Loyola University Medical Education Network Part 8: Cardiovascular System

Slide 1

This capillary running through embryonic mesenchyme has a wall consisting solely of a single layer of endothelium. Notice that the lumen of the vessel is only slightly larger than the diameter of the r.b.c.'s within.

Slide 2

This sinusoid, like a capillary, has only an endothelial wall, but its lumen is characteristically considerably wider. Also, in some locations in the body (such as bone marrow, liver, and spleen) the endothelial cells of sinusoids are rather loosely joined together, thus permitting passage of blood cells between them.

Slide 3

In the middle of the field is a sinusoid (filled with orange-colored r.b.c.'s) in the marrow cavity of spongy bone. (The larger empty circles are fat cells.)

Slide 4

A capillary lying in the endomysium between skeletal muscle fibers. This one shows very dark endothelial nuclei and has 3 pink r.b.c.'s lined up in a row inside.

Slide 5

EM of different capillary endothelia. Note: basal lamina (1) under each one. Also pinocytotic vesicles (PV) and fenestrations (arrows).

Slide 6

Diagram of routes of transport across capillary or sinusoidal endothelial cells. (Notice that we now also consider discontinuous endothelium as well as the types you saw on the previous slide.) "Spikes" on the outer leaflet of membrane = glycocalyx layer.

Slide 7

Endothelium (simple squamous epithelium) lines the lumen of all blood and lymph vessels, as well as the heart. Here endothelial nuclei are seen ringing a venule (so identified because there is only a thin coat of connective tissue outside the endothelium, the lumen is too large to be a capillary, and because sinusoids don't occur in ordinary connective tissue areas like this.)

Slide 8

The smallest arterioles have a single layer of smooth muscle outside the endothelium, and a lumen hardly wider than a capillary. Toward the upper center of this field is a round, cross-cut arteriole with just one or two layers of muscle in the media. To the right is the more irregular, wider shape of a venule with only a thin adventitial wall.

Slide 9

EM of capillary. The inner nucleus and ring of cytoplasm is the endothelial cell around the lumen. The outer nucleus and ring of cytoplasm is a single pericyte wrapped around the vessel. Notice the thin gray line of basal lamina between the cytoplasm of the two cells and shared by both cells. There is also a basal lamina along the outer surface of the pericyte as well.

Slide 10

Small blood vessels, with 3-layered walls: These vessels are often found running together in the c.t. coats of body organs.

Slide 11

A small artery cut longitudinally. Notice the circular arrangement of smooth muscle cells cut tangentially at the left end. For most of the vessel, the muscle is cross-cut, looking almost like an epithelium. The real epithelium, however, is the simple squamous endothelium immediately lining the lumen, with thin, flat nuclei oriented longitudinally along the vessel.

Slide 12

A medium-sized (muscular) artery, showing the typical 3 wall layers:

Slide 13

Another medium-sized, muscular artery (also called a distributing artery). This is typical of the arteries you dissected in the arm; it usually runs with a vein and nerve. There is a characteristic inner elastic membrane (dark pink with arrow pointing to it) and a heavy circular muscle in tunica media. Note that a = adventitia.

Slide 14

EM photo of inner elastic membrane (white band). Endothelial cells are bunched up because the wall is contracted.

Slide 15

Medium-sized vein with a much less compact muscle layer than you saw in the preceding arteries. The tunica media is indicated by bar "a". Bar "b" = adventitia, which is at least as wide as the media, and often even wider. There is no evident inner elastic membrane. (Blood in the lumen stains red here.) To the right, compare sizes and walls of one small artery (d) and two very small veins (c) and (e).

Slide 16

Lymphatic vessel with a c.t. wall even thinner than a vein. There is a cut leaflet of a valve across the lumen. Material in the lumen contains no r.b.c.'s, mostly just structureless lymph and some lymphocytes. There are some fat cells and lymphocytes in the surrounding connective tissue.

Slide 17

Higher power of valve made of a core of fine c.t. with endothelium covering both surfaces. Valves in veins are constructed similarly.

Slide 18

Low power view of wall of aorta, an elastic artery:

Slide 19

Detail of inner portion of aortic wall: "a" bar = depth of tunica intima (next to lumen). In the media there are many layers of wavy, dark-stained elastic membranes, alternating with the paler pink smooth muscle and areolar c.t. (Note that these are elastic membranes, not just fibers. Think of many layers of rubber sheets enwrapping the vessel, and you have cut across them and are looking at the cut edges. These sheets are fenestrated; i.e., they have holes in them, thus allowing passage of nutrients diffusing from the blood in the aortic lumen out into the tissues of the wall.) Arrows = nuclei of smooth muscle cells.

Slide 20

Detail of outer portion of aortic wall, showing blood vessels (vasa vasorum) in the adventitia. These vessels bring nutrients only to the outer one-third or so of the vessel wall.


Slide 21

Inner surface of the heart, with pale, large Purkinje fibers lying in the subendocardial layer. Endocardium (or intima) is above. The beginning of the myocardium (media, cardiac muscle) is below.

Slide 22

The heart wall, like blood vessels in general, has three main layers, though they are not called intima, media, and adventitia. As in vessels, however, the innermost and outermost layers are primarily connective tissue; the middle one is muscle --- in this case, cardiac muscle. From left to right, then, in this picture of ventricle wall, there is first a very thin endocardium, which consists primarily of an endothelial lining and a very small amount of connective tissue underneath it. The muscle layer, or myocardium is next and is by far the thickest layer and constitutes the bulk of the heart. To the far right is the epicardium, which contains considerable fat. In gross anatomy the epicardium is called the visceral layer of the serous pericardium; it has an outermost lining layer of mesothelium.

Slide 23

A high magnification reminder of the appearance of cardiac muscle cut longitudinally, with central nucleus, branching fibers, and cross-striations. Muscle fibers spiral around the heart in all directions and can thus exert the necessary squeezing action as the heart contracts. Remember that these muscle cells are attached end to end by junctions at the intercalated disc. Axon terminals of autonomic neurons innervate some of the muscle cells, and the stimulus is spread to neighboring muscle cells by the intercalated discs and by gap junctions along the side walls of the cells.

Slide 24

Cardiac muscle in cross-section. Note also the many cross cut capillaries in the connective tissue endomysium between muscle fibers. As you might expect from the constant work the heart performs, it is a highly vascularized organ. Capillaries in this (or any) muscle have endothelium that is continuous and non-fenestrated.

Slide 25

The surface of the ventricular lumen is very irregular because of the presence of papillary muscles in the wall. These irregularities are, of course, lined with endothelium.

Slide 26

Low power of a Mallory-stained heart, showing two channels (above) that are continuous with the lumen of the left ventricle (below). The left-hand channel is the aorta, with some blue connective tissue in its wall. There is also one cusp of the semilunar valve, with its blue core of dense collagen. Remember that valves are lined over their entire surface by endothelium which is continuous with aortic endothelium above and the ventricular endothelium below. To the right in this picture is the atrioventricular channel, with chordae tendinae extending down from the mitral valve and attaching to the papillary muscles of the ventricle. Like valves, the chordae tendinae are also composed of dense collagenous connective tissue covered by an endothelial lining.

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John A. McNulty Last Updated: August 12, 1996