Author: Dr. Thomas Caceci
This type of artery has a wall very heavily reinforced with elastic fibers to allow it to expand under the pressure generated by the ventricles (in this case, the left ventricle). Normally these aren't easily seen in an H&E preparation, but in this case, the concentric layers of elastic CT show up as distinct iridescent rings, some of which are marked by arrows. There's a large clot of blood in the top of the field.
The bore of this vessel--like the rest of the cardiovascular system--is lined with a simple squamous epithelium. The innermost of the elastic layers is the internal elastic lamina, right up against the tunica intima. It's the first layer of the main thickness of the wall.
As is true of most elastic arteries, the
tunica adventitia is a pretty minor component of the wall's
thickness. Between the elastic layers there is a considerable
amount of collagen and some smooth muscle. This permits the
artery to expand under pressure. Collagenous components in the wall
prevent over-expansion and resist bursting of the vessel.
Hamster aorta; H&E stain, paraffin section, 200x
Very large elastic arteries have their own internal blood circulation system; the fibroblasts and other cells that keep the wall in good shape are so far from the blood supply that diffusion won't serve their needs. The density of the wall itself and its fibrillar components also impede diffusion. Hence the "vessels of the vessels" are a constant feature. Nerve fibers to the smooth muscle components may also be seen, though ot in this image.
Sometimes the wall of an artery--especially a big one like this,
which is subjected to all or nearly all the pressure the heart can
generate--is weakened by disease or malnutrition. The blood, under a good head of pressure, begins to dissect away the layers of reinforcing material, separating them and causing the side of the artery to bulge, exactly as a garden hose does when its wall is weakened. This bulge is an aneurysm. Chronic high blood pressure makes this more likely, though an aneurysm can occur even when systemic blood pressure is lower than normal. Needless to say, an aneurysm that bursts will
really spoil your afternoon. If it's a big one that blows out, such as the aorta, death will be rapid.
Dog aorta; Verhoeff's stain, paraffin section, 100x