Arteries are blood vessels that transport and distribute oxygenated blood from the heart to all the organs in the body (with one exception – pulmonary arteries transport deoxygenated blood from the heart to the lungs for gas exchange as a part of pulmonary circle). The aorta, which originates from the aortic valve in the left ventricle, is the largest artery in the human body. Arteries are responsible for regulating blood pressure, which morphologically translates into a thick smooth muscle layer that facilitates contraction and dilatation in response to physiological stimuli.

Aorta is a unique example of an elastic artery and contains a lot of elastic fibers within its wall that allow for a large variation of the lumen size.

Histology of the Aorta

Before examining the histological sections of a normal human aorta, it is important to consider commonly used histological stains and their interpretation.
Generally, the stains are divided into non-specific – the ones that use dyes that stain all the cells within tissue in a similar manner, and special – the ones that use dyes that selectively bind to specific tissue/cell components.

This section will examine the normal histology of the human aorta using histology slides stained with hematoxylin & eosin (H&E, non-specific) and elastin/trichrome (special) stains.

Please refer to the sections  How Are the Specimens Obtained and Prepared? and Video Lessons on Histology to learn more about the stains and histology slide preparation.

H&E Stain Interpretation 

To better understand changes that occur in the blood vessel wall during the development of atherosclerosis, it’s critical to understand the normal histology of large blood vessels.
Even though the morphology of blood vessels of different sizes and types (arteries, veins, capillaries) exhibits functional adaptations that reflect the location of a blood vessel and local blood pressure, the main features of the blood vessel wall remain the same across various types.

The wall of the aorta consists of 3 layers (tunics):

1. Tunica intima – thin innermost layer that includes:

a) endothelium – a single layer of epithelial cells that forms the luminal surface of the vessel and is in direct contact with blood. Endothelium within the wall of blood vessels is continuous with the endocardium – the inner lining of the heart;

b) loose connective tissue – a thin layer of connective tissue located under endothelium

2. Tunica media – the thickest layer in the aorta, includes:

a) smooth muscle cells that allow for contraction and relaxation;

b) elastin fibers that further expand the ability of aorta to dilate and contract; in aorta, numerous elastin fibers are located within tunica media, and some within tunica intima;

c) connective tissue fibers and nerves.

3. Tunica adventitia -the outermost layer that includes:

a) connective tissue for structural support and protection;

b) nerve fascicles (nervi vasorum) and blood vessels (vasa vasorum) that supply and innervate the wall of the aorta

As an elastic artery, aorta contains a large amount of elastic fibers within the tunica media. This amount of elastin allows aorta to expand and recoil in response to blood pressure fluctuations (similarly to how an elastic band behaves) and conduct large volumes of blood that’s ejected from the heart under high pressure.

In addition to being present within tunica media, elastin fibers also form two structures on the border between tunics:

a) internal elastic lamina – on the border between tunica intima and tunica media;

b) external elastic lamina – on the border between tunica media and tunica adventitia

Both elastic laminas are thick bands of elastin fibers that appear wavy on the histological slide, which reflects the properties of elastin in a relaxed state. Elastic laminas are more easily identified in smaller arteries, where elastin fibers are located only within the laminas. In the aorta, which has numerous elastin fibers within tunica media, elastic laminas would be the fibers on the border between the tunics.

^{Normal histology of aorta.  Created and presented by Tetiana Povshedna.  Illustrations were created under license with Biorender.com. Histology slides are DHPLC specimens PATH 304-023 (H&E) and PATH 304-024 (elastin/trichome)}

Compare and Contrast the Histological Appearance of 3 Tunics in Various Blood Vessel Types

Aorta

 Vena Cava

Section Review

Blood vessels across the vascular tree share a similar histological structure – their wall consists of three layers (also known as tunics). The thickness and characteristics of these layers (tunica intima, tunica media, tunica externa) vary depending on the blood vessel type (artery vs vein) and location (closer or further from the heart). Aorta, a major vessel that carries oxygenated blood from the heart and distributes it to organs and tissues, is an elastic artery that contains numerous elastic fibers across the layers, with the majority of them accumulated within the middle muscular layer (tunica media). This histological feature allows aorta to expand and recoil in response to blood pressure fluctuations, and carry large amounts of oxygenated blood from the heart. 

Review questions