Download (eg Pulmonary vein), which does NOT have large amounts of elastic

CARDIOVASCULAR AND LYMPHATIC SYSTEMS
Roger J. Bick, PhD, MMEd
Reading: Gartner and Hiatt, pp161-180; Gartner, Hiatt, Strum, pp163-177
Objectives:-After this lecture and lab you should be able to distinguish and know
 the layers and cell types of the heart and vasculature
 The differences between large arteries and large veins
 The differences between arterioles and small veins
 Know the different types of capillaries
 Know the origination, termination and architecture of lymphatics
Key Words:- Key words: Tunicae Intima, Media, Adventitia. Myo-, epi-, endocardium. Valves. Elastic.
Veins, Arteries, venules, arterioles, metarterioles, capillaries (fenestrated, continuous, sinusoidal).
Lymphatic. Vasa vasorum. Pericyte. Purkinje
Cardiovascular system is comprised of 3 layers throughout. The heart has 3 layers - endocardium (inside
- a thin layer of endothelium), myocardium (middle - a thick layer of striated muscle), and epicardium
(outer - a thin layer of flat, squamous endothelium), attached to each other by connective tissue. The
vessels (veins, arteries and capillaries) have 3 layers:- a tunica intima (inner, akin to the endocardium:
tunica [L] means coat), a tunic media (middle, akin to the myocardium) and tunica adventitia (outer,
akin to the epicardium).
The heart: A pump for blood; Bulk of the tissue is striated, involuntary cardiac muscle, which pumps
rhythmically (hopefully); can undergo hypertrophy (thickening), atrophy (thinning), necrosis (damageinduced cell death) and apoptosis (programmed cell death). In the lab you will have a section of heart that
will look something like this:-
There are 4 valves in the heart, tricuspid (right AV), mitral (left AV), and semilunar (pulmonary artery
and aorta) - folds of endocardium covered by endothelium; Tricuspid and mitral valves thicker than
semilunar; all contain smooth muscle, lymphatics, blood vessels and a central connective tissue core. The
Pericardium is probably not present in your lab specimen.
Pericardium (Outer, fluid filled sac covering the heart): Parietal pericardium (outer-nearest the skin) – thin mesothelium that adheres to fat and
connective tissue of pericardial cavity.
 Pericardial Fluid – Plasma ultrafiltrate from visceral pericardium; Proteinaceous in nature;
lubricant for the contraction/relaxation of the heart; 15-50cc.
 Visceral pericardium – thin mesothelium, sometimes cuboidal, that adheres to epicardium.
Epicardium – Thin layer of flat to cuboidal cells, covering fibrous and adipose connective tissue;
Contains nerves and vasa vasorum (vessels) of both heart and of coronary vessels found in this
connective tissue mass (equivalent to tunica adventitia of the vessels).
Myocardium – thickest layer of the heart (usually); Composed of bundles of cardiac muscle of varying
thickness (easily distinguished by striations, intercalated discs, branched fibers and centrally located
nuclei); strands of connective tissue and some vascular tissue course through it (equivalent to the tunica
media of the vessels).
Endocardium – Simple squamous epithelium, sometimes cuboidal, over a layer of variable thickness, the
subendocardial layer (thicker in the atria than in the ventricles), containing collagen fibers, elastic fibers
and smooth muscle (equivalent to the tunica intima of the vessels). Purkinje fibers associated with the
Conduction System are found here.
Heart is supported in chest cavity by dense collagenous and elastic connective tissue fibrous skeleton
(composed of the annuli fibrosi at the atrioventricular orifices, the arterial foramina, the trigonal fibrosa
and the septum membranaceum).
Impulse generating and Conducting System Pathway - SA node (pacemaker) to AV node via Bundle
of His, then out to left and right anterior and posterior bundle branches, which goes to Purkinje
fibers and Purkinje cells. (You won’t be able to discern any of the features of these areas in your slides,
except maybe some vacuous cells (empty-looking) that might be part of this system (Purkinje) so here’s a
diagram and a picture of Purkinje cells.
GENERAL STRUCTURE OF BLOOD VESSELS
Gartner and Hiatt, Page 154
Blood vessels have 3 layers - Tunicae Intima, Media, Adventitia. When large veins are compared with
large arteries, major differences are seen in the thickness of, and cell types in, these layers. The largest
artery is the elastic artery, such as the aorta and its major branches, so called because the tunica media
(middle layer) contains copious amounts of elastic fibers (made of Elastin, black squiggles in the
following image) as well as connective tissue and smooth muscle.
Compare with the largest type of vein (e.g. Pulmonary vein), which does NOT have large amounts of
elastic fibers associated with it, just a few here or there to give it some flexibility, but it is supported
against total collapse by lots of smooth muscle. HOWEVER, and this is important, the elastic in the
artery is in the media, while the muscle in the vein is in the adventitia.
Compare the artery (lower vessel) with the vein (upper vessel) in this image:- Note the layers
Next in size comes the muscular artery
Note that the elastic fibers have disappeared for the most part and the media is comprised of mostly
smooth muscle cells.
Compare and contrast this to the next figure, a medium sized vein and note the thicknesses of the tunics
and less muscle in the vein
To summarize the vessels so far:
 Thickest layer (tunica) of the larger arteries is the media; the thickest layer of the larger veins is
the adventitia.
 Bigger arteries contain lots of elastic and a large amount of smooth muscle (but less than large
veins); the larger veins contain lots of smooth muscle and some (but not much) elastic fibers. Here
is a rough comparison of layer thickness, with the artery on the left, the vein on the right:-
Both the artery (left) and the vein have an internal elastic lamina, a structure very evident in big,
muscular arteries that have to accept large, pulsatile volumes of blood. Also present in elastic arteries,
but harder to distinguish as huge amounts of endogenous elastic fibers are present. (Easier to see in
smaller, though still large, arteries where major component is smooth muscle). Large veins are supported
against total collapse by smooth muscle in their tunica adventitia. Largest arteries are elastic arteries,
which then lose the elastic, retain an internal elastic lamina, and become smaller, but still large, muscular
arteries. The largest veins are large muscular veins that gradually lose their muscle component and
become smaller muscular veins. Both large arteries and large veins possess a Vasa Vasorum, which is
a network of small blood vessels in the tunica adventitia. Vasa vasorum means “Vessels of the vessel”,
and is found in the adventitia.
Fortunately, arteries and veins usually travel in close proximity to each other, (immediate comparison.
We are also lucky that a nerve or a lymphatic, and sometimes both, are also usually close by).
The three tunics thin out, the elastic disappears and the component smooth muscle becomes less.
As the veins become smaller they become less round, while arteries maintain a nice circular profile
(USUALLY, but not always).
Small muscular arteries have 5-10 concentric layers of smooth muscle in their tunica media (as seen
above), while veins have a couple of layers of smooth muscle cells in the tunica media and a large, wispy,
ill-defined adventitia. Compare the small muscular artery on the left with the small vein on the right. The
artery has a number of concentric layers of smooth muscle, The vein on the right still has a few concentric
layers of smooth muscle, and a much bigger lumen than the artery, which is usually the case, and when
more smooth muscle is lost, the veins tend to lose their roundness. Note the nerve in the bottom right
hand corner.
The final stages of the ‘big’ vessels (before capillaries) are venules and arterioles. Venules are thin
walled, with a very thin, low cuboidal to squamous endothelium, minimal tunica media (maybe a few
smooth muscle cells and a few wisps of connective tissue), and an ill-defined adventitia that blends with
the surrounding fibrous connective tissue. Arterioles still maintain a well-defined, round profile, with 2-5
concentric layers of smooth muscle cells, a low cuboidal to squamous endothelium and a sparse
adventitia.
Note:
1) The circular profile of the arteriole
2) The larger size of the lumen of the venule
3) The 2 layers of smooth muscle cells surrounding the arteriole
The open arrowhead is pointing to a pericyte, specialized mesenchymal cells found in post-capillary
venules and constituting the tunica media of capillaries. They have contractile abilities and facilitate the
passage of blood.
Now on to the tiny vessel, the capillary. There are 3 different types of these and, if you apply logical
thinking, you should be able to decide in which tissue they are found.
 Continuous (no holes in the endothelium)
 Fenestrated (has holes in the endothelium)
 Sinusoidal or sinusoids (large with big holes in the endothelium)
Logically, continuous in tissue where contamination not wanted, from blood reaching tissue or vice
versa;
Fenestrated where we want select products getting into the blood; and
Sinusoids in tissue where large things, even cells such as macrophages, pass in and out of the blood. Can
you, even now, think of where you would find these particular types of capillaries?
 Continuous capillary - characterized by absence of fenestrae (holes). Common in muscle,
connective tissue, exocrine glands and nervous tissue. Normal to see pinocytotic vesicles on both
surfaces of endothelial cells, responsible for transport of macromolecules.
 Fenestrated with diaphragms are found in tissue where specific products are required to pass
from synthesizing organs into the blood. The thyroid is one example where transport of thyroid
hormones occurs. Capillaries are nestled between follicles.
 Fenestrated without diaphragms, such as are found in the glomeruli of kidneys, allow passage
of larger products i.e. proteins, but there is some restriction.
 Sinusoids are very large capillaries (diameter >30 m), and have no diaphragms, numerous
fenestrae (openings) and a discontinuous basal lamina because of the interruptions. Large
compounds such as proteins pass through and even cells can move in and out of these vessels.
Commonly found in liver (shown in the following example), bone marrow and the spleen.
Capillaries are formed from tiny branches of the arterioles, the metarterioles, which are surrounded by a
discontinuous layer of smooth muscle, and constriction of these metarterioles helps to regulate the blood
flow in tissues.
In some tissues there are arteriovenous anastomoses (connections) that allow blood to empty
directly into venules from the arterial side, so that when vessels in these anastomoses contract, all the
blood passes directly and rapidly straight through. The number of anastomoses and richness of capillary
network is directly related to the metabolic rate of the tissue or organ. For example, the liver, a highly
synthetic and detoxification organ, has a vast capillary network.
To summarize: Size of vessels diminishes through arterial circulation, and then increases back up
through venous return.
Elastic arteries – muscular arteries (with internal elastic lamina) – muscular arteries – arterioles –
metarterioles – capillaries – venules- large venules – small muscular vein – large muscular vein. This is
the loop from the heart to the periphery and back again.
Finally, THE LYMPHATICS. These originate in the tissues as thin closed-end vessels, with a single
cell thick endothelium and an incomplete basal lamina. Often confused with venules, and so the best way
to identify them in the light microscope is to look at the cells inside the lymphatics which will be
lymphocytes (as opposed to RBC’s). There is another point of confusion too; BOTH VEINS AND
LYMPHATICS CONTAIN VALVES!!!!!!!!!!!!!!!!!!
However, the lymphatic is thin walled, held open against collapse by a few smooth muscle cells and a
meager number of elastic fibers, while the larger vein has a substantial tunica adventitia and
accompanying allotment of smooth muscle cells i.e. the vein is much more substantial structure.
LYMPHATIC
VEIN
The thin lymphatics converge and form two large trunks, the thoracic duct and the right lymphatic duct,
that empty into the venous system. The lymph contained within the lymphatics is cleaned via passage
through the lymph nodes en route. Valves are more numerous in the lymphatic system and contraction of
surrounding skeletal muscle bundles aid in ensuring the unidirectional flow of the lymphatic fluid.
Lacteal – so called as it fills with fat. Found in center of intestinal villus. Part of the lymphatic system
CARDIOVASCULAR SYSTEM LABORATORY
Heart (slide #37)
Place slide on a sheet of white paper and compare to the picture that is shown on the first page of
this chapter. Note the thicker ventricular wall, and the thin atrial wall. You might be lucky and have a
valve hanging around.
Put the slide on your microscope and look at: Outer adventitia, the epicardial fat, small vessels (look for venules, arterioles, etc) and connective
tissue.
 Middle media – myocardium – made of thick cardiac muscle. Look for striations, intercalated
discs and capillaries (what sort will these be?)
 Inner intima – thin. Squamous to low cuboidal epithelium with an amorphous connective tissue
sub-endocardium. This continues around the inside of the ventricle and over the flap of the valve,
this one being the tricuspid. There is a core connective tissue here that ends at the top of the left
ventricle and is called the annulus fibrosus. So there. The endocardium continues into the atrial
chamber where it thickens.
 Between the ventricle and the atrium is a large fat pad that contains many smaller vessels, nice
examples of unilocular fat and nerves.
Elastic Artery and large vein (slides 24 and 25)
24 is H & E stain, 25 is Verhoff-vanGiesen stain. These are samples of the aorta showing the thin
endothelial intima, and the concentric smooth muscle separated by wavy layers of the elastic lamellae
in the media. Try moving the condenser up and down (slightly), and you’ll see the pink bands come in
and out of focus and they should be easier to visualize (if only in your minds eye). The adventitia is
lighter pink and contains lots of blood vessels, which are the vasa vasorum.
The collapsed profile on these slides is the vena cava and is a large vein. Note the thin tunica
intima, and the thinner (when compared to the aorta) tunica media with some smooth muscle
that shows up better as grayish-green in #25. The tunica adventitia is thick and contains
prominent longitudinal bundles of smooth muscle. There is also a nice vasa vasorum. (NOTE: It
states longitudinal bundles in the adventitia).
Muscular Artery (slide #10, 43, 6). Also look here for arterioles, medium sized veins and venules,
particularly in #32. #10 is the sole of the foot. Look deep in to the dermis, past all the purple/red stuff and
nearer the fat. You’ll know you have a muscular artery because you’ll see a prominent
what?…………………………Internal elastic lamina is what. This is probably wavy due to shrinkage
artifact. Move the condenser up and down as before to try and make it stand out. Have a look around as
veins and arteries often travel together, and if there is a large vein close by then you can compare, and in
this tissue, where there is a great deal of pressure and not that much in the way of support to keep the vein
open, the vein will look squashed. Have a quick look in #43, which is a section of submandibular gland.
What type of capillaries would you likely find here? Slide #6 is small intestine, and here you will see
many lovely profiles of arteries, veins, arterioles, venules and capillaries. As mentioned above, veins and
arteries often travel together and this is a great place to contrast and compare. Why is the small intestine
so vascularized? Go on, have a guess.
Capillaries. All the above slides will have examples of capillaries of one sort or another. As discussed,
these vessels bridge the arterial and venous systems. Easiest way to determine if a vessel is a capillary is
to look for a hole, surrounded by one cell, containing a single red blood cell (RBC). As it is only one cell
wide the profile will look like a “class ring”.
Continuous: Look in the myocardium in slide 37 and in tongue (38), just below the epidermis.
Fenestrated (with diaphragms): #56. In the thyroid we can’t see the fenestrae at this magnification but the
capillaries are just outside the follicles.
Fenestrated (without diaphragms): #7. In the kidney; Look at the capillary loops in the cortical regions.
OK, go to the outside, move inwards, come to something that looks like a bundle of spaghetti, and that’s
where you want to be.
Sinusoidal: #1. Liver; Sinusoids contain RBC’s and macrophages called Kuppfer cells; they run between
hepatocytes that form hepatic plates; try to pick out flattened endothelial cell nuclei of the sinusoidal
cells.
Lymphatics: often hard to identify as they are thin and flattened and look like venules (what do
lymphatics and veins have in common on a structural level?): But they won’t contain any RBCs, but will
contain lymphocytes, leukocytes and eosinophilic lymph; look for vessels in the above slides that are full
of BB shot; to see a large lymphatic, use slide #6 which is a section showing intestinal villi. In the center
of each villus is a gap that is a lymphatic called a lacteal. It has this name as after a fatty meal it will
swell.
Last one: Thoracic Duct: lymphatic system does not circulate; a blind-ended system that arises in
interstitium and empties into thoracic duct; smallest structure on slide #25 is main lymphatic drain back
into systemic circulation. Note prominent tunica media and tunica adventitia containing vasa vasorum.
CHECK THE FRONT OF THE LAB TO SEE DEMO SLIDE OF PURKINJE FIBERS IS ON SHOW