Introduction The slides for this lab are located in the “Cardiovascular System” folder on the Virtual Microscope. The cardiovascular system includes the heart and vessels that carry to and from it. You will learn about blood and hematopoiesis in a subsequent lab.

Learning objectives and activities Using the Virtual Slidebox:

A Examine the features and organization of the heart:

o Demonstrate the layers of the heart wall in histological sections. o Locate the major structural components of each layer of the heart wall. o Demonstrate evidence of the presence of the heart’s conducting system. o Identify a heart valve and describe its intrinsic histological organization.

B Analyze the histological organization and structure of blood vessels:

o Demonstrate the structural components of the three tunics that form the wall of a . o Use the appearance of vessel ultra-structure to classify and distinguish between different types of blood vessels in the body.

C Complete the self-quiz to test your understanding and master your learning. Examine the features and organization of the heart.

Examine Slide 1 to demonstrate the layers of the heart wall in histological sections.

Match the following statements with the corresponding regionsin the photograph of the gross heart wall and then locate each region in the histological heart wall seen in Slide 1 (31) using the virtual microscope.

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A. Endocardium The thin, inner lining of the heart wall. It does not contain any large vessels or nerve bundles.

B. Myocardium The thick, muscular component of the heart wall. The muscle of the inner aspect is frequently thrown into ridges (trabeculae carnae and papillary muscles) that project into the lumen.

C. Epicardium The outer most layer of the heart wall. It contains fat, nerve bundles and large vessels. Use Slide 1 (31) to locate the major structural components of each layer of the heart wall.

i) Endocardium

This inner layer of the heart wall:

a. A simple squamous lining the lumen of the heart. Find endothelium in You will see later that the endothelium is continuous with the Slide 1a (31) endothelium of the in blood vessels.

b. Subendothelium and Subendocardium The subendothelium lies between the endothelium and Find the myocardium and is composed of loose irregular (areolar) subendothelium/sube and may contain small vessels. The ndocardium in subendocardium is not normally visible unless it contains Slide 1b (31) components of the heart’s conducting system (Purkinje fibers) or blood such as during a subendocardial hemorrhage. ii) Myocardium

The muscular layer of the heart wall. In Foundations you learned how to recognize cardiac muscle as compared to smooth and skeletal muscle in both longitudinal and transverse section. Review this now:

c. Myocardium in longitudinal section Outline a number of typical cardiomyocytes and confirm: - centrally located nuclei - cytoplasm packed full of actin and myosin organized into Examine regions of sarcomeres (striations) myocardium (long - branching of individual cells section) in - intercalated disks that form communicating junctions Slide 1c (31) between cells - numerous containing erythrocytes between cells

d. Myocardium in transverse section Confirm these features of cardiac muscle in cross section: Examine regions of - the long cells appear rounder or more oval in shape myocardium (cross - striations are not visible because they run perpendicular to section) in the plain of the section. Slide 1d (31) - only a few cells have had their nuclei sectioned - when sectioned the nucleus does not occupy the entire cell iii) Epicardium

The outermost layer of the heart wall. It is composed of the visceral layer of serous pericardium which is continuous with parietal layer of serous pericardium (adheres to fibrous pericardium). Note that the parietal serous and fibrous pericardium are absent because the pericardial sac was removed during preparation. Observe the following layers of the epicardium:

e. Visceral serous mesothelium Find the visceral A simple squamous epithelium lining the outer wall of the heart. serous mesothelium in These cells are responsible for secretion of pericardial fluid into Slide 1e (31) the pericardial space.

f. Adipose layer A layer of fat between myocardium and the mesothelium. Note Find the adipose layer that embedded within this fatty layer are small blood vessels. of the epicardium in These are components of the coronary-cardiac circulation. Slide 1f (31)

iv) Purkinje fibers

Use Slide 2 (32) to demonstrate evidence of the presence of the heart’s conducting system

These are modified cardiac muscle cells locted within the subendocardium that form part of the conducting system of the heart. The fibers transmit electrical impulses from the Bundles of His in the septum to the apex and lateral walls of the ventricles resulting in the coordinated contraction of the ventricles. Note that the cells are larger than regular cardiomyocytes and appear paler due to an increase in glycogen within the cells and reduced volume of more peripherally located actin and myosin in the cells. v) Heart valves

Examine Slide 3 (29) to identify a heart valve and describe its intrinsic histological organization

Valves are avascular and composed of two types of connective tissue:

a. Fibrosa Identify this dense irregular connective tissue core that blends with the connective tissue of the fibrous skeleton of the heart.

b. Spongiosa arterialis (semilunar) or Spongiosa auricularis (AV) Find the fibrosa, Identify this thin area of loose irregular connective tissue located spongiosa and beneath the endothelium on the blood vessel side of a semilunar ventricularis of the valve or atrial side of an AV valve. It infers plasticity and flexibility aortic semilunar valve to the valve cusps. in Slide 3a-c (29)

c. Ventricularis Identify this area which has a distinct region of dense irregular connective tissue beneath the endothelium on the ventricular side of a valve. It contains many elastic fibers.

Also in Slide 3 (29) try to identify the and the following features of the heart wall.

d. Aorta The aorta has features common to the structure of a large elastic Find the aorta in (see later). Note that it is cut in longitudinal section here. Slide 3d (29)

e and f. Myocardium and fibrous skeleton In addition to the cardiac muscle cells, notice the abundance of Find the myocardium dense irregular connective tissue in this region. This constitutes in Slide 3e (29) part of the fibrous skeleton of the heart. The collagen fibers you can see are invested in the adjacent myocardium and are Find the fibrous continuous with the core of the valve to support it. skeleton in Slide 3f (29) g. Endocardium Note that the endocardium is continuous with the outer layer of the heart valve cusp although it is given a different name in these Find endocardium in regions (see above). Slide 3g (29) Analyze the histological organization and structure of blood vessels

Use Slide 4 (or the image below) to demonstrate the components of the 3 tunics that form the wall of a blood vessel

All blood vessels have the same structural organization. They all contain a lumen and their walls are composed of series of structural components organized into three layers or ‘tunics’:

1. Tunica intima

1a Endothelium 1b Subendothelium 3a 3b 1c Internal elastic lamina 2a 2. Tunica media 2b 2a 2b External elastic lamina 1a 1b 1c 3. Tunica adventitia 3a Dense irregular CT 3b Vaso vasorum Nervi vasorum

It is important to be able to know the components of each of these regions in a vessel because variations in composition and size can be used to identify and classify different sizes of vessels.

Before examining any slides, familiarize yourself with this diagram (shown on Slide 4) by identifying the tunics and their subcomponents. Examine Slide 5 (28), Slide 6 (27), Slide 7 (54), Slide 8 (79), Slide 9 (18) and Slide 10 (30) and use the appearance of vessel ultra-structure to classify and distinguish between different types of blood vessels in the body.

i. Elastic Artery (Aorta)

Examine Slide 5 (28) and identify the three tunics and their components in an elastic artery.

a. Tunica intima An inner simple squamous endothelium and a relatively thick subendothelium that is continuous with the endocardium you saw in the heart earlier. The internal elastic lamina is the first layer of elastin seen (and often the thickest).

b. Tunica media Elastic have a characteristically thick and prominent tunica media. It is composed of smooth muscle cells arranged in multiple layers that are separated by distinctive elastin fibers.

c. Tunica adventitia The outermost connective tissue coat of the vessel. It is characterized by dense irregular collagen fibers within which are embedded numerous small blood vessels (vaso vasorum) and nerves (nervi vasorum). ii. Distributing/Muscular Artery

Examine Slide 6 (27) and identify the three tunics and their components in a muscular artery.

In this slide, the artery is located within a neurovascular bundle, which contains nerves, fat, connective tissue and other small blood vessels. a. Tunica intima Identify the endothelium, its thin subendothelium and the prominent internal elastic lamina. The internal elastic lamina is the key feature for identifying muscular arteries.

b. Tunica media Identify the smooth muscle cells of the tunica media. Elastin fibers in the tunica media are barely visible but there is often a prominent external elastic lamina.

c. Tunica adventitia Identify the extent of the tunica adventitia in this artery. Do not confuse it with the surrounding fat and connective tissue of the neurovascular bundle with which it blends. iii. and

Examine Slide 7 (54) and find and in the wall of the esophagus.

Blood supply to the esophagus is via branches of the esophageal arteries (muscular arteries) located within the wall of the organ. These distribute blood via arterioles and return it via venules to the esophageal . These structures run together and comparing adjacent vessels of equivalent size is agood way to recognize their differences:

a. Tunica intima Arteriole: Endothelium is visible and large arterioles have an internal elastic lamina. Venule: Endothelium is visible but large venules do not have an internal elastic lamina.

b. Tunica media Arteriole: Composed of smooth muscles cells between 1-4 layers thick (large v small). Venule: Venules of equivalent size to adjacent arterioles have fewer smooth muscle cells. Small venules have an almost imperceptible tunica media.

c. Tunica adventitia The outer tunica of both venules and arterioles is often indiscernible from the surrounding connective tissue.

iv.

Examine Slide 8 (79) and locate several capillaries in the papillary dermis of the .

Capillaries are the smallest diameter vessels, and therefore do not have the tunic organization like the other vessels. They are usually distinguished by the nucleus of an endothelial cell. Observe them in transverse section and (more unusually) in longitudinal section. v. (small to medium)

Examine Slide 9 (18) and distinguish the vein from the muscular artery.

Note that this vein is comparable in size to the accompanying muscular artery. Compare the tunics of the muscular artery with that of the vein to see the differences. a. Tunica intima Identify the location of the endothelium of the vein. There may be a faint internal elastic lamina in medium veins

b. Tunica media This region is thin for the size of the vessel. As a result veins frequently appear collapsed whereas arteries are held open.

c. Tunica adventitia The tunica adventitia is the thickest component of the wall in veins and contains a vaso vasorum in larger veins.

vi.Large Vein (Vena cava)

Examine Slide 10 (30) and identify the distinguishing features of the three tunics of the vena cava.

The vena cava is the largest vein in the body. The tunica media is very thin for a structure of this size while the tunica adventitia is very thick and has a notable layer of smooth muscle in it.

a. Tunica intima The endothelium is present with a thin subendothelium.

b. Tunica media Composed of a thin, circularly arranged layer of smooth muscle cells, there may be an external elastic lamina present.

c. Tunica adventitia The vena cava is recognizable by this thick layer of connective tissue containing longitudinally arranged layers of smooth muscle. In addition to all the basic tissues in slides you must be able to identify the following features of:

The Heart

Endothelium Endocardium Subendocardium Myocardium Epicardium Mesothelium Purkinje fibers Heart valve Fibrous skeleton

Blood vessels

Tunica intima Tunica media Tunica adventitia Endothelium Internal elastic lamina External elastic lamina Vaso vasorum Nervi vasorum

You must also be able to use these features to identify:

Heart Elastic artery (e.g. aorta) Muscular/distributing artery Arteriole Venule Capillary Vein Large vein (e.g. vena cava)