Page 108 - Headlines Histology2024_Neat

• The tunica externa is a collagenous connective tissue layer with few elastic
fibers and fibroblasts. Vasa vasorum and nervi vasorum are prominent.

Muscular arteries (medium-sized arteries)

• They are also called distributing arteries because they are the main distributing
branches to the various body regions.

• The tunica intima consists of endothelium resting on a basal lamina.
• The subendothelium is a thin connective tissue layer with few fibroblasts and

few smooth muscle cells.
• The internal elastic lamina is thick and fenestrated.
• The tunica media: is the thickest layer composed of 2-40 circular layers of

smooth muscle cells.
• A few elastic fibers and collagen fibers are scattered in between the smooth

muscle. The media is separated from the adventitia by discontinuous external
elastic lamina.
• The tunica adventitia consists of elastic and collagen fibers with vasa vasorum
and nervi vasorum.

Small arteries (arterioles)

• They are arterial vessels with a luminal diameter less than 0.3 mm.
• The tunica intima is very thin and comprises endothelium, subendothelial layer

of a thin collagen and elastic fibers that are absent in small arterioles and
Internal elastic lamina, which is thin but distinct.
• The tunica media is composed of six or less circular smooth muscle layers.
• The external elastic lamina is absent.
• The tunica adventitia is a connective tissue layer has the same thickness as the
tunica media and merges with the surrounding connective tissue.

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