Page 107 - Headlines Histology2024_Neat

3. Tunica externa or adventitia

• It is the outermost layer and consists of connective tissue layer of fine elastic
and collagen fibers. It is separated from the media by external elastic lamina.

• The tissues of the wall receive their nutritional support by small arteries called
vasa vasorum (e.g. vessels of vessels). These vessels are especially prominent
in large veins and elastic arteries.

Arterial system

• The primary function of the arterial system is to distribute blood from the heart
to capillary beds throughout the body. The arteries are classified according to
their size and structure into: large or elastic artery, medium-sized or muscular
artery and arteriole or small artery.

Elastic arteries (large arteries)

• The elastic arteries include the aorta, common carotid, subclavian arteries. The
wall of the elastic artery is made up of tunica intima, tunica media and
adventitia.

• The tunica intima consists of endothelium, subendothelium and internal elastic
lamina. The endothelium is a single layer of flattened endothelial cells rests on
a thin basal lamina.

• The subendothelium is a thin layer of collagenous connective tissue containing
few elastic fibers, scattered fibroblasts and smooth muscle-like cells known as
myointimal cells.

• With increasing age, lipid may accumulate in the cytoplasm of the myointimal
cells leading to atherosclerosis.

• The internal elastic lamina is indistinct.
• The tunica media is the thickest of the three tunics and consists primarily of

concentrically arranged, fenestrated elastic laminae separated by collagenous
connective tissue and relatively few smooth muscle fibers.
• The external elastic lamina is indistinct.

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