Veterinary Histology and Embryology

ARTERIES

The wall of an artery is composed of three tunics or coats which are

• Innermost coat - the Tunica intima consisting of endothelium continuous with that of capillaries,
• The middle coat - Tunica media consisting mainly of smooth muscle with varying amounts of elastic and collagenous tissue and
• The outer coat - Tunica adventitia or externa composed of loose connective tissue.
  • The structure of these coats varies according to the artery and it is convenient to distinguish
    • small arteries or arterioles,
    • medium-sized arteries and
    • large elastic arteries.

Small Arteries (Arterioles)

• The three coats are distinguishable as an endothelial intima, a muscular media and an adventitia of connective tissue. An internal elastic membrane is present and seen easily in arterioles of 40 microns diameter and upward. This marks the boundary between the tunica intima and media. The tunica media is made up of smooth muscle fibres circularly arranged formed of one or two layers only in arterioles and gradually increasing in thickness, as the caliber of the vessel increases.
• The Tunica externa is made up of flattened fibroblasts and longitudinally arranged collagenous fibres and as the caliber of the vessel increases elastic and reticular fibres are also present. These vessels serve more for distribution of blood to various capillary beds and to control blood pressure.

Medium sized (Muscular) arteries

• These include all named arteries excepting very large ones. The walls of these vessels are very thick due to large amount of muscular tissue in the media. These are called muscular arteries. They are also called ‘distributing’ arteries because they distribute blood to all organs and regulate the supply according to functional needs. These Tunica intima exhibits three definite layers endothelium, intermediate layer and internal elastic membrane. The intermediate layer consists of delicate elastic and collagenous fibrils and a few fibroblasts.
• The internal elastic membrane is a thick fenestrated band. It marks the boundary between the intima and media owing to the large amount of muscle in their walls. The postmortem contraction of arteries throws the elastic membrane into longitudinal folds hence in transverse sections it has the appearance of a corrugated or wavy band. The media is the thickest coat and has layers of circularly disposed muscle fibres. There circularly disposed elastic nets in the media.
• The adventitia consists of a thick layer of connective tissue containing collagenous and elastic fibres disposed off longitudinally. Between the adventitia and the media is a very well defined external elastic membrane. The outer layer of adventitia blends gradually with the surrounding connective tissue when attaches the artery to other structures.

Larger arteries (Elastic arteries)

•  Also called conducting arteries because they conduct blood from the heart to the muscular arteries. The wall is relatively thin for the size of the vessels. The chief representative of this type is the aorta. The T. intima is made up of endothelium and endothelial cells are short and polygonal. The deeper portion of intima is made up of coarse collagenous fibres; some longitudinally oriented smooth muscle fibres and longitudinally arranged elastic fibres.
• The elastic membrane is usually split into two or more lamellae, which merge with other similar membranes, both in the media, and hence it is difficult to identify it. The muscle tissue is relatively reduced in amount. The adventitia is very thin and is composed of connective tissue mostly of collagenous fibres arranged in longitudinal spirals.