Cardiovascular System

Vessels
Blood vessels are usually composed of three layers: the tunica intima, tunica media, and tunica adventitia. The tunica intima consists of a layer of endothelial cells lining the lumen of the vessel, as well as a subendothelial layer made up of mostly loose connective tissue. Often, the internal elastic lamina separates the tunica intima from the tunica media. The tunica media is composed chiefly of circumferentially arranged smooth muscle cells. The external elastic lamina often separates the tunica media from the tunica adventitia. The tunica adventitia is primarily composed of loose connective tissue with fibroblasts and collagen fibers.

Elastic Arteries

Elastic artery (aorta)
In the aorta, the tunica media constitutes the greater part of the vessel wall. It is made up of multiple elastic lamellae alternating with thin layers of circularly oriented smooth muscle. The boundary between the tunica intima and media is not readily defined, and the internal elastic lamina is merely the innermost of the many elastic lamellae within the wall and is not obvious in this preparation. The tunica adventitia essentially lacks elastic lamellae, and is mainly loose connective tissue and blood vessels (vasa vasorum).

Elastic artery (carotid) and large vein (jugular)
Contrast the artery with the vein. Observe the intima, media, and adventitia. In the artery, the elastic lamellae are readily observed. The aorta compared to the carotid artery has a thicker intima and more elastic lamellae in the media.

Aorta, human 40+ years, Verhoeff Van Gieson stain.
Note that elastic lamellae and collagen fibers are easy to visualize in this specimen.
Examine these specimens to further reinforce your ability to recognize elastic arteries and their features. Note the relationship between age and histology. Note especially the low amount of elastin in the newborn, and the higher amount and thickness of elastic lamellae in the young (7 years) arteries, with a decline in elastic lamellae thickness and an increasing thickness of the tunica intima in the adult.

Aorta, human new born.
Aorta, human new born, Verhoeff Van Gieson stain.
Aorta, human 7 years.
Aorta, human 7 years, Verhoeff Van Gieson stain.
Aorta, human 22 years.
Aorta, human 22 years, Verhoeff Van Gieson stain.

Muscular (Distributing) Arteries

In muscular arteries, smooth muscle becomes the predominant constituent of the tunica media. Internal and external elastic laminae are prominent. The tunica intima of muscular arteries is thinner than the tunica intima of elastic arteries.

Spermatic cord.
This specimen has several muscular arteries mixed in with atypical veins of the pampiniform plexus. These veins contain a greater amount of smooth muscle than most veins of this size. Also note the outer longitudinal arrangement of smooth muscle in these veins. Observe the characteristic three layers in the arteries. Note the large amount of circularly arranged smooth muscle in the tunica media of the muscular arteries.

Submaxillary gland, Verhoeff's hematoxylin; mucicarmine & methyl green.
Find the muscular arteries. The internal elastic lamina and external elastic lamina are stained black and therefore readily observed. Look carefully in the tunica media of the arteries and you should also be able to resolve elastic lamellae.

Arterioles

In arterioles, the tunica intima consists of a continuous endothelium and a very thin subendothelial layer. In addition, a thin, fenestrated internal elastic lamina is present in larger arterioles but absent in terminal arterioles. Generally, the tunica media consists of 2 layers of smooth muscle cells. But in the smallest arterioles there is a single layer. The tunica adventitia is a thin sheath of connective tissue, which is not easily defined.

Uterine tube
Find the multitude of vessels in the connective tissue which invests the oviduct. This specimen is especially good for observing small arteries, veins, arterioles, venules, and capillaries. Identify the layers of the vessels present.

Capillaries & Venules

Capillaries average from 9 to 12 µm in diameter, just large enough to permit passage of cellular components of blood. The wall consists of extremely attenuated endothelial cells. In cross section, the lumen of small capillaries may be encircled by a single endothelial cell, while larger capillaries may be made up of portions of 2 or 3 cells. No smooth muscle is present. Recall that at the EM level, three types of capillaries can be distinguished: continuous, fenestrated, and discontinuous.

Capillaries and venules are the principal vessels where exchange between the blood vascular space and the interstitium takes place. A high amount of exchange of fluid and cells takes place through the wall of the venule. Venules are similar to capillaries in that they have a very thin wall; but unlike capillaries they have diameters that range from approximately 20-60 µm.

Spermatic cord
Spermatic cord is useful for observation of small vessels. Observe venules, capillaries, and arterioles.

Veins

Veins are best studied along with their corresponding arteries. Veins have the same three layers as arteries, but boundaries are indistinct and elastic components are not as well developed in veins. Histologically, veins are usually collapsed because of the thinner, less elastic walls.

Pancreas
Use this specimen to appreciate the contrasting features of a medium sized artery (muscular artery) and vein. The vein is typical of vessels not surrounded and supported by solid tissue. Veins in these locations have a well developed muscular adventitia (muscle cells are oriented longitudinally). This is also the typical structure of large veins such as the vena cava.

Lymph node
This specimen shows a more typical medium sized vein.

Lymphatics

Lymphatics are found in all tissues except the CNS, cartilage, bone and bone marrow, thymus, teeth, and placenta. Lymphatics are sometimes difficult to demonstrate satisfactorily in normal tissues because these large, thin-walled vessels frequently collapse during tissue processing. Lymphatics are often most easily visualized and studied during pathological processes (e.g., inflammation). Lymphatics start as blind-ended lymphatic capillaries which coalesce to form lymphatic vessels and finally empty into the circulation via the lymphatic ducts (thoracic and right lymphatic).

Spermatic cord
Find the lymphatic vessels of the spermatic cord. Similar to the veins (pampiniform plexus) of the spermatic cord, these lymphatic vessels have unusually thick walls. Note the valve leaflets of this vessel.

Heart

Heart and AV valve
This specimen is a full thickness section heart wall which includes atrium, ventricle and an AV valve leaflet. Note the difference in thickness of the atrial and ventricular walls (correlates with differences in pressure and workloads). The musculature of both walls is composed of cardiac muscle cells which exhibit the usual characteristics of cardiac muscle, i.e., branching, anastomosing, central nuclei, myofibrils, cross striations, and intercalated disks. The musculature of the atria is not continuous with that of the ventricles. Note that the atrial endocardium is very thick but that the ventricular endocardium is very thin.
Observe the AV valve leaflet and its attachment to the fibrous skeleton of the heart. The AV valve leaflet is formed by a fold or duplication of the endocardium; note especially the dense connective tissue core which forms the central part of the valve leaflet. The upper or atrial surface of the valve is thick and resembles atrial endocardium, and the lower or ventricular surface of the valve is thin and resembles ventricular endocardium.
Examine the fibrous skeleton and observe that it is composed of very dense connective tissue and that histologically, it resembles a tendon (the fibrous skeleton of the heart is actually made up of circular tendons).

Heart, muscular interventricular septum.
In the subendocardial layer, locate the longitudinally sectioned Purkinje fibers. Note that 1) they are connected by intercalated discs, 2) they are larger than the ordinary cardiac myocytes, and 3) they are separated from the myocardium by a connective tissue layer which is actually a sheath in the intact heart.