Variations of Vessel Wall Structure

Variations of Vessel Wall Structure

Arteries

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

Elastic arteries

The tunica intimaof 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 alamellar 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.

Suitable Slides

sections of the aorta - H&E, elastin

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.

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

Muscular arteries

The tunica intimais 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 muscular arteries.

Arterioles

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).

 
Suitable Slides

sections of arteries - H&E or elastin (in combination with other stains)
Sections of small muscular arteries and arterioles are present in many sections, and the basic features of their structure are usually visible - even in smaller arteries. Large muscular arteries often have their "own section" in teaching collections.

Artery - H&E and elastin & eosin Identifying muscular arteries in sections is rather straight forward. There are two easily recognizable features which distinguish these arteries from veins. If two vessels have a similarly sized lumen, the walls of arteries will be much thicker and more compact than the wall of veins. At high magnification, the internal elastic lamina forms a pink streak immediately below the endothelial cell lining in arteries and even arterioles, while it is difficult to identify in veins. The layer of subendothelial connective tissue is very thin, and the endothelium seems to rest on the internal elastic lamina. Smooth muscle cell nuclei are frequent in the tunica media. The external elastic lamina stains similar to the internal elastic lamina, but it is thicker and appears fibrous instead of forming a continuous band. Collagen fibres and a few connective tissue cell nuclei are visible in the tunica adventitia. If you close the iris diaphragm of the microscope, the elastic layers will stand out very clearly, but remember to open the diaphragm once you have seen them.

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. Draw either one large composite image containing the three tunics and the cellular and fibrous elements which form the tunics. Alternatively, you can draw a low power overview and supplement it with high magnification illustrations of the individual tunics. Focus on an H&E stained section.