Elastic (large) arteries:

Elastic arteries are composed of three layers; tunica intima, tunica media, and tunica adventitia.

The innermost tunica intima consists of a squamous endothelial layer lining a well-defined lumen. The endothelium is difficult to preserve, and is therefore, not easily identifiable in these sections. The endothelium sits on a basal lamina and a thick subendothelial layer of loose connective tissue, smooth muscle, collagen and elastic lamellae. No distinct border between the tunica intima and media exists.

The middle tunica media is composed of a thick layer of circumferential smooth muscle cells, pink collagen and red, wavy, elastic lamellae. No fibroblasts are present in this layer.

The outermost tunica adventitia consists of a thin connective tissue layer containing fibroblasts, macrophages, and blood vessels with little collagen and elastic lamellae.

Note that the elastic artery slide on the right is stained for elastic fibers. The abundance of elastic fibers in the tunica media is prominent in this slide. The aorta slide on the left is stained with H&E.

 

 

Muscular (medium) arteries:

Muscular arteries, like elastic arteries, are composed of three layers; tunica intima, tunica media, and tunica adventitia.

The innermost tunica intima consists of a squamous endothelial layer lining a well-defined lumen. The endothelium sits on a basal lamina and thin subendothelial layer, which contains fewer smooth muscle cells and less elastic tissue than elastic arteries. A prominent internal elastic membrane separates the intima from the media, and may appear wavy due to smooth muscle contraction.

The middle tunica media is predominantly composed of circumferential smooth muscle cells with little fine, wavy elastic material. No fibroblasts are present in this layer.

The outermost tunica adventitia is a thick layer of longitudinally arranged collagen fibers bound by an external elastic membrane. The adventitia blends in with the connective tissue of the surrounding adipose tissue containing blood vessels.

The section of liver on this slide demonstrates multiple muscular arteries. Scan this slide for such arteries.

 


 

Veins:

Veins have the same, but often not as well-defined, layers as arteries. Because of their thin walls, veins often appear to have irregular collapsed lumena. Veins also frequently have valves.

The tunica intima consists of a thin endothelial lining with basal lamina and a thin layer of subendothelial tissue.

The tunica media is relatively thin, containing smooth muscle cells, collagen and fibroblasts. Veins have less elastic tissue than arteries and the internal elastic membrane may be ill-defined or absent.

The tunica adventitia is the thickest layer and is composed of collagen and elastic fibres, as well as fibroblasts and prominent bundles of longitudinally arranged smooth muscle.

The section of liver on this slide demonstrates multiple veins. Scan this slide for such structures.


 

Heart wall:

The heart wall is divided into three main layers; endocardium, myocardium and epicardium.

The innermost endocardium lines the atria and ventricles of the heart. An inner endothelial layer overlies a subendothelial connective tissue layer, a middle connective tissue layer containing smooth muscle cells and an outer subendocardial layer containing purkinje fibres.

The myocardium is composed of branching cardiomyocytes with central nuclei, cross-striations and intercalated discs.

The outer epicardium consists of a layer of mesothelial cells on the outer surface of the heart overlying connective tissue containing blood vessels and nerves. The epicardium is surrounded by adipose tissue.

 

 

Fatty streak:

Accumulation of foam cells (macrophages filled with lipid droplets), smooth muscle cells, and lymphocytes can be identified beneath the endothelial layer of arteries.

 

 

Atherosclerotic plaque:

A central necrotic mass of lipid, cell debris and cholesterol clefts lined by a fibrous cap of thick connective tissue containing smooth muscle cells, macrophages, foam cells, lymphocytes, collagen and elastin.

 

 

Acute myocardial infarction:

A circumscribed area of ischemic coagulative necrosis is seen within the heart wall. Cardiomyocytes appear wavy, have eosinophilic cytoplasm and have lost their cross-striations and nuclei. Neutrophilic infiltrate is seen initially, followed by lymphocytes and macrophages. Granulation tissue will eventually replace the infarct with a fibrous (collagenous) scar. Contraction bands may be identified peripherally in reperfused infarcts.

 

 

Acute myocarditis:

Myocardial fibres are disrupted by interstitial infiltrate of lymphocytes and macrophages.

 

 

Acute bacterial endocarditis:

Vegetations composed of platelets, fibrin, cell debris, and organisms are seen overly an edematous and inflamed valve.