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Histology Study Guide
Cardiovascular System

These notes are an ancillary resource, NOT a substitute for scheduled resource sessions or for textbooks.  If you use this on-line study aid, please refer to your textbooks and atlases for richer, more detailed information.

SAQ -- Self Assessment Questions

SAQ, Cardiovascular and Lymphatic Systems

SAQ, Introduction -- microscopy, cells, basic tissue types, blood cells.

Online slides of the heart -- normal  |  pathology
Online slides of blood vessels and lymphatics -- normal  |  pathology

These specimens at the Virtual Slidebox (University of Iowa Department of Pathology) may be examined with full range of magnification and movement.  Requires Java and fast internet connection



Blood vessels

Blood vessels are basically tubular organs found within other organs.  (Recall that an "organ" consists of two or more different tissue types "organized" to serve a larger function.)

Larger blood vessels may even have smaller vessels within their walls (the vasa vasorum).

Histologically, blood vessels consist of concentric layers or "tunics" of different tissue types.

  • The tunica intima is the inner lining, consisting of endothelium and a relatively thin layer of supporting connective tissue.
     
  • The tunica media is the middle muscular and/or elastic layer, containing smooth muscle and elastic tissue in varying proportions.
     
  • The tunica adventitia is the outer, fibrous connective tissue layer.
     
  • Nervous tissue is generally inconspicuous in blood vessels but serves to regulate smooth muscle function and to mediate pain sensation.

Image © Blue Histology

Blood vessels are categorized by function.  The composition of the wall varies quantitatively among these subcategories.

Comparison of artery and vein.  
Click on an image for enlargement.

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Intima (tunica intima)

The intima is the inner layer of a vessel.  It consists of very thin lining of simple squamous endothelial cells supported by a similarly thin layer of connective tissue.  The integrity of the intima is critical, since damage can lead to atherosclerosis or clotting.

Endothelium

The endothelium is a specialized form of mesenchymally-derived epithelial tissue.  This simple squamous epithelium forms a thin, delicate lining of all blood vessels as well as the heart and the lymphatic system.  The endothelium is arguably the most significant feature of the vessel. 

Capillary endothelium may have closely associated pericytes, cells with contractile properties which might regulate capillary flow.

Endothelial cytoplasm is inconspicuous in routine light microscopy.  Typically only the nuclei are visible, at the boundary between the lumen and the wall of a vessel.  

In cross section, endothelial nuclei typically appear thin and dark.  However, occasionally endothelium lies parallel to the plane of section.  In this case the nuclei may appear very large, round, and pale.

Although endothelial cells appear rather uninteresting under the microscope, these are important cells.  They are situated at a critical location, between the blood and all other body cells.  They secrete substances which control local blood flow and blood coagulation, and they are active participants in white blood cell emigration during inflammation.

Historical note:  The close association of endothelium with macrophages in liver, spleen and lymph nodes (i.e., those organs with elaborate endothelially-lined channels supported by reticular connective tissue) led to the term reticuloendothelial system .  The name reflects former confusion about the distinction between endothelial cells and the scattered population of macrophages (monocytes, histiocytes).  Macrophages can be readily labelled experimentally through their phagocytosis of injected carbon particles.  However, endothelial cells are also labelled by the same procedure.  Although endothelial cells are not dramatically phagocytotic, they do shuttle some materials across the endothelial lining via small endocytotic and exocytotic vesicles.

Continuous endothelium.  Throughout much of the body, the capillary endothelial lining is continuous, with neither large gaps between cells nor holes through cells.  Materials pass through the endothelium either by diffusion or via rapid vesicular transcytosis.  (In most of the brain, a lack of transcytotic vesicles accounts for the blood brain barrier -- the only substances which cross such a barrier are those which can diffuse through plasma membranes or those for which specific membrane channels exist.)  

Fenestrated endothelium.  In a few special locations -- notably in the sinusoids of the liver, in the glomeruli of the kidney, and in most endocrine glands -- the endothelium is fenestrated (i.e., full of holes -- from fenestra, window).  

Consult your histology textbook and/or atlas for additional detail and electron micrographs of endothelial cells, or visit "More about endothelial cells" (from Blue Histology).

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Media (tunica media)

The media is the middle layer of a blood vessel and in most arteries and veins it is the thickest of the three tunics.

The media consists of smooth muscle and elastic tissue in varying proportions.  Elastic arteries have the highest proportion of elastic tissue while muscular arteries have the highest proportion of smooth muscle.

For a good view of elastin in vessel walls, see Virtual Slidebox, Artery and Vein.

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Adventitia (tunica adventitia)

Ordinary fibrous connective tissue forms the outer layer of blood vessels.  This adventitial connective tissue is usually more or less continuous with the stromal connective tissue of the organ in which the vessel is found.  That is, there is not distinct outer boundary to the tunica adventitia. 

Nevertheless, the fibers of adventitial connective tissue tend to be more concentric around the vessel and often somewhat denser than the surrounding connective tissue (fascia).  Adventitia may also contain numerous elastin fibers.  (For a good view of elastin in vessel walls, see Virtual Slidebox, Artery and Vein.

Clinical note:  The presence of adventitial connective tissue tightly adhering to vessels facilitates the surgical isolation and repair of vessels.

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Arteries

The largest arteries, such as the aorta and its larger branches, have a tunica media dominated by elastic tissue.  The elasticity conferred by elastin allows these elastic arteries to smooth out the sharp changes in blood pressure resulting from the pumping heart.

Most arteries are muscular arteries, with a media dominated by smooth muscle.  But elastin is also a substantial component (for a good view of elastin in vessel walls, see Virtual Slidebox, Artery and Vein).

Examples of typical muscular arteries.  
Click on an image for enlargement and comparison with an adjacent vein.

  Arterioles are the smallest arteries.  Note that gross anatomists and surgeons may use the term arteriole for any very small artery.  Histologists tend to use the term arterioles only for terminal arterial vessels (i.e., those immediately preceding a capillary bed) which are characterized by having only a single layer of smooth muscle cells. 

Click on a thumbnail below to enlarge an image
which includes a small artery or arteriole..

EM image of artery (longitudinal section, from Elektronenmikroskopischer Atlas im Internet).

Most arteries have continuous layer of elastin, called the internal elastic lamina, at the boundary between the media with the intima.

In routine histological sections, the internal elastic lamina of transversely-sectioned arteries typically displays a distinctive sinusoidal appearance, resulting from postmortem contraction of the artery's smooth muscle in the absence of normal blood pressure.

The thickness of arterial walls is typically not much less than the diameter of the lumen.  With such relatively thick walls, arteries tend to retain a round cross-section in postmortem histological preparations (in contrast to veins, which tend to appear more flattened).

Note that arteries of pulmonary circulation (which convey blood of lower pressure than systemic circulation) have relatively thinner walls, similar to systemic veins.

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Veins

Veins have a wall similar to that of arteries but with a thinner tunica media

The thickness of vein walls is typically much less than the diameter of the lumen (i.e., proportionately much thinner than arteries carrying a similar volume).  With such relatively thin walls, veins tend to appear flattened or collapsed in cross-section in postmortem histological preparations (in contrast to arteries, which tend to appear more round).

Note that vessels of pulmonary circulation (which convey blood of lower pressure than systemic circulation) have relatively thinner walls than systemic vessels of comparable diameter.

Examples of typical veins.  
Click on an image for enlargement and comparison with an adjacent artery.

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Capillaries and sinusoids

Capillaries and sinusoids are the smallest (microscopic) vessels, providing communication between arteries and veins

By definition, capillaries are vessels whose diameter is so small (less than 10µm) that red blood cells must pass through in single file (thumbnail at left).

Capillaries often form an interconnected plexus, or "capillary bed" (thumbnails at right, within kidney or stomach). 

Historical note:  The existence of capillaries was predicted by William Harvey in 1616, as a necessary corollary of his theory of blood circulation.  Capillaries were actually observed in 1661 by Marcello Malpighi using a newly invented instrument, the microscope.)

In contrast to capillaries, sinusoids are typically more voluminous than capillaries (for an example, see liver).  Although the lumen of a sinusoid is commonly narrow in one dimension, it may be extensive in another dimension, like the space between two walls rather than the hollow of a tube.

The wall of capillaries and sinusoids consists of little more than endothelium.  A single thin, flattened endothelial cell forms a segment of capillary, like a sheet of paper rolled into a tube (with a small bulge at the site of the nucleus).  These single-cell tubes are then attached end-to-end to form a capillary.

In histological preparations, capillaries tend to be quite inconspicuous unless they contain red blood cells.  Capillary endothelial nuclei are commonly difficult to distinguish from fibroblasts and other connective tissue cells, unless the preparation is fine enough to resolve the thin endothelial cytoplasm surrounding a small lumen.

Wrapped at intervals around capillaries are poorly-studied pericytes, cells with contractile properties that can apparently adjust capillary blood flow.  This effect has been demonstrated for local microvascular control of blood flow in the brain.  [Reference:  MacVicar & Salter, Neuroscience: Controlled capillaries, Nature 443, 642-643 (12 October 2006) | doi:10.1038/443642a.]

In most parts of the body, capillaries and sinusoids are more or less "leaky".   The degree of "leakiness" is determined by variations in the endothelial lining of the capillaries. 

In ordinary connective tissue, plasma leakage is typically balanced by re-absorption and/or lymph drainage.  A shift in the balance (e.g., increased vascular permeability due to inflammation) can result in accumulation of tissue fluid, or edema

Images © Blue Histology

 

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Heart

The lining of the heart is similar to that of blood vessels, with an intima including endothelium

The pericardium (the heart's outer surface) is a serosa lined by mesothelium.

Mesothelium is a simple squamous epithelial tissue, derived from mesoderm, which forms the surface lining not only of the pericardial cavity but also peritoneal and pleural cavities (i.e., all major body cavities). 

The bulk of the heart consists of cardiac muscle, one of the three distinct types of muscle in the body.  (The other two muscle types are skeletal muscle and smooth muscle.)

Compare muscle fiber types
Cardiac muscle

Cardiac pathology yields characteristic changes in the structure/appearance of cardiac muscle.

Cardiac muscle has several distinct characteristics.

For more information about cardiac muscle cells, see your textbook.

For more electron micrographs see Elektronenmikroskopischer Atlas im Internet.

Specialized cardiac muscle cells form bands called Purkinje fibers, which conduct electrical signals to coordinate heart contractions (see your physiology resources).  Purkinje fibers have a larger diameter and paler cytoplasm (i.e., fewer contractile filaments) than ordinary cardiac muscle fibers.  Both of these features are adaptations for reducing electrical resistance and increasing the speed of electrical conduction.

See WebPath Myocardial Infarction for some cardiac pathology.

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Lymphatic System

The lymphatic system consists of:

The lymphatic system serves two principal functions.

  1. The lymphatic system provides a route for excess interstitial fluid ("lymph") to return to the blood.
     
  2. The lymphatic system provides the means for immune system cells (most notably lymphocytes) to travel, communicate, and proliferate; thus the lymphatic system is instrumental in the process of inflammation.

For more in-depth treatment of the immune system, see outside links.

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Lymphatic vessels

Lymphatic vessels (often just called lymphatics) are channels which drain excess fluid ("lymph") from tissues.

In most peripheral tissues, some plasma seeps out of capillaries.  A portion of this is taken back up in venules while the rest drains into terminal lymphatic channels, also called lymphatic capillaries.  A shift in the balance between fluid entering and leaving tissues (e.g., increased vascular permeability due to inflammation) can result in accumulation of tissue fluid, or edema

All lymphatic vessels eventually lead "downstream" to the thoracic duct, which empties into the vena cava (a point where blood pressure is quite low; higher pressure would impede drainage).

Lymphatic vessels resemble blood vessels with exceptionally delicate walls (and, of course, without red blood cells).  Smaller lymphatic vessels consist of little more than endothelium.

If, while examining a histological specimen, you encounter a flattened, endothelially-lined passage that seems too delicate to be a vein but too large to be a capillary, it is probably a lymphatic.

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Lymph nodes

Lymph nodes are small organs, found in scattered locations, along the lymphatic drainage system of the body.

Some lymph nodes can be palpated, where they are sometimes called "glands", especially when swollen in reaction to infection.

Lymph nodes provide "gathering places" through which wandering lymphocytes (which are principal elements of the immune system) must pass as they travel from peripheral tissues on their return to systemic circulation.  Within the lymph nodes, activated lymphocytes (i.e., those which have encountered an antigen which they recognize) proliferate, producing more lymphocytes able to respond to the same antigen.

Metaphorically, lymph nodes might be considered to be "filters" for "used" interstitial fluid.

Lymph nodes consist of a fibrous connective tissue capsule containing many lymphocytes and other immune-system cells, into and through which flow both lymph and blood.  Some of the lymphocytes are organized into lymph nodules, also called lymphoid follicles, which are sites where lymphocytes congregate.  At the center of each lymph nodule is a germinal center where the lymphocytes proliferate. 

Lymph nodes are supported within by a network of reticular fibers.

For additional information, see your text or visit "Lymphoid Tissues" (from Blue Histology).

Key for practical differentiation of lymphoid tissues (courtesy M. O'Keefe, MSI)

(For more about the immune system, see outside links.)

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Mucosa-associated lymphoid tissue ("MALT")

Lymphoid tissue occurs in lamina propria of most mucosal organs, conspicuously in the gastrointestinal tract, where it is sometimes referred to as MALT, for Mucosa-Associated Lymphoid Tissue.

The most characteristic features of mucosa-associated lymphoid tissue are lymph nodules, also called lymphoid follicles, which are sites where lymphocytes congregate.  At the center of each lymph nodule is a germinal center where the lymphocytes proliferate.

The connective tissue around a lymph nodule is usually heavily infiltrated with lymphocytes migrating to and from the germinal center.  

Lymph nodules may occur in lamina propria anywhere along the GI tract.  Large aggregations of lymph nodules may extend beyond lamina propria and intrude into the submucosa.  Nearby epithelial tissue may also be infiltrated by lymphocytes.

Aggregations of lymph nodules are characteristic of tonsils, Peyer's patches, and appendix.

For more on MALT, consult your histology text or visit "Lymphoid Tissues" (from Blue Histology).

(For more about the immune system, see outside links.).

Key for practical differentiation of lymphoid tissues (courtesy M. O'Keefe, MSI)

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Spleen

The spleen is an organ in which lymphoid tissue and blood are intermixed and served by a network of vascular sinusoids.   The loose lymphoid and blood cells are contained within a fibrous connective tissue capsule and supported by strands of fibrous connective tissue ("trabeculae").  Lymphoid tissue forms the so-called "white pulp" while surrounding tissue rich in blood forms "red pulp". 

The color names for red and white pulp reflect their natural (unstained) color, with red pulp colored by red blood cells while white pulp takes on the creamy color of massed lymphocytes.

For more detailed description of spleen structure and function, consult your text or visit "Lymphoid Tissues" (from Blue Histology).

(For more about the immune system, see outside links.)

Key for practical differentiation of lymphoid tissues (courtesy M. O'Keefe, MSI)

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Thymus

The thymus is the primary organ for maturation of lymphocytes.  It plays a central role in developing self-tolerance (i.e., the immune system's ability to avoid attacking the body's own cells).  "Promiscuous gene expression" in stromal epithelial cells leads to deletion of T-cells that react to self-antigen.  (A similar process has been detected in peripheral lymph nodes; see Science 321:776 [2008].)   [more, outside link]

Superficially, the thymus resembles a gland subdivided into numerous small lobules.  But the thymus has no ducts and no proper secretory tissue.  In an active (i.e., young) thymus, the cortex is densely packed with lymphocytes while each lobule has a paler medullary core where cells are dividing.  Thymus also contains cells called "reticular cells", "epitheliocytes", "dendritic cells", and "stromal cells."  Although such cells are generally inconspicuous in routine histological specimens, clusters may be evident as "Hassall's corpuscles."

For more information on thymus structure and function, consult your text or visit "Lymphoid Tissues" (from Blue Histology).

  Key for practical differentiation of lymphoid tissues (courtesy M. O'Keefe, MSI)

See additiional outside Links for more in-depth treatment of immunology

  

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Cardiovascular pathology

WebPath offers an extensive image collection for blood and bone marrow smears, both normal and pathological.

WebPath also provides a tutorial on myocardial infarction and an extensive set of images associated with atherosclerosis
Note especially the following specific examples.

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Image index

 

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David KingComments and questions: dgking@siu.edu

SIUC / School of Medicine / Anatomy / David King

http://www.siumed.edu/~dking2/crr/cvguide.htm
Last updated:  1 October 2012 / dgk