Atherosclerosis and Angina

Gross Anatomy and Histology of Normal Aorta

Tetiana Povshedna

Learning Objectives

By the end of this chapter, you will be able to:

  • compare and contrast the main histological features of tissues  that can be identified via H&E and elastic/trichrome stains
  • identify three main layers (tunics) in the wall of the aorta and their main components
  • relate the structural features of aorta to its function

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.

One the gross anatomy specimen that features ascending aorta, identify:

  • smooth luminal surface, which is required for uninterrupted blood flow;
  • wide diameter (around 20-30 mm in ascending aorta), necessary for allowing large volumes of oxygenated blood to be conducted from the heart into the vascular tree;
  • thick walls (2-3 mm in the ascending aorta), which reflect high blood pressure in the aorta;
  • color: aorta appears yellow/white due to a high amount of connective tissue fibers (collagen, elastin).

Please note: the classic depiction of normal heart anatomy usually examines the heart chambers and vessels from the anterior side (Figure 8.10). In this gross anatomy specimen, however, the heart chambers and vessels are examined from the posterior side (Figure 8.11). Thus, the left ventricle is on the left, and the aorta is in front of the pulmonary trunk.

 

an illustrated heart is opened so all 4 chambers of the heart are visible. The great vessels (pulmonary trunk and aorta) fill the top middle of the image and they are connected to the ventricles taking up the bottom of the image - identifiable by their thick walls. Separating the ventricles from the upper atria are the tricuspid and mitral valves which are attached to the ventricle by thing stands of chordae tendinae
Figure 8.10. Normal heart anatomy created by Tetiana Povshedna using creative commons license associated with smart.servier.com under a Creative Commons Attribution 3.0 Unported License)

 

A pig heart has been opened so that the inner chambers are visible. The left ventricle is on the bottom left with a very thick (2-3cm) wall whereas the right ventricle (bottom right corner) has a thinner wall The ventricles are separated by thin white translucent tissue that make up the mitral and tricuspid valves. Thin white strands of chordae tendinae connect these valves to the papillary muscle nestled in the ventricles. Both the pulmonary trunk and aorta take up the top middle: both are thick white tissue that are thick enough to hold their luminal space open.
Figure 8.11. Normal heart anatomy (pig). DHPLC Specimen B0001. All rights reserved

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 

a microscopic image of skin tissue stained pink (with eosin) and purple (hematoxylin). There are 3 clusters of cells, each surrounding a central opening. The cells are mostly pink with a purple ovoid at the basolateral side of the cell (away from the opening)
Figure 8.12 H&E staining of skin DHPLC slide PATH 304 004, created by Tetiana Povshedna under the CC-BY-NC 4.0

 

microscopic view of the aorta stained blue, black, red, and dark pink. The image looks like multiple parallel waves: the top most being light blue suggesting presence of elastin/collagen. The lower image shows lots of black and purple (blue +red) suggesting muscle tissue with lots of collagen
Figure 8.13 Elastin/trichome stain of normal aorta DHPLC slide PATH 304 – 024

 

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

Please note the variation in the thickness of tunica media and the amount of elastic fibers between the two large vessels – aorta and vena cava.

These morphological adaptations reflect blood vessel location and blood pressure levels – both thickness of tunica media and amount of elastin fibers decrease in veins and venules compared to arteries. Note that tunica media is the thickest layer in aorta, but not in vena cava. Tunica externa of the vena cava is the thickest layer that also contains some smooth muscle fibers.

AORTA 

layers of the aorta is visible and enumerated with 1 (tunica intima) at the top of the image, 2 (tunica media) which fills up the middle of the image), and 3 (tunica externa) which occupies the bottom. All layers are a light pink colour with a sense of parallel layers. The tunica intimal layer show discrete layers of cells atop each other. The medial layer has many dark pink dots suggesting nuclei of sm muscle cells. The externa has a variety of tissues including fat (all white with no stain), and dark pink with purple nuclei.
Figure 8.14 H&E stain of normal aorta DHPLC e-slide PATH 304-023. Created by Tetiana Povshedna under a CC-BY-NC licence

 

layers of the aorta is visible and enumerated with 1 (tunica intima) at the top of the image, 2 (tunica media) which fills up the middle of the image), and 3 (tunica externa) which occupies the bottom. The top 2 tunica stain a dark blue/black with a background colour of pink, with a sense of parallel layers. The tunica intimal layer show a light blue with discrete layers of cells atop each other. The medial layer is dark blue/black with pink suggesting a lot of collagen and elastin. The externa has a variety of tissues including fat (all white with no stain), and dark pink with purple nuclei.
Figure 8.15 Elastin/trichome stain of normal aorta. DHPLC e-slide PATH 304-024. Created by Tetiana Povshedna under a CC-BY-NC license

 Vena Cava

layers of the vena cava is visible and enumerated with 1 (tunica intima) at the top of the image, 2 (tunica media) which fills up the middle of the image), and 3 (tunica externa) which occupies the bottom. All three tunica stain an intense dark pink. The tunica intima and top half of the media are round clusters, arranged in layers. The bottom half of the tunica media shows long dark pink cells with purple nuclei. The externa has a variety of tissues including fat (all white with no stain), and dark pink with purple nuclei.
Figure 8.16 H & E stain of vena cava. DHPLC e-slide PATH 304 025. Created by Tetiana Povshedna under a CC-BY-NC license.

 

layers of the vena cava is visible and enumerated with 1 (tunica intima) at the top of the image, 2 (tunica media) which fills up the middle of the image), and 3 (tunica externa) which occupies the bottom. The tunica intima and half of the media stain a dark blue/black with a background colour of pink. The lower half of the the tunica media stains dark purple, with a sense of parallel layers, suggestive of a lot of collagen and elastin. The externa has a variety of tissues but mostly fat (all white with no stain).
Figure 8.17 Elastin/trichrome stain of vena cava. DHPLC e-slide PATH 304 026. Created by Tetiana Povshedna under a CC-BY-NC license.

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

 

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Pathology Copyright © 2022 by Tetiana Povshedna is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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