School of Anatomy and Human Biology - The University of Western Australia
|Blue Histology - Vascular System|
Lab Guides and Images
Aorta - elastin & eosin and H&E
Artery - elastin & eosin and H&E
Variations of Vessel Wall Structure
Capillaries - cardiac muscle, Whipf's polychrome
Vein - elastin & eosin and H&E
Vein Valve - H&E
Lymph Capillary -Lacteal - jejunum, H&E
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The cardiovascular system is concerned with the transport of blood and lymph through the body. It may be divided into four major components: the heart, the macrocirculation, the microcirculation and the lymph vascular system.
Essentially, the macrocirculation comprises all vessels, both arteries and veins, that would be visible to the eye. The vessels of the macrocirculation supply and drain a network of fine vessels interposed between them, the capillaries. This network is also called the capillary bed. Water and other components of the blood plasma which exude from the blood vessels form the interstitial fluid, which is returned to the circulation by the lymph vascular system.
You have already seen blood vessels of various sizes and types in preparations available in other lab sessions, and you should be aware that the histological appearances of vessels of different sizes (arterioles vs. arteries) and different types (arteries vs. veins) are different from each other. These differences are the result of quantitative variations of a common structural pattern that can be seen in all blood vessels with the exception of capillaries, i.e. the division of the walls of the blood vessels into three layers or tunics.
about .... Endothelial Cells
by Professor John McGeachie
All arterial vessels originate with either the pulmonary trunk (from the right ventricle) or the aorta (from the left ventricle). Specialisations of the walls of arteries relate mainly to two factors: the pressure pulses generated during contractions of the heart (systole) and the regulation of blood supply to the target tissues of the arteries. The tunica media is the main site of histological specialisations in the walls of arteries.
Vessels close to the heart (aorta, pulmonary trunk and the larger arteries
that originate from them) are
The tunica intima of elastic arteries is thicker than in other arteries. A layer of loose connective tissue beneath the endothelium (subendothelial connective tissue) allows the tunica intima to move independently from other layers as the elastic arteries distend with the increase in systolic blood pressure. Distension of the walls is facilitated by concentric fenestrated lamellae of elastic fibres in a thick tunica media. In adult humans, about 50 elastic lamellae are found in the tunica media of the aorta. The energy stored in the elastic fibres of the tunica media allows elastic arteries to function as a "pressure reservoir" which forwards blood during ventricular relaxation (diastole). Smooth muscle cells and collagen fibres are present between the layers of elastic fibres. Both fibre types are produced by the smooth muscle cells. Each elastic lamella forms together with interlamellar fibres and cells a lamellar unit. The external elastic lamina is difficult to discern from other layers of elastic fibres in the tunica media. The tunica adventitia appears thinner than the tunica media and contains collagen fibres and the cell types typically present in connective tissue.
The walls of these large arteries are so thick that their peripheral parts cannot derive enough oxygen and nutrients from the blood of the vessel that they form. Larger vessels are therefore accompanied by smaller blood vessels which supply the tunica adventitia and, in the largest vessels, the outer part of the tunica media of the vessel wall. The vessels are called vasa vasorum. In macroscopic preparations vasa vasorum are visible as fine dark lines on the surface of the larger arteries.
Aorta, human - H&E ,
elastin & van Gieson
The thin endothelial lining of the aorta corresponds to that of other vessels. The flattened cells are easily damaged during preparation and it may be difficult to identify the endothelium. The subendothelial layer of connective tissue is characterised by a lower density of cells, i.e. fewer nuclei, a fibrous appearance of the tissue and the absence of well-defined elastic layers. Because the lamellae of elastic fibers diffract light differently from the remaining tissues they should also be visible in H&E stained sections. Elastic lamellae become visible in the tunica media. The majority of cells in the tunica media are smooth muscle cells. Smooth muscle cells and collagen fibres are found between the layers of elastic fibres. If you scan the periphery of the aorta you may find small blood vessels, the vasa vasorum, in the tunica adventitia and penetrating into the outer part of the tunica media.
Draw the aorta at low magnification and label the three tunics. Draw part of the tunica media at high magnification and identify collagen fibres, layers of elastic fibres and smooth muscle cell nuclei in your drawing.
The diameter of individual arteries decreases as we follow them further into
the periphery. However, their total diameter increases, which leads to a fall
in blood pressure. Also, the properties of the elastic arteries have to some
extent evened out differences in diastolic and systolic blood pressure. The
amount of elastic fibres in the tunica media decreases with these physiological
changes. We now find a type of arteries which are termed
The tunica intima is thinner than in elastic arteries. Subendothelial connective tissue other than the internal elastic lamina is often difficult to discern. The internal elastic lamina forms a well defined layer. The tunica media is dominated by numerous concentric layers of smooth muscle cells. Fine elastic fibres and and a few collagen fibres are also present. The external elastic lamina can be clearly distinguished although it may be incomplete in places. The thickness and appearance of the tunica adventitia is variable.
The basic structure of the walls of arteries does not change much as we come
to the next type of arterial vessels. Size is used to differentiate them from
are arterial vessels with a diameter below 0.1 - 0.5 mm (different
values in different textbooks). Endothelial cells are smaller than
in larger arteries, and the nucleus and surrounding cytoplasm may 'bulge'
slightly into the lumen of the arteriole. The endothelium still rests on
a internal elastic lamina, which may be incomplete and which is not always
well-defined in histological sections. The tunica
media consists of 1-3 concentric layers of smooth muscle cells.
It is difficult to identify an external elastic lamina or to distinguish
the tunica adventitia from the connective tissue surrounding the vessel.
The smooth muscle of arterioles and, to some extent, the smooth muscle of small muscular arteries regulate the blood flow to their target tissues. Arterioles receive both sympathetic and parasympathetic innervation. The final branching of the arterioles finally gives rise to the capillary network (microcirculation).
Artery - H&E and elastin & eosin
In addition to the inner and outer elastic laminae, elastin
stains will show fine
elastic fibres in the tunica media and coarse elastic fibres between
the collagen fibres of the tunica adventitia. The appearance of other
structures will depend on the stain used together with the elastin stain.
Eosin, the E in H&E, gives a pink colour
to both collagen fibres and the cytoplasm of cells. Nuclei
are not stained if the H is omitted from the H&E.
The sum of the diameters of all capillaries is significantly larger than that of the aorta (by about three orders of magnitude), which results in decreases in blood pressure and flow rate. Also, capillaries are very small vessels. Their diameter ranges from 4-15 µm. The wall of a segment of capillary may be formed by a single endothelial cell. This results in a very large surface to volume ratio. The low rate of blood flow and large surface area facilitate the functions of capillaries in
These functions are also facilitated by a very simple organisation of the wall of capillaries. Only the tunica intima is present, which typically only consists of the endothelium, its basal lamina and an incomplete layer of cells surrounding the capillary, the pericytes. Pericytes have contractile properties and can regulate blood flow in capillaries. In the course of vascular remodelling and repair, they can also differentiate into endothelial and smooth muscle cells.
Three types of capillaries can be distinguished based on features of ethe endothelium.
Cardiac Muscle, sheep - Whipf's
The walls of veins are thinner than the walls of arteries, while their diameter is larger. In contrast to arteries, the layering in the wall of veins is not very distinct. The tunica intima is very thin. Only the largest veins contain an appreciable amount of subendothelial connective tissue. Internal and external elastic laminae are absent or very thin. The tunica media appears thinner than the tunica adventitia, and the two layers tend to blend into each other. The appearance of the wall of veins also depends on their location. The walls of veins in the lower parts of the body are typically thicker than those of the upper parts of the body, and the walls of veins which are embedded in tissues that may provide some structural support are thinner than the walls of unsupported veins.
Venous vessels originate from the capillary network which coalesce into the smallest venous vessels the
They are larger than capillaries. Small venules are surrounded by pericytes. A few smooth muscle cells may surround larger venules. The venules merge to form
Small to medium-sized veins
The largest veins of the abdomen and thorax
do contain some subendothelial connective tissue in the tunica intima, but both it and the tunica media are still comparatively thin. Collagen and elastic fibres are present in the tunica media. The tunica adventitia is very wide, and it usually contains bundles of longitudinal smooth muscle. The transition from the tunica adventitia to the surrounding connective tissue is gradual. Valves are absent.
Vasa vasorum are more frequent in the walls of large veins than in that of the corresponding arteries - probably because of the lower oxygen tension in the blood contained within them.
Vein, human - H&E
Vein Valve - H&E
Small arteries and veins often form anastomosing networks, which provides routes for alternative blood supply and drainage if one of the vessels should become occluded because of pathological or normal physiological circumstances. Some arteries are however the only supply of blood to their target tissues. These arteries are call end arteries. Tissues which are supplied by end arteries die if the arteries become occluded.
The segments of the kidney and the heads of the gastrocnemius muscle are examples of tissues supplied by end arteries.
Arteries and veins may also form arteriovenous shunts, which can shunt the blood flow that otherwise would enter the capillary network between the vessels. These shunts usually contain specialisations of the smooth muscle in the region of the shunt. Arteriovenous shunts are frequently seen in the blood supply of distal parts of the limbs and the nose (thermoregulation) and in the blood supply of endocrine organs.
Parts of the blood plasma will exude from the blood vessels into the surrounding
tissues because of transport across the endothelium or because of blood pressure
and the fenestration of some capillaries (this process
is partly counteracted by the higher osmotic pressure of the blood).
The fluid entering tissues from capillaries adds to the interstitial fluid normally
found in the tissue. The surplus of liquid needs to be returned to the circulation.
Lymph vessels are dedicated to this unidirectional
flow of liquid, the lymph. Three types
of lymph vessels can be distinguished based on their size and morphology.
are somewhat larger than blood capillaries and very irregularly shaped. They begin as blind-ending tubes in connective tissue. The basal lamina is almost completely absent and the endothelial cells do not form tight junctions, which facilitates the entry of liquids into the lymph capillary. Temporary openings in the endothelial lining of the lymph capillaries also allow the entry of larger particles into the lymph capillaries (lipid droplets, which are absorbed from the lumen of the gut do not enter blood capillaries, but enter the circulation via lymph vessels which are found in the villi of the ileum and jejunum). Lymph capillaries merge to form
which are larger and form valves but otherwise appear similar to lymph capillaries. The lymph is moved by the compression of the lymph vessels by surrounding tissues. The direction of lymph flow is determined by the valves. Lymph vessels empty intermittently into lymph nodes from which the lymph continues in efferent lymph vessels.
Only very little lymph is returned from the limbs if they are immobilized, which illustrates the importance of muscular action in lymph transport. This is also the reason for immobilizing limbs that are either infected or that have been bitten by venomous Australians. The effect can also be observed after long intercontinental flight when you may feel that your shoes and socks are just about one number too small. Finally, impeded lymph drainage is one of the problems associated with surgery which requires the removal of lymph nodes and which thereby interrupts the lymph collecting vessels.
Eventually the lymph collecting vessels merge to form
which contain one or two layers of smooth muscle cells in their wall (some textbooks call this layer the tunica media of lymph vessels). They also form valves. The walls of the lymph ducts are less flexible in the region of the attachment of the valves to the wall of the duct, which may give a beaded appearance to the lymph ducts. Peristaltic contractions of the smooth muscle contribute to the movement of lymph towards the heart in addition to the compression of the ducts by surrounding tissues.
The largest lymph duct of the body, the thoracic duct, drains lymph from the lower half and upper left quadrant of the body and empties the lymph into the circulation by merging with the vascular system close to the junction of the left internal jugular and subclavian veins. That it is the largest lymph duct does not mean that it is a large vessel when compared to the large arteries and veins. It actually is not much larger (about 5mm in diameter) than one of the superficial forearm veins.
Jejunum, baboon - H&E
The jejunum slide is also good for revision. You should be able to find columnar epithelium, goblet cells, smooth muscle, small nerves, a few ganglion cells and, of course, lots of loose connective tissue and blood vessels.
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last updated: 6/08/09
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