Follow link . Using the drop-down menu find the endocardium. The endothelium-lined endocardium is analogous to the vascular intima. In fact, the endocardium is continuous with the intima of the blood vessels entering and leaving the heart. Continue with [1,2]. Using the mouse-over find the endocardium. Do you see the red blood cells next to the squamous epithelial cells? You should be able to recognize both cell types.
Beneath the endocardium is the middle layer, the myocardium, which is composed of cardiac muscle. This section shows part of the ventricular wall. Zoom out once and mouse-over to find cardiac muscle in longitudinal and cross section. Click link . The heart is very well vascularized and the many "holes" between the muscle fibers are capillaries.
Zoom out once and click link [3,1]. This is a high magnification view of the cardiac muscle cells (fibers). Similar to skeletal muscle, cardiac muscle is striated. Striated muscle is so named because of the striations, the patterns of light and dark bands seen at microscopic level. Look at the slide and notice the bands that run horizontally across each long muscle fiber. Some horizontal bands are particularly thick. These are the transverse portions of the intercalated disks. This particular slide was stained using a Weigert stain in order to visualize these disks. Cardiac muscle cells are joined to each other by these intercalated disks. Some features of these disks are only visible at EM level. These elaborately interdigitated junctions contain desmosomes and fasciae adherentes. Portions of the intercalated disks also run longitudinally between muscle fibers for a short distance. This longitudinal portion contains many gap junctions.
Using the drop-down menu highlight the Z-lines. The Z-lines delineate the sarcomeres, the functional units in striated muscle tissue. Also notice the cardiac muscle nucleus. The nuclei in cardiac muscle are centrally located within the muscle fiber (as opposed to skeletal muscle, where the nuclei are in the periphery).
Click the thumbnail and then link [1,1,1,1]. This Weigert stain also allows us to see mitochondria, which are very numerous in cardiac muscle. A number of cardiac muscle nuclei are also labeled here.
Click the thumbnail, and click link . Using the mouse-over find the myocardium and the pericardium. The visceral pericardium is also called the epicardium. It is a serous membrane covered externally by a single layer of mesothelial cells, beneath which is a relatively thick layer of adipose tissue containing the coronary blood vessels and nerves. Follow [1,1] and find the adipose cells and a lymphatic vessel. Now follow [1,1] to see the simple squamous epithelium (mesothelium).
This section, stained with our usual stain, is nearly adjacent to the previous section, w18a. Click link  . The myocardium and endocardium are labeled. The papillary muscles are also lined with endocardium. Follow [2,1] for a higher magnification view of the endocardial lining.
Click link . This slide also reveals the striated nature of cardiac muscle. Using the drop-down menu find each structure. The A-bands are darkly stained. A-bands contain the overlapping portion of the thick and thin filaments. The I-band is lightly stained and contains only thin filaments. The H-band is the very thin, light streak in the middle of the A-band. The Z-lines (in the middle of the I-bands) are the attachment points of the thin filaments and are the boundaries of each sarcomere. Notice again the centrally located nucleus.
Click the thumbnail and click link [1,1,1]. The connective tissue surrounding the muscle fibers, the endomysium, is labeled here. The endomysium is part of the cardiac skeleton. For a higher magnification view of endomysium, click link . Cardiac cells (and neurons) do not divide in the adult; therefore they accumulate lipofuschin pigment, as seen here.
Click the thumbnail and click link . Using the mouse-over find the pericardium and a branch of a coronary artery. The larger blood vessels of the heart run within in the pericardium. Follow [1,1]. Within the pericardium are also nerves, adipocytes, lymphatics and capillaries. Notice that the capillaries are, in general, smaller in diameter than the adipocytes. Follow [1,1] and notice the simple squamous epithelium. The presence of red blood cells in the pericardial sac is an artefact.
The Purkinje fibers, part of the conduction system of the heart, are specialized cardiac muscle cells. (Do not confuse them with Purkinje neurons of the cerebellum.) These fibers are situated in the subendocardium, which is a layer of loose connective tissue that binds the endocardium to the myocardium. Follow  to see the relationship of the Purkinje fibers to the myocardium and endocardium. Follow [1,2]. The Purkinje fibers are larger than the ordinary cardiac muscle fibers. They contain big, empty spaces where glycogen was stored, but was dissolved away during fixation. Find the small labeled fibroblast nucleus. Notice that the collagen fibers surrounding the individual Purkinje fibers form the endomysium. The endocardium also contains collagen, as is apparent here.
Orient yourself using the drop-down menu.
The semilunar valve is located at the origin of the aorta and, thus, separates the left ventricle from the aorta.
Follow [4,1]. The valves in the heart are folds of the endocardium. So, both sides of this semilunar valve are lined with endothelium. The tough connective tissue core of the valve helps it to withstand the powerful shear forces of the pumped blood. Use the mouse-over to find these structures. Follow [1,1]. The endothelial nuclei on the aortic side are labeled. Zoom out once and click . Despite its pleated appearance, the endothelium on the cardiac side is also squamous. Elastic fibers are also labeled here.
Click the thumbnail. Click link . The cardiac skeleton, composed of dense, fibrous connective tissue, allows the heart to work in a coordinated fashion as a pump. The valves are anchored directly to this framework. The endomysium and the perimysium, both part of the cardiac skeleton, anchor the muscle fibers. Using the mouse-over highlight the semilunar valve attaching to the cardiac skeleton.
Now that we have looked at the heart we will look at the remainder of the cardiovascular system. The heart is the pump, the arteries and the veins the conducting system, and the capillaries (and portions of the smallest venules) the site of nutrient and gas exchange. Follow link . This is a good example of a muscular artery and a vein.
Arteries can be classified into three major types: elastic arteries, also called conducting arteries (e.g., aorta), muscular arteries, also called distributing arteries (e.g., radial) and arterioles. Remember that blood vessels form a continuum; there is not an abrupt change from one type to another.
Arteries are said to have three concentric layers of tissue or tunics that make up the wall structure. These are best seen in the muscular arteries. The innermost layer, the tunica intima, is composed of squamous endothelial cells resting on a basal lamina and supported by a delicate fibroelastic layer. In muscular arteries, the tunica intima contains an internal elastic lamina. The middle layer is the tunica media. This contains helically or circularly oriented smooth muscle interspersed with elastic lamellae in varying proportions depending on the function of the vessel. In general, the amount of elastic tissue decreases in the tunica media as the vessels become smaller and smooth muscle becomes the dominant component. The outermost layer, the tunica adventitia, consists of longitudinally oriented connective tissue. In larger muscular arteries there is an external elastic lamina separating the tunica media from the tunica adventitia.
Follow links [4,2,1]. Using the mouse-over highlight the three layers of this artery. The adventitia has collagen and elastic fibers. In the media you can see the darker, elongated oval-shaped nuclei of the smooth muscle cells. In the tunica intima the nuclei of the endothelial cells can be seen. The light pink line within the intima is the internal elastic lamina.
Click the thumbnail, then follow links [1,6,2]. Using the drop-down menu highlight an arteriole. Arterioles are the smallest arteries, usually 0.1 mm or less in diameter. The arteriole walls are relatively thick in comparison to the lumen diameter. Within the tunica intima, there may be a thin internal elastic lamina. The tunica media consists of one to three layers of smooth muscle cells depending on the size of the arteriole. The tunica adventitia is a fairly thin layer of loose connective tissue.
Capillaries are found in almost all tissues, the cornea being the most notable exception. They are the distal continuations of the arterioles and are composed of a single layer of squamous epithelial cells forming a tube 7 to 10 mm (microns) in diameter. Click the thumbnail and follow link [1,1,1] to see capillaries, some of which are within a nerve fiber bundle. Follow link  for a higher magnification view.
Follow [4,1]. In the center of the slide is a muscular artery with its characteristic wavy, pink internal elastic lamina. The lumen is partially filled with red blood cells. Click link . Using the drop-down menu identify the internal elastic lamina, the endothelial cell nuclei, the smooth muscle (tunica media), and the adventitia. Notice that the smooth muscle doesn't have any striations. Click link  to see a high magnification view of the internal elastic lamina and how it separates the intima from the media. The smooth muscle cell nuclei are centrally located.
Most of the tongue is made up of skeletal muscle. Click the thumbnail and then follow . The skeletal muscle fibers run in many different directions. This is the only place in the body that skeletal muscle is so arranged. Continue by clicking link . One muscle fiber very clearly shows the striations. Using the drop-down menu, highlight the muscle nuclei in the cell periphery. Remember that cardiac muscle and smooth muscle have centrally located nuclei.
Endomysium and perimysium (both of which are connective tissue) connect muscle fibers to tendons, thus allowing the muscle to function in a coordinated fashion. Click the thumbnail and then follow [5,1] to see endomysium labelled. Click  to see a higher magnification view.
Click the thumbnail, then follow [3,1,1]. Using the drop-down menu highlight the different bands within the skeletal muscle. The A-band, I-band and Z lines have already been discussed. The M line (seen in the EM only) is a dark band in the center of the H-band. The M-line holds adjacent myosin thick filaments in register. Zoom out twice and follow [2,1] to see more labeled bands.
Follow [1,1,2]. This is another good example of a muscular artery. The internal elastic lamina is wavy and pink. The smooth muscle layer is well demarcated and if you look closely at the nuclei, small dark nucleoli can be seen. The adventitia of this artery is not clearly distinguishable from the surrounding connective tissue.
The aorta is an elastic artery. Click link . Using the drop-down menu highlight the tunica intima, media and adventitia. In the aorta, the intima is one-fourth to one-fifth of the thickness of the whole wall. The demarcation between the intima and the media is not clear (characteristic to elastic arteries). Click [1,3]. The pink wavy lines in the media are elastic laminae. A characteristic feature of elastic arteries is the concentrically arranged elastic lamellae throughout the media. Smooth muscle cells snake in and around the collagen fibers and elastic lamellae. Click  to see a higher magnification view. Use the drop-down menu, and make sure that you can distinguish between the muscle cells, collagen fibers, and elastic lamellae.
Zoom out twice and click link . Using the mouse-over highlight vas vasorum. This is a small blood vessel supplying the artery wall with nutrients and oxygen. Zoom out once and use the drop-down menu to identify many vasa vasorum within the adventitia.
Follow [1,1]. This has features of both elastic and muscular arteries; that is why it is called a mixed artery. Using the mouse-over, highlight the three layers. Between the intima and media, notice a defined internal elastic lamina. In the aorta there is no clear demarcation between the intima and media. Follow . Highlight the elastic laminae, which are not as numerous as in the aorta. The smooth muscle in this mixed artery is less prominent than in the muscular arteries. Continue with . Highlight the internal elastic lamina.
Zoom out twice and click link  to see the vasa vasorum in the adventitia.
In this oblique section, we have captured a coronary artery as it branches away from the aorta. The coronary arteries generally branch off the aorta above the semi-lunar valve.
Follow [1,1]. The aorta is an elastic artery. There are many elastic laminae within the media.
Click thumbnail and follow . Notice the abrupt change in the wall thickness between the aorta and coronary artery. Now click link . The coronary artery is a muscular artery. There are not as many elastic laminae within the media as in the aorta.
We return to this slide to discuss veins. Follow [1,4]. Veins are grouped into three categories: small, medium and large, although the structure of veins varies even more than that of arteries. The muscular and elastic tissues are not as developed in veins. In this slide, we can see that there is an internal elastic lamina in the artery and that the muscular layer (media) is much thicker in the artery than in the vein. Follow [1,1] for a higher magnification view of the venous wall.
There are valves in many veins that prevent the backflow of blood. These valves are especially abundant in the veins of the legs, where they act against the force of gravity. The valves are paired, semilunar folds of the intima that project into the vein lumen. Click the thumbnail and follow [1,3,1]. Using the mouse-over find the large vein lumen and the lumen of the small tributary vein. Continue with  and highlight the valve leaflets of the tributary vein. Notice that the leaflets are extensions of the intima of the tributary vein.
Click thumbnail and follow [1,5,1,2]. This is a lymphatic vessel. Lymphatic vessels start as blind-end capillaries within the tissues. They collect the fluid that has leaked from the capillaries into the interstitium and eventually drain this fluid into the venous system. The lymph capillaries, like blood capillaries, are lined with a continuous endothelium, but are irregular in diameter and wider than blood capillaries. In EM, it would be apparent that the basal lamina is discontinuous. The lymph capillaries lead into larger vessels. An important characteristic of lymphatic vessels is the presence of many valves that prevent backflow of lymph. Using the drop-down menu highlight the lymphatic valve and the lymphatic vessel. This is a cross-sectional view of the lymphatic valve as opposed to the longitudinal section of the venous valve. Click the thumbnail and follow [1,2,1,1] to see the wall of a much larger lymphatic vessel for comparison. As the lymphatic vessels become larger a thin layer of smooth muscle surrounds the vessels.
Click link [1,1]. In this very exciting section, we can see a lymph valve in operation. Using the drop-down menu, locate the lymphatic valve. Observe how the lymph has collected upstream of the closed valve and the downstream portion of the lymphatic is, therefore, empty.
The vena cava inferior is a large vein. Follow [2,1,1,1]. Using the mouse-over, highlight the layers. In large veins the tunica media is underdeveloped or missing entirely. Instead there is a well-developed tunica adventitia. The tunica adventitia contains smooth muscle, collagen, elastic fibers and vasa vasorum. In this longitudinal section, you can also see dark staining smooth muscle fiber bundles.
Click the thumbnail and follow [1,1] to see a cross-sectional view of the vena cava. Using the drop-down menu highlight the structures, noticing the abundant collagen and smooth muscle bundles.
Pulmonary veins and the vena cava near the heart have cardiac muscle in their adventitia. Click link  to see the relative amounts of adventitia to media/intima in this vein. Follow [1,1,2]. Highlight the cardiac muscle in cross-section in the adventitia. The endothelial cell nuclei are clearly seen lining the vein. Zoom out once and click .
Follow [1,1,2] to see a vein, some capillaries and some small lymphatic vessels. The lymphatic vessel walls are one cell layer thick. The blood capillary walls are also one cell thick, but are thicker than the lymphatics. The presence of red blood cells aids in the identification of some of the blood capillaries in this slide. Blood capillaries cannot be distinguished as continuous or fenestrated at the light microscopic level. Zoom out once and click . Here, you can find a venule and a couple lymphatics. Venules are the smallest veins and, like capillaries, are leaky vessels with thin walls. However, they can be distinguished from capillaries by their larger lumen. The walls of lymphatic vessels are even thinner than those of venules.
Follow [1,2,1]. Using the drop-down menu highlight the pericytes. Pericytes are pluripotential cells of mesenchymal origin that surround the endothelial cells of small capillaries and venules. They share a basal lamina with the endothelial cells. There is evidence that after injury to the vessel these cells differentiate to become smooth muscle cells and endothelial cells. Pericytes give rise to macrophages and, in the CNS, to microglia.
Follow [1,5,1,1]. Using the drop-down menu hightlight the high endothelial cell postcapillary venules. The venule wall is composed of one layer of endothelial cells that are tall, almost cuboidal, and tend to bulge into the lumen. These venules play an important role in allowing lymphocytes to enter the lymph nodes.
Click the thumbnail and follow [2,1]. This is a longitudinal section of a large lymphatic vessel with valve leaflets visible in the lumen. Continue with link . Using the drop-down menu, highlight the valve leaflets and the smooth muscle within the lymphatic wall.
Follow [1,1,3]. Highlight the sinusoids. The sinusoids of the liver are discontinuous capillaries. They have a wider diameter than other capillaries and are very irregular in outline. In addition to liver, they are found in bone marrow, spleen and some endocrine organs. They pose little barrier between the blood and the parenchymal cells (hepatocytes in the liver). The features responsible for this can only be seen in EM and include the gaps between and the fenestrae within individual cells, as well as the discontinuity or absence of basal lamina. Continue with . Using the drop-down menu identify the endothelial nuclei.
Now is a good time to review macrophages, neutrophils, lymphocytes and red blood cells. You should be able to identify these cell types in this magnification without help.
This is a good slide for reviewing the material we have just covered. Click link . These are the villi of the jejunum. In the villi are blood capillaries and lymphatic capillaries (called lacteals in the small intestine). Click link . Before using the mouse-over try to find the blood capillaries (hint, look for the red blood cells) and the lymphatic. The lymphatic capillary has a very wide lumen compared to the blood capillaries; in addition, the lymphatic vessel wall is even thinner than the blood capillary wall.
Click thumbnail and follow [3,1,1]. Highlight the isolated smooth muscle cells. They are longitudinally oriented and act to shorten the villi. Although not labeled in this view, try to find a few capillaries by recognizing the red blood cells in their lumen. (As a refresher, look for the mast cells and lymphocytes.)
Click the thumbnail and follow [2,1]. This shows the two layers of smooth muscle in the muscularis externa. These fibers are much larger than the smooth muscle fibers in the arteries and arterioles. In the cross-sectional view it is easy to see that the nuclei are centered within the muscle fibers. Also, in the longitudinal section, there are no striations, unlike skeletal muscle.
We direct you to the following links for further review of the vessel types: arteries, capillaries, veins, and lymphatics. Click the thumbnail and follow [3,2]. Now click the thumbnail and follow . Check links , , , and . Be aware that even a trained histologist will not be able to identify every vessel in a given section.
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