Blood vessels are organs composed of different tissues (endothelium, connective tissue and smooth muscle) which are organized in a specific manner that is distinctive for a particular sized vessel. Although the arrangement of the various components is such that there are distinctive differences between arteries and veins and between different sized vessels in these categories, blood vessels as a whole have a basically similar structure. It is the amounts of connective tissue and muscle that vary according to the size and function of particular vessels.
Except in the smallest vessels, the walls are made up of three tunics or coats: the tunica intima, the tunica media, and the tunica adventitia. The intima, adjacent to the lumen is always very thin and always has endothelium (which lines the entire circulatory system). The thickness of the media and adventitia varies in different vessels. Smooth muscle is usually the prominent component of the media. The adventitia is largely collagenous fibers that act as a brace to prevent over-extension of the vessels, yet permit some flexibility.
General differences between arteries and veins of corresponding sizes. For most corresponding vessels, the following differences can be applied:
- Veins usually have a larger overall diameter than arteries.
- Veins usually have larger lumens and thinner walls than arteries.
- The three coats are not as distinct in veins as in arteries.
- Veins have valves.
- Arteries have a more uniform structure; structure of veins is more variable.
- In tissue sections, arteries generally remain rounded; veins are more apt to be collapsed.
Small Blood Vessels
Slide 44, Skeletal Muscle (H&E)
Capillaries are thin walled tubes whose wall consists of a layer of endothelial cells. No other tissues are incorporated into the wall, although the intermittent strands of connective tissue that are applied closely to the wall are sometimes referred to as an “adventitia.” In areas on Slide 44 where the muscle is cut in perfect cross section, look with high power at the endomysium, which is artifactually separated from the muscle fibers. Many cross sections of capillaries are present. Some appear only as very small, evenly outlined “circles.” Others exhibit an endothelial cell nucleus “to one side” and/or have a red blood cell filling the lumen.
Look in the perimysium to identify blood vessels much larger than capillaries (arterioles, venules, and small arteries), but still very small blood vessels.
Slide 144, Colon (PASH)
Identify small blood vessels in the submucosa (the connective tissue just below the mucosa). In this section, the smooth muscle “stands out" prominently in the arterioles and arteries.
Identify metarterioles seen as being “lopsided” because of single smooth muscle cell creates an enlargement on only one side of the vessel when seen in cross section. Metarterioles have a lumen about the size of a capillary and serve to control the flow of blood into capillary beds.
Note arterioles and small arteries exhibiting PAS-positive internal elastic nets or membranes. Identify such vessels in longitudinal section. Observe that when the smooth muscle is cut in cross section, it is so regularly arranged that it appears almost as a cuboidal epithelium.
Surrounding each smooth muscle fiber are PAS-positive reticular fibers and an external lamina.
Identify venules and small veins. Venules may lack smooth muscle or have only very few smooth muscle cells widely scattered. The small veins will have single layer of loosely arranged smooth muscle cells in the tunica media.
Slide 150, Pylorus-Duodenum
Slide 150, Pylorus-Duodenum has well preserved small arteries and veins in the submucosa of the pylorus. Study these vessels for their characteristic features.
Medium-Sized Blood Vessels
Slide 35, Lymph Node (H&E) (Monkey)
This section has a variety of small and medium- sized blood vessels that should be identified. Note also the numerous sections of nerves coursing through the connective tissue. Some of the vessels containing valves are lymphatics draining lymph nodes.
A medium artery and a medium vein lie in close proximity of each other. Both are cut in cross section and partially collapsed (not very “round”). Identify features of each.
- The medium artery has very thin attenuated endothelium, essentially no subendothelial connective tissue, internal and external elastic membranes, a compact tunic media of smooth muscle, and an extra wide tunica adventitia (wider than most human medium arteries).
- The medium vein has perhaps 5 or 6 layers of smooth muscle in its tunica media.
Note that the muscle is much less compactly arranged than that of the artery.
This vessel lacks both an internal and an external elastic membrane.
The tunica adventitia is thicker than the media, characteristic of most small and medium veins.
- Some of the vessels containing valves are efferent lymphatic vessels that drain the lymph nodes. Such vessels may be easily recognized when they contain many lymphocytes but lack red blood cells.
Slide 157, Parotid Gland (H&E)
Study blood vessels on this slide noting in particular the medium artery and medium vein which can be seen with the unaided eye in the section.
Compare and contrast the two vessels. Note how much thicker and more compact the tunica media of the artery is than the media of the vein.
The artery has an internal and an external elastic membrane but such structures are lacking in the vein.
Note that the tunica adventitia of the vein is thicker than its media, whereas in the artery just the opposite feature occurs. However, here in this artery, the tunica adventitia is very compact and clearly defined from the less dense surrounding connective tissue.
Identify smaller blood vessels and nerves in the connective tissue septa of the gland. Caution: DO NOT confuse blood vessels with excretory ducts of the parotid gland. The latter have a lining of simple columnar or pseudostratified columnar epithelium.
Slide 82, Spermatic cord (H&E)
The spermatic cord, which contains the ductus deferens, is an extremely vascular tissue, containing vessels of medium size as well as innumerable small ones.
Locate the ductus deferens as a thick-walled, muscular tube lined by pseudostratified columnar epithelium with stereocilia (poorly defined).
In the upper left microscopic field, locate a group of several veins and one artery.
The artery is a small-medium-sized vessel. Identify:
- Intima of endothelium, a thin but a definite subendothelial connective tissue layer with fibroblasts, and the internal elastic membrane.
- Muscular media. Many layers of smooth muscle coursing circularly, compactly arranged, fine elastic fibers interspersed.
- Adventitia. (It may be torn off in part.) The external elastic membrane is distinct in places. Note mergence of the adventitia with surrounding connective tissue.
Most of the veins in this group are medium-sized with several layers of muscle in the media arranged loosely in connective tissue. There may be some muscle bundles in the adventitia.
- With low power, compare the artery with the veins. Note the compact orderly arrangement of the muscle in the media (as in all arteries) with the more loosely arranged muscle and connective tissue in the veins.
- In the veins, note the absence of an internal elastic membrane and the lack of a distinct intima-media and media-adventitia junction. Smooth muscle may also be present in the adventitia, seen as groups of fibers in cross section. The muscle courses longitudinally.
- The artery may contain a little blood, but some of the veins are filled. Red blood corpuscles are densely packed in them. Identify the nucleated white blood cells. Neutrophils have lobulated nuclei and lymphocytes have small, round dark nuclei and no visible cytoplasm.
Slide 83, Medium Artery and Vein (Masson’s stain) from the Lower Extremities
With this stain, collagenous and reticular fibers are blue, all nuclei are red, elastic fibers are essentially colorless, and muscle cytoplasm is light red.
The artery. This is an example of a transition type of medium-sized artery in which the elastic fibers in the media are thicker than in a “typical” artery such as on slide 82, and some muscle is present in the inner adventitia.
Endothelial cell nuclei are prominent, possibly thin strands of cytoplasm can be distinguished.
The very thin layer of subendothelial connective tissue is visible in places.
Internal elastic membrane is prominent.
Distinguish nuclei and cytoplasm of the compactly arranged muscle fibers.
Focus to see the fairly thick elastic fibers between muscle fibers. The blue stain represents reticular fibers.
An external elastic membrane is quite prominent but there are one or more other equally prominent membranes in the inner adventitia with some smooth muscle between them.
Outer adventitia is collagenous fibers only. Note its mergence with adjacent connective tissue.
The medium-sized vein. This is an example of a medium-sized vein in which the thin media has several layers of fairly compact muscle and the adventitia is very wide with scattered bundles of smooth muscle.
Compare it with the artery.
Note the thin media and the thick adventitia, and the absence of an internal elastic membrane. Crinkly elastic fibers may be seen, and in places are suggestive of a membrane, but is not as continuous or as thick as that in the artery.
Some subendothelial connective tissue is visible.
There are 4 or 5 layers of muscle in the media with interspersed connective tissue.
There is no external elastic membrane.
The adventitia is wide. It appears to be primarily collagenous but contains scattered very small groups of smooth muscle (in cross-section, since adventitial tissues course longitudinally).
Note red blood corpuscles in the lumen. They are round on surface view, thin discs in profile. A few white blood cells may be present; identify them.
Extravasated blood is seen in the connective tissue surrounding the adventitia-the result of blood spilling out of the cut vessels when the section of tissue was removed.
Slide 83, Medium Artery and Vein (Masson’s stain) from the Lower Extremities
Slide 51, Aorta, Human-a large, elastic artery (H&E)
Look at the slide grossly and note the huge lumen and thin wall.
With low power, note that the media comprises almost the whole wall. The adventitia is a very thin layer; it merges with surrounding connective tissue.
There is no distinct intima-media junction. Elastic membranes begin in the subendothelial layer and gradually merge with those in the media.
Distinguish the large elastic lamellae throughout the media.
Between them, note thin layers of smooth muscle. Dark nuclei of the smooth muscle cells are seen but cell outlines are not distinguishable. Focus to see fine elastic fibers.
The thin collagenous adventitia may be torn off, but it should be distinguishable in places.
Look for vasa vasorum in the adventitia and outer media (small arterioles, venules, or capillaries).
In the surrounding connective tissue, identify the blood vessels. Look for small nerves.
Slide 10, Aorta (Verhoeff’s elastic stain)
Structure is as above in slide 51.
Note especially the heavily stained elastic lamellae and the networks of finer fibers between these. (Other tissues take only a background stain).
Slide 93, Pulmonary Artery (H&E)
Again note typical large artery structure in this section of pulmonary artery as it is entering the lung.
The artery follows a tortuous course in the lung, therefore it may be seen sectioned in more than one plane. Look for areas where it is sectioned somewhat obliquely or tangentially. Here it may be possible to see sheet-like lamellae and perhaps a bluish ground substance due to the presence of chondroitin sulfate.
Slide 11, Large Vein (portal vein) in the Hilus of the Liver (H&E)
The vein is the large U- shaped structure in the center of the section.
Select a good cross-sectional area of the vein to see the typical structure for a large vein.
- Look at the endothelium and subendothelial connective tissue. A thin internal elastic membrane, which is often present in large veins, is not apparent. (See below on slide 81.)
- The media is virtually absent. Occasional smooth muscle cells may be seen intermingled with connective tissue.
- Note the broad adventitia with numerous bundles of smooth muscle in cross-section (since they course longitudinally) with prominent collagenous fibers between them (and fine elastic networks, not readily visible).
- The outermost adventitia is principally collagenous fibers, which blend into the loose connective tissue that merges with the capsule of the liver. Vasa vasorum are present.
Slide 81, Large Vein, Inferior Vena Cava (Verhoeff’s elastic stain)
Elastic fibers are black, collagenous fibers have a yellowish-tinge, and muscle is brownish. Details are not demonstrated with this stain.
Structure is like that of the portal vein on slide 11. Intima, thin media, and thick adventitia are clearly indicated.
Note especially the internal elastic membrane close to the endothelium and the distribution of smaller elastic fibers throughout the media and adventitia. It is not prominent as in arteries.
Slide 92, Section through the sinus of the kidney showing several blood vessels, nerves, and a section of ureter, all embedded in loose connective and adipose tissue (H&E)
This is a good summary and review slide.
- Identify the long section of large vein (renal vein) showing features as described previously. (Two sections of this large vein are present).
- Identify the cross section of a typical medium artery (renal artery) and distinguish its three coats. Details of structure are seen well. Where the plane of section is appropriate, elastic fibers and fine elastic networks in the media are seen clearly.
- Look for smaller vessels in the surrounding connective tissue. Identify types and sizes.
- Prominent sections of nerves are seen accompanying the blood vessels.
- Locate the section of ureter. Identify the epithelium that lines it and the tissues comprising the wall. Note the very small blood vessels in the wall; most of the venules are greatly congested, that is, filled with red blood corpuscles. (There are also areas of extravasated red blood corpuscles).
In the left wall of the ureter (in the microscopic field) is an area of infiltrated cells.
Identify lymphocytes and occasional plasma cells.