Veterinary Histology and Embryology

VEINS

• The caliber of veins is as a rule much larger but the walls are thinner than arteries due to a great reduction of the muscular and elastic elements.
• The collagenous connective tissue is present in much larger amount and constitutes the bulk of the wall. The three coats are present but their boundaries are often indistinct. The whole wall is flabbier and tends to collapse when not filled with blood.
• The histological structure of the veins varies greatly and the variations are not always related to the size of the vessels but depend on local mechanical conditions. The structure may be quite different in veins of same caliber and even in different portions of the same vein.

Small veins or venules

• These are endothelial tubes. Tunica intima surrounded by collagenous fibres and fibroblasts, with a few isolated, circularly disposed, plain muscle fibres forming the Tunica media.
• As the caliber of the vessel increases, circular smooth muscle fibres forms a continuous layer and may also become many layered, the individual layers being separated by loose connective tissue.
• The Tunica adventitia is relatively thick and is made up mostly of longitudinally disposed collagen fibres

Medium sized veins

• These include all the named veins and their immediate branches excepting the main trunks.
• The Tunica media is composed of circularly arranged plain muscle fibres and collagen fibres.
• The media is thickest in the veins of the limbs and is very thin in the views of the head and abdomen.
• The Tunica adventitia is well developed and forms bulk of the wall. It consists of collagenous and elastic tissues.

Large veins

• The Tunica intima is made up of endothelium.
• The Tunica media is usually thin with the reduction in circularly arranged smooth muscle fibres.
• Tunica adventitia is very thick and among collagenous and elastic fibres, there is dense arrangement of longitudinal bundles of plain muscle.

Valves

• Veins over 2 m.m. in diameter are provided with valves at intervals. These are semilunar flaps, which project into the lumen, their free margin being projected towards the heart.
• These are derived from the intima and consist of loose connective tissue covered by a single layer of endothelium.