VASCULAR SYSTEM
Objectives
You should become able to:
1. find and recognize the blood vessels in a tissue section.
2. recognize the layers of the walls of arteries and veins.
3. distinguish the following
structures on your slides:
| muscular arteries | venules = small veins | |||
| tunica intima | medium-sized veins | |||
| endothelium | vena cava | |||
| tunica media | heart | |||
| smooth muscle cells | atrium | |||
| tunica adventitia | ventricle | |||
| internal elastic membrane | endocardium | |||
| external elastic membrane | subendocardial layer | |||
| elastic arteries / aorta | myocardium | |||
| arterioles = small arteries | coronary artery | |||
| capillaries | heart valves | |||
| pericyte | Purkinje fibers | |||
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 |
Blue Histology |
Slides
| D-2 | Adipose tissue (H&E) | |
| D-18 | Left atrium (elastic stain) | |
| D-19 | The heart (H&E) | |
| D-20 | Purkinje fibers (H&E) | |
| D-22 | Aorta (elastic stain) | |
| D-23 | Aorta (H&E) | |
| D-27 | Vena cava | |
| D-72 | Seminal vesicle (H&E) | |
| D-121 | Pancreas, monkey (Masson stain) | |
| D-162 | Scalp (H&E) |
Optional
slides
| | D-16 | Heart muscle (H&E) | |
| D-24 | Carotid sheath (elastic stain) | |
| D-25 | Brachial artery and vein (Masson stain) | |
| D-26 | Subclavian artery (H&E) | |
| D-114 | Colon (H&E) | |
| D-122 | Pancreas, monkey (H&E) |
Prologue
Arteries
Arteries have to cope with the pressure pulses generated by the heart. The large arteries close to the heart contain very large amounts of elastica, which allows them to function as a temporary pressure reservoir. These include the aorta and some of the first arteries that branch from it. Their walls have dozens of sheets of elastica arranged circularly. Further down the vascular tree the arteries are dominated by smooth muscle in their middle media layer. These arteries are called muscular or distributing or, simply, medium sized arteries. Their main function is to regulate the amount of blood that flows to various parts of the body. The smaller arteries still are called arterioles or small arteries. Their function is also to distribute blood at controlled rates to specific regions of tissue. They also regulate blood pressure, since the small arterioles are the ones that provide the greatest resistance to blood flow. There are no concrete criteria for distinguishing the smallest distributing arteries from arterioles. Generally, if an artery can always be found in a particular place it is called an artery, whereas those that are too variable to be named in a gross anatomy text are dismissed as arterioles. Thus, some named arteries of the eye are called arteries even though they are much, much smaller than "arterioles" in the upper thigh.
Veins
Three categories of veins are distinguished: small, medium and large. This subdivision is somewhat unsatisfactory because the caliber of the wall does not always correlate with structure. Individual veins vary much more than do arteries. In general, veins running in dense connective tissue and those that drain downwards to the heart have thinner walls than others. Some running through very dense connective tissue may have nothing in their walls except for a layer of endothelium. Another generalization is that arteries and veins of corresponding size tend to run together and are named accordingly. If you are examining a small artery, the accompanying veins will be small as well.
Veins with diameters less than 2 mm are referred to as small veins or venules. These have two main layers: an intima and adventitia. A few smooth muscle cells may be present as a media in larger small veins. The adventitia consists of loose connective tissue with thick bundles of longitudinally oriented collagen and elastic fibers. The smallest venules, less than 50 microns across are sometimes called postcapillary venules. Their structure is similar to that of a capillary, consisting of only very squamous endothelium and a few pericytes. They share an exchange function with capillaries. Much of the tissue fluid resorbed at the venous end of capillaries actually is taken up by postcapillary venules. In some places the endothelium of postcapillary venules is specialized to permit certain types of white blood cells to leave the blood stream and enter the surrounding connective tissue.
Medium-sized veins (2-9 mm) include the cutaneous and deeper veins of the extremities. Their intima consists of an endothelium and an inconspicuous connective tissue layer. The media usually is visible but much thinner than in corresponding arteries. It consists mainly of circularly arranged smooth muscle and collagenous fibers. The adventitia is thicker than the media. It contains connective tissue with thick longitudinal collagenous bundles and elastic fibers.
Large veins have an intima similar in structure to that of the medium-sized veins. In the larger vessels the connective tissue layer under the endothelium may be prominent and thick. The media also is similar in structure to the medium-sized veins but with more smooth muscle. The adventitia is well developed. It contains prominent longitudinal layers of smooth muscle (see D- 26). It also has thick elastic fibers (see D-24). All of these layers are well demonstrated in D-27.
The most obvious structural difference between arteries and veins is the thickness of the tunica media. The function of this layer is distinctly different in the two types of vessels. The smooth muscle layer of arteries contracts to limit the flow of blood or to increase blood pressure. Veins maintain the tone in their walls to regulate the volume of blood that is held in them. Veins contain about 70 percent of the blood volume of the body and can alter this reserve by contraction.
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Slide descriptions
D-121 Pancreas, monkey (Masson stain)
This section contains a typical medium sized, muscular ( = distributing) artery. If your section was cut from a different block of tissue, obtain a replacement which follows the description here. Hold the slide up and locate the artery off to one side as a hole about a millimeter across ringed in blue. Examine it through the 10X objective with which the entire artery should fit into view (illustration). The staining procedure colored collagen blue-green, elastica white and cells red, in particular the muscle cells in the artery;. The radial lines in the arterial wall are artifact where the section of tissue has buckled instead of lying flat on the slide. Such problems are not uncommon for tissue with a lot of elastica.
The tunica media of the arterial wall is instantly recognizable because it is composed mainly of red-colored smooth muscle with some collagen. It ends internally in a colorless sheet of elastica, the internal elastic membrane. That structure and the traces of tissue inside of it constitute the intima. Along one side of the artery the intima is pathologically thickened. Smooth muscle cells have invaded the intima to form an arteriosclerotic lesion.
The tunica media is surrounded by the tunica adventitia, composed of dense connective tissue. The adventitia is about 1/4 - 1/2 the width of the media. The whole artery runs in a seam of (blue) connective tissue between bright red lobules of pancreatic parenchyma.
Examine the individual layers of the wall more closely at high (400 X) power (illustration). The smooth muscle cells of the media are circularly arranged, as are the collagen fibers. This layer is very dense. The internal elastic membrane is prominent. It has puckered up the inner surface but if the artery were distended that surface would be smooth. In most places the intimal surface is well preserved and you can see the flattened nuclei of the endothelial cells resting on a minimal layer of connective tissue.
Knowing what elastica looks like in this stain, scan around the media to find very thin scattered sheets of it appearing as thin white or pale lines. If you have very sharp eyes you may be able to detect some elastica in the adventitia, again as wavy colorless lines.
Turning to the adventitia, its boundary with the tunica media is characteristically very sharp. The outer edge of the adventitia is less obvious but still recognizable. One of the reasons that you can distinguish the adventitia from the surrounding connective tissue is that its fibers are more densely packed together. Another is that the fibers of the adventitia are oriented predominantly along the axis of the artery instead of randomly. This orientation is a bit hard to see because the fibers are so closely jumbled. However, note that the nuclei of the fibroblasts in the adventitia are squat and scrunched up. This is because they are elongated in the direction of the fibers and so have been cut transversely. In the surrounding irregular connective tissue the nuclei are elongated in various directions. Let me reiterate this important structural generalization about arteries and veins: the components of the media layer are arranged circularly while those of the adventitia run mainly longitudinally.
Finally take a look at that arteriosclerotic plaque. It is more developed on some of the class slides than others. The lesion does indeed lie in the tunica intima where it has interrupted the internal elastic membrane. The pink structures in it are smooth muscle cells running in various directions but primarily longitudinally. Your large arteries are probably riddled with plaques such as this. Eventually some will grow so large as to impede the flow of blood. If this happens in your coronary arteries you may welcome a bypass operation. Better push your plate away from a second helping of Australian mutton stew and learn to like bread without butter.
Before you leaving the slide go back to low power and scan around for the large duct in the middle of the section (illustration). It too is a hollow tube running in a seam of connective tissue. A glance at high power immediately distinguishes it from an artery. It is lined with tall epithelium instead of very squamous endothelium.
D-72 Seminal vesicle (H&E)
This slide happens to include a good sized tab of connective tissue full of small muscular arteries, stained with (H&E). It also has comparably sized veins with thin walls, little smooth muscle and irregular lumens. Ignore the sections through the seminal vesicle for now. We will come back to them as a part of the male reproductive system
Find an artery that has been cut transversely so as to appear round in cross section. Then go to high power (illustration). The media should look like a sheet of smooth muscle caught in the plane of section, such as you saw in the last slide. If this not the case then you probably are looking at a vein (illustration). Go back to low power and find an artery. Gauge where the outer limit of its adventitia is. That boundary is not nearly as sharp as its boundary with the media. The adventitia is as thick as, or a bit less than, the media. You may be lucky enough to be looking at an artery that shows the internal elastic membrane well but probably the vessel has contracted so much that the whole intima is a thick mess.
Now look at the veins. Notice how they typically collapse in an irregular fashion. Imagine how large their lumens would be if they were outstretched with blood. Of course you could easily overestimate if the section were oblique instead of transverse.....how would you be able to tell if this were the case.
D-22 aorta (elastic stain)
The largest arteries in the bodies are of the elastic variety. Their media is modified by having many sheets of elastica wrapped around. Smooth muscle and collagen lie between these sheets. The aorta is the prime example, stained here to reveal the elastica. Its resilient elastic wall is quite thick, but in proportion to the size of the lumen it is thinner than the wall of muscular arteries. The elastic stain readily distinguishes the three tunics in the wall (illustration). The tunica media contains an especially large amount of elastica. The intima and adventitia, with less elastica are distinctly paler and redder. What is the significance of the artifactual radial dark lines in the walls?
Go to 100 magnification. The 30 or so concentric, fenestrated elastic membranes show up well (illustration). Fenestrated means that they have holes in them, a necessity to allow nutrients to pass to the buried cells. Most of the cells are smooth muscle cells also arranged circularly. These smooth muscle cells secreted most of the collagen and elastica of the media. The outermost elastic sheet marks the boundary between the media and the adventitia. The adventitia is composed primarily of thick, longitudinally oriented bundles of collagen. These are cut transversely leaving the adventitia with a crumbly appearance. The elastica here is also mostly in the form of longitudinal fibers. These show up as black dots and dashes at 400X.
The intima is less satisfactorily presented on these slides. Its endothelium has been lost from large regions of the aorta. Also there is more substantial arteriosclerotic pathology than in slide D-21. Smooth muscle cells have invaded the intima and proliferated to produce a layer of variable thickness above the media.
D-23 Aorta (H&E)
Compare the aorta stained with familiar (H&E) with that on the last slide. First distinguish the intima, media and adventitia layers (illustration). Then examine the media at higher magnification. You should be able to discern the elastic lamina and to distinguish them from the cytoplasm of the cells and the collagen (illustration). The elastica appears as a blank pale wavy band, because it is made up of a protein (hence acidophilic) that is quite hydrophobic (hence relatively impervious to aqueous dye molecules). The cytoplasm of the smooth muscle cells extends past the ends of their nuclei and stains darkly. The collagen appears as irregular wavy wisps accompanied by spaces resulting from tissue shrinkage. The important point is to recognize that this wall looks substantially different from that of a muscular artery even when stained with H&E.
D-162 Scalp
The scalp is a good place to look for small blood vessels. You are already familiar with this slide from your sessions on epithelium, connective tissue and skin. Determine which edge of the section is the skin surface (it is covered with epithelium) and which is the internal edge (the ragged edge where the connective tissue holding the skin to the underlying tissues was separated). Look for blood vessels close to the internal edge. For some reason a number of arterioles here also are expanded and round instead of contracted and puckered as is usually the case.
Start of by picking out a small, but not tiny, arteriole with several layers of smooth muscle in its wall (illustration). The intima consists of the flattened endothelium, an all but invisible trace of connective tissue and the internal elastic membrane. There should be a discernible adventitia, although the outer extent of that layer may be indistinct (illustration).
Next, pick out several venules (illustration). They will be in various stages of collapse. Their thin walls are mostly adventitia. The endothelial cells should show up well along the edge of the lumen. The somewhat elongated nuclei farther out belong to fibroblasts. You may find a smooth muscle cell, but probably not unless the venule is quite large. Conveniently, arterioles (and arteries) often are accompanied by comparably sized venules, making the comparison between the two easy (examples #1, #2, #3)
Now look at the smaller vessels. A variety of characteristics can help to distinguish small arterioles from venules - the relative thickness of the wall to the lumen, a puckered appearance to the lumen, an internal elastic lamina and so forth. I find that the most definitive is the presence or absence of smooth muscle cells in the media. Small venules are devoid of these cells. Large venules may have a scattering of smooth muscle cells but MANY fewer than arterioles of comparable size. A good exercise is to try to identify the cell types of the nuclei in the blood vessels that you come across. You should be able to make good guesses about most of them (view 1, view 2).
The very smallest arterial structures are called metarterioles. They do not have a continuous layer of smooth muscle in their walls. The gaps between muscle cells are easily seen in longitudinal sections (illustration, example2). However, cross sections may or may not cut through a muscle cell. If they do not then it is almost impossible to tell metarterioles from venules. The point is, do not worry if you cannot identify some of the structures slightly larger than a capillary.
Finally you might (or might not) be able to recognize a lymphatic vessel (example2). Its wall will be much thinner and its lumen much larger than a venule (example). Lymphatic capillaries usually are collapsed and their endothelial cell nuclei look like scattered fibroblasts lying between collagen fibers.
By the time you finish with this slide you should be able to identify most of the blood vessels that you come across. Test yourself on this image .
D-2 Adipose tissue
Adipose tissue is well supplied with very small arteries and venules (second example)and is especially rich in capillaries. The capillaries consist of a single layer endothelial cells, sometimes with an associated pericyte. They often run where three fat cells come together. You can find nice examples of longitudinal as well as transverse sections. Capillaries are large enough in diameter for only one red blood cell to pass through at a time. The larger structures, big enough to hold two or more blood cells abreast but still with only a layer of flattened endothelium for their wall, are venules (longitudinal example) (cross-section example). Be warned, many people who should know better sloppily call small venules "capillaries". If you accost them they might rationalize their carelessness by pointing out that small venules allow a significant amount of exchange between blood and connective tissue (especially the uptake of water). However, this is no more reason to misidentify them as capillaries than to call Geoff Meyer an American just because he teaches here for the quarter and likes hot dogs.
This last picture of a venule also shows a pericyte in the wall of a small venule. You can only identify these cells under ordinary light microscopy when there is an endothelial cell nucleus nearby. Then, by comparing the location of the two you can detect that the pericyte is outside of the endothelial cells. This may require careful focusing as the two nuclei may be in different planes in the section (illustration). However, since you are keeping your fingers on the focusing knob of your microscope and continually shifting the plane of focus slightly up and down this will happen automatically.
Arterioles, of course, have one or more layers of concentrically arranged smooth muscle cells wrapped around a simple endothelium. You will see few here, so do not get your hopes up.
D- 27 Vena cava (Masson)
The vena cava is the largest vein in the body. Its media is easily visible as a relatively narrow layer of smooth muscle (red) and collagen (blue) arranged circumferentially. Like all veins, the adventitia of the vena cava is much thicker than the media. Here it is composed mainly of bundles of smooth muscle arranged longitudinally. Connective tissue fills in between the bundles. There is a pathological thickening along one side. Either ignore it or note that it is formed by circularly arranged smooth muscle cells invading into the inner layers of the vessel.
D-19 The heart
Hold up this slide as shown in the picture above and then rotate it 90 degrees counterclockwise. Now it is in its ordinary orientation. The atrium is at the top and the ventricle below. The outside of the heart is to the left and the lumen of the chambers, lined with endothelium is to the right. The heart valve is the thin line of tissue extending downwards. It is a thin sheet of tissue, and in sections shows up as a narrow band. Note that it attaches to the wall of the heart at the junction between the atrium and the ventricle. Note also that the muscle of the ventricle does not actually contact with that of the atrium (illustration), a point of some physiological importance. Ask your lab partners if they know what that significance is and if they do not know, tell them.
It is obvious that a substantial ring of connective tissue lies between the upper and lower chambers of the heart. It has a gristly texture - almost a cartilage - although this is not apparent visually. The prominent round structure in it is a section of a coronary artery. This artery is unusual in that it has two discrete layers of smooth muscle in its media. Its intima also has yucky pathology. (I told you to stay away from Australian mutton).
Begin by examining the wall of the ventricle under the 10X objective. The main layer is the myocardium, which corresponds to the tunica media of an artery. It is a thick band of darkly staining cardiac muscle. The endocardium is a thin layer, consisting of an endothelium and some underlying connective tissue. Check to see that it varies in thickness from place to place, and is more developed in the atrium than in the ventricle.
The epicardium is a fairly thick layer of loose connective tissue. It has a large number of fat cells, making it stain palely. (Fat cells are a dead giveaway for distinguishing the epicardium from the endocardium.) A single layer of mesothelial cells covers the free surface of the epicardium
Examine the atrium. Note that it has a much thinner myocardial layer and a thicker endocardium than does the ventricle. There loose connective tissue between the two layers, spoken of as a subendocardial layer.
Examine the valve and decide which tissue layer it is composed of. What type of tissue is it? What consistency do you think it has? What type of muscle is found in it (do not ask the instructor - look for yourself at the tissue)?
D-18 Left atrium (elastic stain)
The atrium has the same three layers as the ventricle. However, the myocardium is thinner and the endocardium thicker.
Examine the endocardium. It is very well developed. In fact, its lower part is often recognized as a "subendocardial layer". This is where most of the elastica of the atrium is found. Another curious feature is the presence of smooth muscle cells in the subendocardium. You will have to look carefully to recognize them (illustration). This is a challenging exercise if you are gung-ho, but don't bother if you are running out of time.
And, as the law of compensation would have it, cardiac muscle also extends for a short distance from the heart into the media of the aorta. Do not worry about remembering these idiosyncratic details. They are mentioned just to emphasize that distinctions in real life are never as hard-and-fast as in your class note - "Only cardiac muscle in the heart, and in the heart only."
D-20 Purkinje fibers
This slide has the interventricular septum cut horizontally. It demonstrates a specialized type of cardiac muscle cell, the Purkinje fiber. Purkinje fibers form the conduction system of the heart. Bands of these cells run from the atrium down along the interventricular septum to the apex of the heart to make the ventricles contract from the bottom up. Read about them in your text. Because the interventricular septum is within the heart, both sides of the septum you see are covered with endocardium. The Purkinje fibers, cut transversely, appear subendocardial on both sides (illustration, high mag.). They may be distinguished from normal cardiac muscle cells by the following criteria :
| l. | Purkinje fibers are wider and lighter-staining in H & E. |
| 2. | Except for the nucleus, the central part of a Purkinje fiber appears empty in H & E. This is due to an abundance of glycogen, which does not stain well in H&E. |
| 3. | Myofibrils are confined to the periphery of a Purkinje fiber. How should they look? |
| 4. | Purkinje fibers are often arranged in intimate groups of four or five. Such a group is surrounded by connective tissue. |
| 5. | A substantial proportion of Purkinje fibers have two nuclei. |
Find a bundle of Purkinje fibers and verify these four properties.
By the way, the heart tissue on this slide looks peculiar because it is from a newborn lamb. This is the classic preparation for demonstrating Purkinje fibers because here they are so distinct from the unspecialized heart muscle cells. However, as is obvious, the cardiac muscle of the lamb looks very different from that of adult Homo sapiens. It is somewhat harder to pick out Purkinje fibers in human tissue but "give it a go" (as the Australians say) in the optional slide D-16 described below.
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Optional slides
D-26 subclavian artery (H&E)
You have two slides (D-26 and D-24) of smaller elastic arteries that more or less repeat the structure of the aorta. D-26 has been stained with H&E. Examine the artery to assure yourself that you can make sense of the its wall and can tell that it is an elastic instead of muscular artery.
One additional feature of the artery that can be seen well is the vasa vasorum (illustration). Large blood vessels have enough tissue in their walls that they need a blood supply. Arterioles and venules run along the outer edge of the adventitia. They happen to be expanded here, making them easy to see. Capillaries run from these vessels up to the edge of the media.
This slide also contains a large vein, which has been split open and collapsed. Find it by taking your slide out from your microscope and comparing it with the labeled diagram of the whole tissue above. Scan along the wall of the vein at low power noting how relatively thin it is compared to the artery. Its thickness seems to vary from place to place. This is just because it has been sliced perpendicular to the surface in some places and somewhat tangentially in others. Do you remember considering this problem in the lab session on epithelium? A wall appears thinnest where it has been cut perpendicularly and this reflects its true thickness. This is also where the organization shows best and so is where you should examine any tubular structure.
Go to 400 magnification. The vessel has been cut obliquely but it is irregular in shape in its collapsed condition so that some of the wall is caught longitudinally. Scan for such an area by locating where the muscle cell nuclei show up as small round circles (illustration). Here you can see that the media is only five or six smooth muscle cells deep. These cells are dispersed as clumps or bands, separated by collagen. Most of the muscle fibers have not been sectioned through their nuclei but their cytoplasm stands out more darkly than the surrounding collagen. Now look at the adventitia. It is composed of very thick bundles of collagen mostly in a longitudinal orientation.
If you just cannot put this slide away, you might look for the few scattered clumps of smooth muscle in the inner part of the adventitia of the artery. As you might predict they run longitudinally instead of circularly as in the media.
D-24 Carotid sheath (elastic stain)
This elastic artery is stained for elastica. Review it as you did the aorta. The important feature to look at is the disposition of the elastica in the adventitia. Find the junction between the tunica media and tunica adventitia at 400 X (illustration). The elastic in the media is in the form of sheets which show up as thick wavy black lines. That in the adventitia and extending into the outer part of the media has the form of elastic fibers. These are longitudinally arranged and therefore show up in cross section as black dots. If you focus up and down on these dots they seem to move slightly. This is because you are viewing them at different planes in the section. Their shape does not change, showing that they are fibers of fairly constant cross section. Farther out the fibers become fewer, smaller and less regularly oriented. The longitudinal fibers in the inner adventitia would correspond to the external elastic membrane of a muscular artery.
This use of focusing has a lesson. Changing the focus gives you access to the third dimension in your sections. A good histologist always keeps his hand on the focusing knob, moving the stage up and down as he looks here and there. The apparent movement of the elastic fibers immediately alerts you to their shape. It will also call attention to subcellular details, such as the Golgi, which will be important later in the course. Continual movement in the third dimension will let you find structures that you might otherwise gloss over because they were out of the particular plane that you happened to be statically focused on. It does not "cost" you anything to emulate a professional. I would advise you to develop the habit of always grabbing onto the focusing knob all of the while that you are looking through your microscope. Some histology teachers say that you should focus continually with your left hand so that you can draw what you see with your right. We do not require you to draw structures, but if we were not so lazy as teachers we would encourage it and discuss your drawings with you in the lab.
To get back to your slide, two small muscular arteries lie off to one side (illustration). Use the above picture to locate them in your section. The larger, rounder one offers an excellent view of internal and external elastic membranes (illustration). The former membrane is a single fenestrated lamina, as you saw before. The breaks in this wavy black line represent fenestrations (pores) in the sheet. The external elastic membrane has an entirely different nature. It is made up mainly of longitudinally arranged elastic fibers in the inner part of the adventitia. With H&E you can usually see the internal elastic membrane in even small arterioles but not the outer elastic membrane.
Now take the slide from the microscope and find the vein. It is huge compared to the diameter of the artery and yet its wall is much thinner. As explained earlier the part of the wall to look at is where it appears to be the thinnest. Pick out that part by eye (on my slide it is the lower right hand part of the vein when the slide is held with the label to the right). Examine this part with the 10 X and then 40 X objectives. The elastic fibers in the media run only circularly while fibers in the adventitia run predominantly longitudinally (illustration). The outer part of the adventitia has almost no elastic fibers, just very thick bundles of collagen.
D-25 Brachial artery and vein
Hold this slide up to the light and locate the section of the l) brachial artery (the structure with a thick wall and open lumen), 2) one or two sections through the brachial vein (collapsed vessel with thin walls and very irregular lumen), 3) ulnar nerve (rounded masses). All three of these are cross-sections of elongate structures.
Large muscular arteries look different from smaller ones because they contain more elastica, some as sheets. You can think of them as in transition from elastic to muscular, although this artery is definitely muscular. It lacks the orderly lamella upon lamella of elastica.
The intima is thin with a squamous endothelium and a sparse amount of underlying connective tissue, as you would expect (illustration). Immediately beneath, a thick band of elastic fibers forms the elastica interna. It gives a folded or crenellated appearance to the intima. Note the blank pinkish appearance that Masson stain imparts to elastica, so that you will recognize this material in the other tunics of the artery.
The media is dominated by smooth muscle cells (stained red) arranged in concentric layers, but with a substantial number of small elastic sheets and fibers and collagen fibers between the cells. The elastica can be recognized as light pink wavy bands arranged concentrically. The small bundles of collagenous fibers stain green.
The adventitia is a thick dense connective tissue. Its external elastic lamina is visible as a density of cross sections of stout elastic fibers.
Masson stain emphasizes the difference in wall structure between arteries and veins (illustration). The media of the fairly good sized vein on this slide is much thinner than that of the corresponding artery. Nevertheless it does have a significant amount of smooth muscle, as bands separated by sheets of collagen (illustration). Confirm that the muscle fibers run circularly. The adventitia is almost entirely collagenous, except for a sprinkling of small blood vessels. Now, with your prowess at picking out elastic fibers under Masson staining, satisfy yourself that there are some in the media and, less prevalently, in the adventitia (illustration).
D-122 Pancreas, Monkey (H&E)
This is an excellent slide for examining small and medium sized arteries. Look in the connective tissue between the pancreatic and lymphatic tissues. Maybe you should have looked at this slide first instead of the others.
D-114, Colon
Hold the slide up to the light and locate the submucosa layer. It is the band of connective tissue with a grainy appearance to the unaided eye. It has many small blood vessels, mainly venules. These venules happen to have relatively thick walls with abundant fibroblasts (illustration). You will have to distinguish them from arterioles by looking for smooth muscle cells in their walls (illustration). Most of the arterioles are expanded with blood in them so the standard criterion will not work (that arterioles have walls thicker than their lumen while venules do not). Here you must look at cellular architecture, and make judgments, since some venules have a trace of smooth muscle. You might also find lymphatic capillaries expanded enough to recognize, or you might not.
As an addendum another good place to see small arteries are to be seen in the fragment of loose fatty connective tissue attached to the outside of the section of colon. You will probably find it easier to distinguish arterioles from venules here than in the submucosa.
D-16 Heart muscle (H&E)
This section of heart has fibers cut longitudinally and transversely. Some of the longitudinal bundles contain Purkinje fibers. You can hunt for them using the features listed above, or, if impatient, use the picture above to guide you to them. Look at these Purkinje fibers and see that the criteria listed above for transversely sections apply equally to the fibers running in the plane of section.Compare the Purkinji fibers them with unspecialized cardiac muscle fibers.
Some slides have only obliquely sections Purkinje fibers*. If so, find the ones sectioned most nearly longitudinally and most nearly transversely to examine.
| * | If this is the case take it as a friendly warning. Other students are not having this problem. Shape up and try leading a cleaner life for a change. I can tell you guys were not boy scouts or girl scouts. Clean up your act if you expect to get through dental school. |