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Medical Histology and Virtual Microscopy Learning Resources

The University of Michigan Medical School

Cardiovascular System

Atlas Wheater's, pgs. 152-66, Circulatory system
Text Ross, Chapter 13, Cardiovascular System
Lab Resources


  1. Be able to distinguish successive parts of the circulatory pathway, and explain how the structure of the vessel wall meets the functional needs that are present in each of the parts.
  2. Know how structural differences in capillaries influence the passage of diverse material across the endothelium.
  3. Recognize features in the heart (such as cardiac muscle, valves, cardiac skeleton) that allow it to serve as a pump.
  4. Be able to describe the path of the cardiac impulse from sinoatrial node to the ventricle, and recognize the cells in the ventricle that are involved

I. Vascular System

A. Muscular (medium) Artery and Vein (W pg 148, 8.8; pg 155, 8.20)
Slide 30 (mesentery, H&E) WebScope ImageScope
Slide 42
(mesentery, H&E) WebScope ImageScope
Slide 95M
(mesentery, trichrome) WebScope ImageScope
Slide 303
(artery & vein, Verhoeff) WebScope ImageScope -elastic fibers stain purple
Slide 304
(femoral artery & vein, canine, vonGieson) WebScope ImageScope -elastic fibers stain brown

Additional slides (not referenced below, but they show more examples of muscular arteries and veins):
Slide 95
(mesentery, H&E) WebScope ImageScope
Slide 92
(artery & vein, H&E) WebScope ImageScope
Slide 92F
(femoral artery & vein, canine, trichrome) WebScope ImageScope (same as 304 above, but trichrome-stained)

Study the arteries first in slide #30 [example], #42 [example], and/or #95 (trichrome) [example]. Technically, these vessels are actually "small" arteries rather than "medium" arteries (the only difference is that medium arteries usually are given all those pesky anatomy names like "left pericardiacophrenic artery" whereas small arteries are not named). You do NOT need to worry about distinguishing between small and medium arteries histologically, but you should be aware of the organization of the typical muscular artery, of which these are excellent examples. Note the thin intima, the distinct internal elastic lamina (IEL) and the media composed of smooth muscle cells. The media also contains some elastin and abundant "reticular" collagen, as well as specific proteoglycans. There is sometimes a condensation of elastic fibers in the outer portion of the media that may be an "external elastic lamina", which varies from artery to artery. The adventitia is the dense, irregular connective tissue surrounding the media. Remember that in blood vessels the components of the media are arranged circularly, while those of the adventitia are oriented longitudinally. In vessels where the media is too thick to be supported by simple diffusion, vasa vasorum (e.g. shown here in slide 95M) may also be observed in the adventitia.

The structure of the companion veins in slide #30 [example], #42 [example], and/or #95 (trichrome) [example] is much less regular and may be difficult to understand at first, but still consists of the same basic layers as arteries with a tunica intima, media, and an adventitia, although the media is usually much thinner compared to the companion artery. Conversely, the adventitia is usually thicker in veins and may often have some bundles of longitudinal smooth muscle [example] as well as vasa vasorum [example]. In some sections, you may be able to see a thin internal elastic lamina beneath the venular endothelium (particularly in slide #95M). While the presence of such a structure is not common, it can be found occasionally in some veins, so don't use the presence of an IEL as the sole diagnostic criterion for an artery.

Slides 303 and 304 are of actual medium (i.e. named) arteries and veins. The artery in slide 303 [example] exhibits some intimal thickening (the endothelium is not evident, but you can see a crescent of tissue in between the IEL and the lumen on the right side of the artery) and deep purple-staining elastic fibers of the IEL and within the media can be readily seen. The vein in slide 303 [example] is irregularly shaped and also shows some intimal thickening. The media is thinner compared to the artery whereas the adventitia is a bit thicker --several vasa vasorum may also be seen. The "loose" appearance of the connective tissue in the adventitia is an ARTIFACT of specimen preparation --in reality, the adventitia is a sheath of dense irregular connective tissue. In slide 304, the elastic lamellae (they look like "fibers," but they are actually sheets of elastin --you're just looking at them edge-on) in the tunica media of the artery [example] appear brown-black whereas all of the cells and collagen fibers are stained with eosin (no hematoxylin was used, so no nuclei can be seen) allowing you to again see the distribution of elastic tissue in the IEL, the media, and the adventitia. The vein in slide 304 has been sectioned at a point where it is joined by a tributary, so it is a bit difficult to make out the layers in some places, but if you look in the area right next to the artery, you'll see a relatively thin media (the endothelium is not that evident) and an extensive adventitia. At the junction where the tributary empties into the femoral vein, there are two valve leaflets [example], which are extensions of the intima. You may also notice that there are more elastic fibers on the side of the valve facing the blood flow which helps the valve close to prevent backflow.

B. Small Arteries and Veins (W pg 158-9, 8.11-12; pg 163, 8.19)

Slide 30 (mesentery, H&E) WebScope ImageScope
Slide 40 (trachea, H&E) WebScope ImageScope
Slide 42
(mesentery, H&E) WebScope ImageScope
Slide 93
(mesentery, H&E) WebScope ImageScope
Slide 95M (mesentery, trichrome) WebScope ImageScope

In addition to the small arteries and veins in slides #30, #42, and #95, you should also look for arterioles and venules [example]. Side-by-side examples present are excellent. Side-by-side comparisons of wall thickness rather than luminal diameter is usually a better way to differentiate arteri(es/oles) from veins/venules as the apparent diameter may be vary with the plane of section (an obliquely sectioned arteriole may appear to have a larger lumen than a companion venule cut in cross section). Many of the rather large vein-like vessels in slide 30 are actually very large lymphatic vessels [example] (W pg. 166, 8.25-26), some of which contain easily recognizable valves. These large lymphatic vessels may be differentiated from veins by the conspicuous ABSENCE of red bloods cells (lymph vessels usually contain ONLY white blood cells). In slide 40 of the trachea, arterioles and venules [example] may be found in the connective tissue between the respiratory epithelium and the numerous glands. Slide 93 offers an opportunityto look at a small artery [example] and vein [example] in longitudinal section.

C. Capillaries and Lymphatics (W pg 153, 8.3; pg 160, 8.14; pg 163, 8.19; see also section on cardiac muscle in R pg 313, Plate 20, Fig 4)
Slide 98HE
(heart wall, H&E) WebScope ImageScope
Slide 98-N (heart wall, trichrome) WebScope ImageScope
Slide 102HE
WebScope ImageScope
Slide 305
(heart wall, H&E) WebScope ImageScope
Slide 29
(small intestine, H&E) WebScope ImageScope

The myocardial connective tissue of the heart (especially in the ventricles) in slides #98, #102, and #305 contains excellent examples of capillaries in various planes of section. With a little looking you'll find a circular endothelial cell sometimes with a single red blood cell [example] contained within the lumen. Find several examples.

As you have seen in the mesentery samples above (slides #30, #42, #93, and #95), lymphatic vessels of varying size can be found in association with nearby lymph nodes. However, these channels are also present in just about every other organ. Slide 29 of the small intestine is a good example in which lymphatics may be seen in the connective tissue in the wall of the gut tube (example). The main identifying feature of lymphatic vessels is that they usually have a VERY thin wall compared to the lumen.

D. Aorta (W pg 148, 8.7)
Slide 36 (aorta, aldehyde fuchsin) WebScope ImageScope
Slide 88
(aorta, H&E) WebScope ImageScope

Look at both stains. Note that a precise junction between the intima and media is not easy to recognize in either the trichome [example] or H&E-stained [example] sections. Unfortunately, the endothelial layer is mostly absent in both slides. The transition from the media to adventitia, however, can be easily recognized in both trichome [example] and H&E-stained [example] sections. What kinds of cells are found in the aortic media? (CV1) NOTE: The radial folds [see orientation] in these sections are artifacts (folds) due to the elastic nature of the vessel and are not vasa vasorum, although examples of these small vessels may be found usually at the transition from the media to adventitia (look for spaces containing RBCs) [example].

II. Heart (W pgs 144-7)

A. Heart wall
Slide 98HE (heart wall, rt atrium & vent, H&E) WebScope ImageScope
Slide 98-N (heart wall, rt atrium & vent, trichrome) WebScope ImageScope

This section is stained either with H&E or with Aldehyde Fuchsin-Masson. You should look at both stains. Locate the atrioventricular sulcus that contains a branch of the coronary arterial system (a muscular artery that exhibits moderate intimal thickening) embedded in the epicardial fat. Look at the connective tissue present between the ventricle and atrium. This is part of the cardiac skeleton into which cardiac muscle inserts. A leaflet of an A-V valve takes origin from the cardiac skeleton. What is the composition of the valve? (CV2) Look at the atrial and ventricular endocardium, both lacking endothelium for the most part. Is this layer thicker in one chamber than the other? In this section use the endocardium just superior to A.V. sulcus as "typical" atrial endocardium. With low power, locate the Purkinje fibers present immediately beneath the ventricular endocardium in the H&E [example] and trichrome-stained [example] sections (note the appearance of these fibers in cross and longitudinal orientations). These conducting fibers are larger and paler staining than the cardiac muscle fibers. Note the arrangement of the cardiac muscle (myocardial) fibers and the investing connective tissue.

B. Interventricular septum and valves
Slide 99HE
(heart septum, H&E) WebScope ImageScope
Slide 99HM
(heart septum, H&Masson) WebScope ImageScope
Slide 99M (heart septum, trichrome) WebScope ImageScope

The interventricular septal connective tissue is present, and, in most sections, a distinct unit of specialized cardiac muscle, the A-V bundle (of His), traverses the septal connective tissue (a thin group of muscle fibers surrounded by dense c.t.). The A-V bundle is easiest to see in slide 99HM, [example], although although you should also be able to recognize it the H&E-stained section [example] as well. In these slides, the bundle fibers are cut in cross section and they are similar in size and staining to that of normal cardiac muscle fibers, although in some of your sections the fibers may more closely resemble Purkinje fibers (which is what they are). On one side of the section, a leaflet ofthe aortic valve [example] is present. On the other side, portions of an A-V valve [example] are present, as are bits and pieces of collagenous chordae tendinae. In slide 99HE, there is a piece of chorda tendinae actually attached to the valve [example], whereas in slide 99M, the pieces are unattached and out in the ventricular lumen (the attachment site is out of the plane of section [example].

C. Heart aortic valve (W pg 156, 8.7-8)
Slide 100HE
(left vent wall, H&E) WebScope ImageScope
Slide 100W
(left vent wall, Wiegert's aldehyde fuchsin) WebScope ImageScope
Slide 100 HM
(left vent wall, H&Masson) WebScope ImageScope
Slide 102
(left vent wall, H&E) WebScope ImageScope

There are 3 stains for this section (H&E, aldehyde fuchsin only, mislabeled Masson, and Masson with hematoxylin counterstain, labeled H & Masson). Note the histology of the semilunar valve and the wall of the aorta [example] at the root of the aorta, both of which may be seen in close association with the fibrous cardiac skeleton. What is the distribution of elastin? (CV3) Check the heart slide 102. This H&E stained section provides another good histological study of the heart including part of the aortic valve.

Electron Micrograph Wall Charts

#69 EPICARDIUM & MYOCARDIUM WebScope ImageScope Note the very thin, squamous mesothelium with numerous microvilli. The epicardium (visceral layer of pericardium) is rather thin, but contains blood vessels as well as lymphatics. One such lymph capillary (which is impossible to identify as such based upon this one micrograph) and part of an epicardial vein are seen here. Observe the dense arrangement of myocardial cells, and the numerous blood capillaries which permeate the myocardium.

#70 ENDOCARDIUM WebScope ImageScope Observe how thick the endocardium is in comparison with the epicardium. At this magnification, it is not possible to see the structural difference between smooth muscle cells and fibroblasts. However, it should be remembered that the endocardium does contain smooth muscle cells in addition to some nerves and impulse conducting cells (not seen here) usually located immediately adjacent to the myocardium. Is there a difference in endocardial thickness between the atria and ventricle? (CV4)

#71 ATRIOVENTRICULAR VALVE WebScope ImageScope Note that the valve is composed of two apposing layers of endocardium. The core of the valve contains loose connective tissue near the surface of the atrioventricular orifice, and a thick, dense connective tissue plate on the opposite side. Note the absence of smooth muscle cells or capillaries within the substance of the valve. What covers the valve leaflets? (CV5) PATHOLOGY: The small dense spherules in the connective tissue represent the beginning of a calcification process - an aging phenomenon.

#72 CHORDA TENDINAE WebScope ImageScope In this electron micrograph, study the arrangement of collagenous and elastic fibers in this small tendon. The endocardium is reduced to the layer of endothelial cells. Where are the cardiac muscle fibers? (CV6)

#73 IMPULSE-CONDUCTING SYSTEM - PURKINJE CELL (FIBER) WebScope ImageScope Note that Purkinje fibers are made up of Purkinje cells in the impulse condcting system. Observe the great width of the Purkinje cells in this part of the conducting system, and compare them with the width of the ordinary myocardial cells. The Purkinje cells have more glycogen but fewer myofibrils. Note the extensive lateral contact between neighboring Purkinje cells. It is believed that this relationship increases the rate of impulse propagation.

#74 ELASTIC TYPE ARTERY - AORTA (cross section) WebScope ImageScope Note the alternating layers of connective tissue and smooth muscle cells in the media. If there are no fibroblasts in the media, which cell is involved in the synthesis and maintenance of the collagen and elastic fibers as well as vascular proteoglycans? (CV7) The junction between the intima and media is difficult to identify!

#75 MUSCULAR TYPE ARTERY (cross section) WebScope ImageScope Note that the intima in this type of artery consists of only the endothelium. Note also, the obvious internal elastic lamina, the paucity of elastic components within the media, and the arrangement of smooth muscle cells.

#76 INTIMA & MEDIA WebScope ImageScope Endothelial cells are held together by intercellular junctions (mostly tight junctions). Find the interruption (fenestra) of the internal elastic lamina. This is also said to facilitate transport of metabolites to and from the media. Review the structure of the smooth muscle cell.

#78 ARTERIOLE WebScope ImageScope Note that the media in an arteriole consists of only one or two layers of smooth muscle cells. An adventitia may not be present. CAUTION: Although this arteriole (afferent arteriole of the renal corpuscle of the kidney) contains a complete, albeit thin, internal elastic membrane, many arterioles do not.

#79 CAPILLARY NETWORK WebScope ImageScope The capillary bed consists of highly anastomosing endothelia-lined tubes, seen here in a longitudinal view. It is not difficult to understand the basic function of a capillary when one sees the extremely thin endothelial wall separating the blood from the extravascular tissue space.

#80 CAPILLARY (continuous) WebScope ImageScope Note abundance of micropinocytotic vesicles.

#16 CAPILLARY (fenestrated) WebScope ImageScope Note the diaphragms within fenestrations.

#81 CAPILLARY (continuous, blood-brain barrier) WebScope ImageScope Note the lack of vesicles, a reflection of a true blood-brain barrier. Remember that true tight junctions restrict transendothelial passage of metabolites and fluid.

#82 POSTCAPILLARY VENULE WebScope ImageScope This is an electron micrograph of a portion of a microcirculatory bed. Observe the addition of pericytes to the wall of the venous capillary (those capillaries that empty into venules) and the postcapillary venule -- as opposed to the situation in true capillaries. (You do not have to distinguish subtle differences in microcirculatory bed structure!) Remember that this is the segment of the microvascular bed where lymphocytes and polymorphonuclear leukocytes exit the vascular systems by traversing the vessel wall by the process of diapedesis.

Review Questions

CV1: What kinds of cells are found in the aortic media? Answer

CV2: What is the composition of the AV valve? Answer

CV3: What is the distribution of elastin in the aortic valve? Answer

CV4: Is there a difference in endocardial thickness between the atria and ventricles? Answer

CV5: What covers the valve leaflets? Answer

CV6: Where are the cardiac muscle fibers in the chordae tendinae? Answer

CV7: Which cell is involved in the synthesis and maintenance of the collagen and elastic fibers as well as vascular proteoglycans in the aorta? Answer

Practice Questions

1. The vessel indicated by the arrow is:
click here for image
A. muscular artery
B. muscular vein
C. arteriole
D. venule
E. capillary answer


2. The section shown is from the wall of a:
click here for image
A. heart ventricle
B. heart atrium
C. muscular artery
D. elastic artery
E. muscular vein
F. large lymphatic vessel


3. The group of cells indicated by arrow:
click here for low mag image
click here for high mag image
A. synthesizes and maintains components of the cardiac skeleton.
B. initiates contraction of atrial myocardium.
C. attaches to atrioventricular valve leaflets via chorda tendinae.
D. conducts action potentials from the A-V node to the ventricular myocardium.
E. contracts vigorously when stimulated and therefore contributes significantly to overall cardiac output.


4. The section shown is from the wall of a:
click here for image
A. heart ventricle
B. heart atrium
C. muscular artery
D. elastic artery
E. muscular vein
F. large lymphatic vessel


5. In this TEM of an A-V valve leaflet, endocardium is indicated by:
A. bracket A
B. bracket B
C. bracket C
D. brackets A and B
E. brackets A, B, and C


6. Which statement regarding vascular smooth muscle cells is NOT true:
A. They are never found in the tunica intima.
B. They produce reticular and elastic fibers in the tunica media of muscular (medium) arteries.
C. They produce reticular and elastic fibers in the tunica media of elastic arteries.
D. They are oriented circularly in the tunica media of arteries.
E. They may be present as longitudinal bundles in the tunica adventitia of veins.


7. The structure indicated by the arrow in the TEM is comprised primarily of:
A. type IV collagen and laminin
B. elastin and fibrillin
C. type I collagen
D. smooth muscle cell cytoplasm
E. endothelial cell cytoplasm


8. In this section of heart tissue, which of the brackets correctly demarcates the endocardium?
A. A
B. B
C. C
D. D