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.
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.
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.)
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.
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.
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.
A medium-sized (muscular) artery, showing
the typical 3 wall layers:
(1) tunica intima, consisting of endothelium lining the lumen, and a small amount of areolar
c.t., a black and wavy inner elastic membrane divides this layer from the tunica media. (Only the
elastic membrane shows up at this magnification.)
(2) tunica media = smooth muscle (bluish color
here) and some elastic fibers (black).
(3) tunica adventitia = dense irregular c.t. (pink collagenic fibers, black elastic fibers).
There is fat outside the adventitia.
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.
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).
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
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.
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.
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.
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.
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.