[doi: 10.5505/2017ichc.PP-225]

Review of the histological structure of veins

¹Departmant of Histology and Embryology, Gulhane Military Medical Academy, Ankara, Turkey
²Departman of Anatomy, Gulhane Military Medical Academy, Ankara, Turkey

Venous system is responsible of carrying blood from organs to the heart. This system begins from post capillary venules and ends with inferior and superior caval veins at the right atrium. Veins are classified as large-sized veins such as caval veins, medium-sized veins such as saphenous vein, and small-sized veins. In this presentation we aimed to review histological structure of the veins.
As in the arteries, walls of the veins are composed of three layers. Tunica intima is the innermost layer and it consists of a thin layer of endothelium, basal membrane and subendothelial connective tissue. Tunica media is the middle layer and it is composed of circularly arranged smooth muscle fibers between collagen and elastic fibers. The tunica adventitia is the outer layer of loose connective tissue containing collagen, fibroblasts and vaso vasorum.
Tunica adventitia is the thickest layer of the veins. It is well developed in large-sized veins and three zones are distinguishable. The outer zone composed of dense connective tissue of collagen and elastic fibers. Vaso vasorum are mostly seen here but they can be found in all layers. Middle zone mainly consists of smooth muscle fibers. They are longitudinally arranged and this zone is most-developed in inferior vena cava. It consists of numerous smooth muscle fibers and distinguishes inferior vena cava from superior vena cava.
Tunica intima is similar in small, medium or large-sized veins. Femoral and mesenteric veins have specialized tunica intima that is subendothelial connective tissue contains longitudinal smooth muscle fibers.
Tunica media is thinner and it is not well developed as in the arteries. Due to the relative amount of the muscle fibers, veins cannot hold the shape of the lumen and they are seen irregularly according to the arteries. Large veins consists of a few layers circular smooth muscle fibers in tunica media but superior vena cava have more than 10 layers of circularly arranged smooth muscle fibers. Maternal veins of placenta and retinal veins doesn’t have any smooth muscle in tunica media layer on the contrary, thickening of tunica media is well observed in umbilical vein.

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Reprehensive figures of specified structure of vena cava superior, femoral and umblical veins.

Figure 1: (A) The superior vena cava contains abundant smooth muscle fibers in the thick tunica adventitia layer. (B) The femoral vein has longitudinal smooth muscle fibers as subendothelial connective tissue in tunica intima. (C) The umbilical vein contains thickened tunica media layer consist of smooth muscle fibers (Masson trichrome, Scale bar 500-200-500 µm).