Cardiovascular System From Veterinary Histology PR

7/24/2019 Cardiovascular System From Veterinary Histology PR

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Cardiovascular System: Myocardium and Heart

The cardiovascular system can be anatomically subdivided into the heartand the

blood vessels, and the latter category is further subdivided into different vessel

types.

Arteries are blood vessels which conduct blood away from the heart veins are blood

vessels which conduct blood towards the heart. These two ma!or categories are

bro"en down further, and there are subcategories of arteries and veins based on

si#e, construction of the vessel wall, etc. However, the direction of blood flow is the

criterion which separates arteries from veins, and which defines a vessel as being in

one or the other ma!or class. $ou may have heard the hoary old chestnut that

%Arteries carry o&ygenated blood, and veins carry deo&ygenated blood.% TH'S 'S

()T T*+. Some arteries carry deo&ygenated, and some veins carry o&ygenated

blood. The direction of flow is the only criterion for classification.

.

The Heart

-egin with slide /, and start by

holding it up to the light. This is

an entire heart from some small

animal 0most li"ely a rat1 which

has been sectioned from the ape&

to the cranial end of the atria.

After orienting yourself, place it
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on the microscope under

low power. $ou will easily

be able to ma"e out at

least two, and probably

three of the chambers if

you have a favorablesection, you may be able

to see all four. The atria

and ventricles are

separated from each

other. *unning from the

ape& to the cranial end

you will see the septum

dividing the right and left

sides. There should be at least one of the heart valves in your section, connecting the

atria and ventricles. &ternal to the heart proper, you should have a cross section of

one of the great vessels 0the pulmonary artery1 and some adipose connective tissueas well.

Turn now to slide . This is the heart of yet another unfortunate rodent, sectioned

transversely, below the level of the atrioventricular !unctions. This slide graphically

illustrates the thic"ness of the wall of the left ventricle and ventricular septum. The

cross section at right shows a %slug% of blood fro#en in its passage from the left

atrium to the left ventricle, and the left A23 valve is open. )n this slide you should

be able to ma"e out most of the features identified in the previous slide, e&cept those

which are out of the plane of the section.

4hile both atria and ventricles are composed of the same type of speciali#ed musclecells, those of the atria tend to be less numerous, thinner, and more elongated than

those of the ventricles. The ventricles of the heart are more stoutly constructed than

the atria because they have more wor" to do. All the atria do is pass blood to the

ventricles below: the right ventricle sends blood to the lungs against the resistance of

the pulmonary circulation, and the the left ventricle has to deal with the entire

system circulation5s resistance to flow. 4hile the resistance of the lung capillaries to

blood flow is considerable, that of the entire peripheral circulation is much higher.

Myocardium

The heart is a large mass of muscle. Cardiac muscle is a variant form of striated muscle,with distinct differences from the skeletal form; and some unique structures that make it

work properly, day in and day out, until death. The term "myocardium," (from Greek,

myos muscle ! kardio heart is specifically the mass of muscle that comprises the#ulk of the organ.

)n slides / and at low power, you can easily observe the pattern of the muscle

cells of the myocardium. Myocardial cells 0cardiac myocytes1 are much smaller than


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the myofibers of s"eletal muscle, and they don5t form long cylindrical structures the

way myofibers do.4hile s"eletal myofibers can in some cases be metersin length,

it5s a rare cardiac myocyte that e&ceeds /66 7m in length, and perhaps 86296 7m

wide. The shape of the myocyte is pretty irregular, with stumpy pro!ections coming

off of it where it contacts other cells. Myocytes also differ from s"eletal myofibers in

that cardiac myocytes are mononuclearand not syncytial.)ne nucleus per myocyteis the rule, not hundreds as in a s"eletal myofiber, and that one nucleus is centrally

located.

Here5s scanning

electron

microscope

image of a

single cardiac

myocyte, which

has been

mechanicallyseparated from

the mass of the

myocardium.

This beautiful

image 0at about

9666&1 shows

the irregular

shape of the

myocyte uite

well, and the

points at whichthis cell is in contact with others via the intercalated discs are indicated by arrows.

Also visible as wispy strands of material on the surface are fine collagen fibrils of

the intercellular collagen networ". ;ust as in s"eletal muscle, the 'C( transmits

force throughout the mass, and the hierarchical arrangement of endomysium2

perimysium2epimysium applies.

(ote that the surface of the myocyte appears to be ridged or grooved. These ridges

are the sites of


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but remain connected to each other: one is more heavily stained than its partner, so

that the boundary where the two are !oined 0an intercalated dis", see below1 is clear.

As is obvious in this image, each cell has a single centrally located nucleus. +nli"e

s"eletal muscle, neither myocardium nor the cells of which it5s composed are

syncytial. The cells are physically isolated from each other, a fact which was in

dispute before the electron microscope settled the matter once and for all about/=96. 0(evertheless, than"s to the >aw )f ?ersistence )f rroneous 'nformation,

you5ll stillfind the statement in te&ts that %the heart is functionally syncytial,%

something that sets my teeth on edge.1 Though there is undoubtedly constant

communication and coordination between heart muscle cells, they are structurally

distinct entities, and the term %syncytium% is inappropriate in this conte&t.

Than"s to the difference of cellular architecture, the histological appearance of

myocardium is very different from that of s"eletal muscle.$ather than forming neat#undles of parallel fi#ers with well%defined striations, myocardium forms an

anastomosing network with a sort of "spongy" appearance. The contacts #etween

myocytes are such that a #ranching network with #lood vessels in the spaces #etweenthem is the result.

$ou will see some areas where the myocardial cells are oriented end2to2end in long

rows parallel to the plane of the section 0i.e., they are cut longitudinally1 and others

where they run at right angles 0and thus are cut transversely1. There will also be

areas where the orientation with respect to the plane of the section is less well

defined these are obliue sections. The myocardial muscle bundles are oriented in

such a way as to ma"e most efficient use of the force of contraction.

These two images give a pretty good idea of the appearance of myocardium in the

light microscope. The one at left is stained with toluidine blue at about @66& the one

at right with H at roughly B66&.

Myocardium has a %stringy% loo" compared to s"eletal muscle. 4hile s"eletal

muscle cells are very large and lie ne&t to each other in parallel bundles, the smaller

cardiac muscle cells are butted together at their ends, and irregularly shaped,


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numerous blood vessels between them. The effect is to create an anastomosing

networ" of fibers rather than a solid phalan& of muscle fascicles. The single nucleus

of each myocardial cell is uite clearly visible in these images, especially on the

right.

This drawing will clarify the architecture of the tissue. (ote the branching, and

compare it to the actual specimens shown in the longitudinal view. At lower right in

the drawing, the cells are shown in cross section. Compare the drawing to the actual

specimen shown at right, and to the sections of smooth muscle in &ercise /6: at first

glance this field could be confused with smooth muscle, but they can be told apart

fairly easily. An important difference between the appearance of this tissue

compared to smooth muscle is that in cardiac muscle almost all the cell profiles are

appro&imately the same si#e, which isn5t true of smooth muscle. urthermore, most

of the cell profiles will have a nucleus inside. Cardiac myocytes are so short and the


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nucleus ta"es up such a proportionally greater percentage of their length 0compared

to smooth muscle cells1 that the chances of intersecting a nucleus with the plane of

the section is pretty good. The individual cells are clearly defined by their CT

envelope 0the endomysium1 and most of them show a nuclear profile.

The smaller, denser nuclei outside the cells are fibroblasts that ma"e and maintainthe intercellular collagen networ". There5s a blood vessel crossing the field about

one uarter of the way down from the top edge. Cardiac muscle is even better

served by the circulatory system than s"eletal muscle is. +nli"e s"eletal muscle,

cardiac muscle can5t incur an %o&ygen debt,% because it doesn5t have the lactic acid

pathway to generate AT? when o&ygen levels are low. Conseuently it5s uite prone

to ano&ia, and the capillary beds are e&tensive to minimi#e the possibility of it

happening.

'ntercalated Dis"s

The most prominent feature of myocardium, and the one that5s absolutelydiagnostic for it, is the intercalated disk. The 'D is a speciali#ed cell2to2cell

adhesionEcommunications site. 't demarcates the beginning of one myocyte and the

end of the ne&t, and information is pass across it from cell to cell. These structures

are found only in cardiac muscle.

The 'D5s are vital to normal myocyte function and understanding how they are put

together is important. 'n these special areas there5s a considerable degree of

interdigitation of myocyte plasma membrane, but there is noactual fusion of

cytoplasm. The speciali#ed apparatus of the 'D allows myocytes to act as ifthey

were a true syncytium there is in fact no actual cytoplasmic continuity these are

distinct and individual cells.

The true nature of the 'D wasn5t fully understood until the transmission electron

microscope was available for their study. This instrument revealed that 'D5s are

physically held together by large numbers of desmosomes, with gap !unctions

between the myocytes forming a region that permits electrical communication in the

form of ion flu&es. This flu& maintains the coordination of the waves of contraction

between one cell and the ne&t.


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'ntercalated discs are readily seen on slide 9@9, in regions where the myocardium is

cut longitudinally 0they won5t be visible in cross sections1. Since the 'D5s are located

at the ends of the cell, and since that5s where the first and last sarcomeres of any

given myofibril are, you can thin" of the 'D as a sort of thic"ened %terminal


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&amine the lining of the chambers on slide /. The entire cardiovascular system,

including all chambers of the heart and all blood vessels, is lined with a simple

suamous epithelial covering.This lining is among the most metabolically active

tissues in the body. 'n blood vessels, this is 0by convention1 called %endothelium,%

but in the heart the special term endocardiumis used instead. 0There isn5t a nic"el5s

worth of difference between them, but the terminology is so entrenched it couldnever be eradicated.1 'n any

event, %endocardium% is an apt

name because it5s literally

%inside the heart.% The

endocardium covers the valve

cusps, too, as you can easily

verify on this slide. 'f you

traverse the thic"ness of the

myocardium and e&amine the

outersurface of the wall, you

will see a similar 0though lesswell defined1 layer of simple

suamous epithelium. Again, by

convention, this has a special

name it5s the epicardium, which

translates as %upon the heart.%

Anatomically, this is the visceral

layer of the pericardial sac.

The epicardium is the inner portion of the

CT envelope that surrounds the heart. 'tcorresponds to the peritoneum of the

abdominal cavity in origin 0from the

mesoderm lining the embryonic coelom1

and function 0to allow the heart to move

freely in its cavity without adhesion to

surrounding structures1. Here you see it as

a thin serous membrane overlying the

outer surface of the organ. A stain for

connective tissue would reveal a very

delicate sub2epicardial CT layer, mainly

reticular fibers.

?ur"in!e ibers

Slide 9F@ is another chun" of myocardium. Most of the features of myocardium are

visible, but this section also shows ?ur"in!e fibers0;ohannes vangelista von

?ur"in!e, /GFG2/F=, a C#ech anatomist, physiologist and microscopist1. These fibers

are part of the conducting system of the heart.
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The conducting system transmits the signal to contract from its origin in the

%pacema"er% to the rest of the myocardium. The ?ur"in!e fibers are not neurons.

They5re speciali#ed muscle cells, and they loo" li"e it. They can be distinguished

from the myocardium by their si#e 0they5re much larger than regular myocytes1 and

by their

staining0they tend

to be more

lightly

stained

than true

myocardium1. 4ith the ?AS stain they5re strongly positive, since they contain large

amounts of glycogen.

?ur"in!e fibers are not contractile, at least not to any significant e&tent. They

contribute nothing to the force generation of myocardium. The cells of the ?ur"in!e

fibers are organi#ed into several tracts that lead away from the atrioventricularnode and form nerve2li"e structures. There5s a bundle along both sides of the

septum, one distributing to the right ventricle and one to the left ventricle. The

?ur"in!e fibers are larger and less well stained than the ordinary contractile

myocardium because of the relatively protein2poor cytoplasm, which doesn5t ta"e up

the eosin stain very well. Hence the fibers loo" pale and washed out in comparison

to the protein2pac"ed myocytes. ?ur"in!e fibers do have some of the other features

of cardiac myocytes, including a centrally2located single nucleus, and even

intercalated dis"s.

-ut despite their strictly conductive function, they are muscle cell derivatives, so

you5re uite li"ely to see striations and other indications of %contractility% if youe&amine them at higher power. There should be two areas of ?ur"in!e fibers in your

slide, one at each edge of the section. 'n this slide also note the ramifications of the

muscle, and the intercalated discs.

The ?ur"in!e fibers ramify into the mass of ventricular myocardium, and from their

termini the signal is spread through the 'D system. *ecall that all muscle tissue is

%e&citable% and this property can be used to carry information. That5s what5s


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happened here. 4hat is a secondary function in contractile myocytes has been

transmuted into the main role of these not2uite2muscle cells. The fibers run

through the sub2endothelial connective tissue they can5t generate any force because

internally they have only a few disorgani#ed actin and myosin fibrils, nothing li"e

the very regular arrays of sarcomeres in the contractile cells.

Slide =, shown at left, is

interesting because it contains the

atrioventricular node, the

beginning of the ?ur"in!e fiber

conduction apparatus.

This image is ta"en with the atrial

region at the top of the field. At

the upper left is the base of one of

the great vessels leading from the

heart 0probably the aorta1 andbelow that a triangular2shaped

region, the trigona fibrosa. The

trigona fibrosa is part of the so2

called %cardiac s"eleton,% the

dense fibrous CT to which the

intercellular collagen networ" is

ultimately anchored. 'f you loo"

at this structure at high

magnification it5s very similar in

appearance to cartilage. The

trigona fibrosa lies between theatrial and ventricular parts of the

organ and the fibrous rings of the

great vessels are attached to it so

is the top level of the 'C(. 4hen

force is e&erted by contraction of

myocardium, this is the %#ero

point% against which the force is e&erted. 4hen contraction occurs, the different

directions of muscle tracts impart a compressive, twisting motion to the heart

grossly 0you will be able to see this in surgery1. The motion has been compared to

%wringing out% a wet cloth, and it serves the same function: e&pulsion of the

ma&imum amount of blood per stro"e. Contraction of the myocardial cells leads to

reduction in volume of the heart chambers, and conseuently to e!ection of all 0or

almost all1 of the blood therein. 'mpairment of this action results in decreased

efficiency of pumping and eventual failure.

Alongside the trigona, and lyingas its name impliesbetween the atrium and the

ventricle, is the A23 node. 't loo"s li"e a bit of atrial myocardium, splon"ed up

against the denser and more massive ventricular type. 0This slide also gives you a


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good comparison of the two types of myocardium side by side1. This isn5t the

%pacema"er,% 0that5s the sino2atrial or S2A node1 but it receives signals from it and

transmits them via the conducting fibers to the mass of ventricular myocardium.

Histologically it loo"s li"e the rest of the atrial myocardium, and in the absence of

the landmar"s used here, would be very difficult to identify in a section.

3alves of the Heart

Slide /6@ is of a valve cusp. >oo" at it at low magnification and at high

magnification and e&amine its construction. The cusp of a valve has a core of CT,

much of it composed of elastic fibers. There

may be some smooth muscle wor"ed into it as well. The outer surfaces, e&posed to

the blood flow, are covered with the epithelium of the endocardial lining, li"e the

rest of the system. (ote that there5s an e&cursion of myocardium into the base of the

valve cusp this terminates before the end of the cusp. 't5s continuous with the

myocardium of the ventricle.

-lood Supply

' hope it hasn5t escaped your notice that the entire mass of the myocardium is shot

through with blood vessels of varying si#es and shapes, and that there5s an e&tensive

degree of vasculari#ation. 'n fact, the heart pumps blood to itself before it sends any

to other parts of the body. The first branches off the aorta are the coronary arteries

of the myocardial circulation. After you have completed the section on blood vessels

you may want to come bac" and classify some of these.

Arteries

>et us now leave the pumping station and e&amine some of the pipes: the arteries

and veins. *emember the definition: arteries pump blood away from the heart

veins carry blood towards the heart. (o other criteria for classification of a vessel as


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one or the other e&ists.Capillaries are those small vessels within the tissues from

which o&ygen transfer ta"es place.

Arteries may be subclassified by type. Conducting or elastic arteries are large ones,

with very strong and relatively elastic walls, whose function is to %conduct% the bul"

of the blood to regions of the body where it5s to be distributed. &amples include theaorta, subclavian, and pulmonary arteries. )nce the blood has reached the region of

distributionsay, the limbsit will be handled by smaller 0but still fairly large1

distributing or muscular arteries, which send it to sub2regions.As the distribution

area gets more and more limited the arteries become smaller. 'n very local areas you

will see small arterioles, essentially mini2arteries with a wall considerably less

muscular than the larger ones %upstream.%

lastic or Conducting Arteries

lastic arteries are constructed li"e fire hoses. *ightly so, because they have the

same function: to carry a stream of liuid under high pressure. Hence they5redesigned to minimi#e internal friction and flow resistance and to ma&imi#e the

strength of the wall.

The lumen is lined with a thin suamous epithelial layer 0the tunica intima1 which

may be li"ened to the rubber bore of the hose li"e the rubber, it offers a smooth and

unimpeded passage for the flow of blood. The tunica mediais a region of elastic and

collagen fibers. lastic arteries 0the aorta most of all1 must withstand an enormous

head of pressure to pump against the peripheral systemic resistance: conseuently

the wall is heavily reinforced to prevent bursting, !ust as the wall of a fire hose has

reinforcing cords in it. The elastic fibers allow some stretching and %springiness% in

response to the pressure, and the collagen fibers limit the degree of stretchpermitted. 0'n some disease or deficient nutritional statesfor e&ample lathyrism, a

copper deficiency caused by certain plantsthe wall may be wea"ened, resulting in

an aneurysmwhich may lea", or burst with fatal effects.

To complete the analogy, the collagenous tunica adventitiaof the aorta is the fabric

covering on the outside of the hose. ;ust as the fireman needs a firm grip to control

the hose, so must elastic arteries be anchored down to the surrounding structures, to

prevent them from moving around as pressure varies internally. The tunica

adventitia of conducting arteries is scanty, and collagenous in nature.

Slide 86 is a fine e&ample of an elastic or conducting artery. 't is, in fact, a section ofthe aorta. The section on this slide is stained with the 3erhoeff5s stainfor elastic

fibers. 't renders these fibers blac", and you will see that the wall of this vessel is

shot through with elastic fibers. The spaces between the elastic fibers are mostly

occupied with collagen, and some small amount of smooth muscle 0see below1. The

amount of elastic fiber infiltration is so great that no internal or e&ternal elastic

laminae can be identified.


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This is a section of the aorta, a nice e&ample of an elastic artery. The elastic fibers

normally aren5t easily seen in an H preparation li"e this one, but in this case, the

reinforcement is so heavy that the concentric layers of elastic CT show up as distinct

iridescent rings, some of which are mar"ed by arrows. There5s a large clot of blood

in the field. The bore of this vesselli"e the rest of the cardiovascular systemis

lined with a simple suamous epithelium.

The innermost of the numerous elastic layers in this artery has its own name: the

internal elastic lamina.'t5s the one right up against the tunica intima. The internal

elastic lamina is much more easily seen in muscular arteries 0see below1 than in the

elastic ones, though. 'n an elastic artery li"e this one it gets lost in the %bac"ground%

of do#ens of similar layers.

As is true of most elastic arteries, the tunica adventitia on this one is a relatively

small contributor to the wall5s thic"ness.

There5s more than elastic fibers in the wall of this type of artery. -etween the elasticlayers there is a considerable amount of collagen and some smooth muscle,

permitting the artery to e&pand under pressure and recoil to original diameter when

the pressure drops again. Collagenous components in the wall prevent over2

e&pansion and resist bursting of the vessel.


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+sing stains specific for elastic fibers reveals how e&tensive the reinforcement of the

wall can be. Here are two e&amples, again from the aorta. The one at left has been

stained with a combination of the 3erhoeff and the 3an Iieson stain at about B66&,

the e&tent of the elastic component 0in blac"1 and the collagenous reinforcementbetween the elastic fibers is easily visible. This field is from the outer edge of the

tunica media: the almost2completely red area at the bottom is the tunica adventitia.

At right, the 3erhoeff stain has been used and at about @66& it clearly show elastic

fibers but not the collagen or smooth muscle, which is unstained by this method.

3ery large elastic arteries have their own internal blood circulation system and

nervous supply. They have to: the fibroblasts and other cells that "eep the wall in


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good shape are so far from the blood supply that diffusion won5t serve their needs,

and the density of the wall and its fibrillar components also impede diffusion. The

smooth muscle in the wall is innervated so that the C(S can control blood pressure

and initiate contractions when needed. The vasa vasorum0and in the case of nerve

fibers, the nervi vasorum1 i.e., the %vessels of the vessels% and the %nerves of the

vessels% are a constant feature of big vessels.

Muscular Arteries

)nce you get

the blood out

to the ma!or

regions of

the body,

there5s a

transition in

the structureof the

arterial wall.

The

proportion

of elastic

fibers

decreases,

and the

proportion

of smooth

muscleincreases.

Some elastic

fibers and

collagen

fibers will

always be

present, but

eventually

the great

bul" of the tunica media will be smooth muscle, and at that point we5re dealing with

muscularor distributingarteries, whose function is to %distribute% blood supply to

their regions of responsibility, such as a limb. The artery on slide /68, at right, is a

dandy e&ample of what a muscular or distributing artery should loo" li"e. This is

the femoral artery the femoral vein 0see below1 and the femoral nerve are there as

well. There may be a branch point off this artery on your slide.

(ote that the wall of the artery is mostly tunica media, and that this tunic is almost

entirely smooth muscle.lastic CT is present, to be sure, but not nearly to the e&tent


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that it is in the elastic arteries. There is also a noticeable internal elastic lamina,

stained bright pin", and a less well defined e&ternal elastic lamina, though at the low

magnification of this image it5s hard to ma"e out. The endothelium of the tunica

intima is easily visible. The tunica adventitia grades off into the surrounding

connective tissue, but is fairly sharply defined. The internal elastic lamina is !ust

barely visible in this image. 't5s the undulating pin" line immediately below thelining endothelium. 't mar"s the innermost limit of the muscular tunica media. The

e&ternal elastic lamina is less regular and can5t be made out at all. (ote the

prominent collagenous tunica adventitia here. 'n muscular arteries the tunica

adventitia is often most of the wall5s overall thic"ness. The e&ternal CT investment

anchors this artery to the surrounding CT.

The internal elastic lamina is most easily seen in smaller muscular arteries. 'n theseit usually stands out as a bright pin" undulating band !ust below the lining epithelial

cells and their supporting CT. 'n life, the artery is always under some pressure, but

when death occurs the tonus of the wall causes a partial collapse, so that the

undulation is something of an artifact.
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'n the e&ample at left, the '> is indicated this e&ample also clearly shows the

separation of the lining epithelium from the '> by the sub2endothelial CT. A few

layers of smooth muscle constitute the tunica media in this small artery.

The smooth

muscle of the wallof distributing

arteries ma"es

them very

e&tensible, and

also provides for a

counter force to be

e&erted against

the pressure of

filling. As the

vessel e&pands the

smooth musclecells are stretched:

in reaction to this

they begin to

contract. The

pea" of their

contraction comes

at about the point where systole ends and diastole begins thus the contraction of the

arterial walls dampens out the pulsations of the flow to provide a more or less steady

supply of blood at normal pressure into the capillary beds. (ervous input can also

control this to some e&tent, independent of the mechanical force of stretching. As

distance from the heart increases, the force reuired to dampen the oscillations isless, and smaller arteries can handle it the interval between pea"s also lengthens

and there is a much more uniform flow rate.

't5s

useful to

put the

two

ma!or

arterial

types

side by

side,

stained

to reveal

the wall


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components. )n slide 86, this has been done using the 3erhoeff stain. A small

muscular artery is near the large elastic one, to ma"e the point that as the artery

si#e decreases, the proportion of elastic fibers in the wall decreases as well. The wall

of the small artery has almost no elastic tissue in it, but the the internal elastic

lamina, however, is very clearly defined against the bac"ground of the muscular

tunica media.

't5s possible to combine several staining methods to show allthe ma!or components

of the wall, including the smooth muscle. 'n the image below, the 3erhoeff and

Masson5s stains have been combined. lastic tissue is stained blac" smooth muscle

is red and collagen is green. The tunica media 0TM1 is almost entirely muscle, with

a few minor strea"s of green collagen in it. The inner elastic lamina 0'>1 stands out

prominently and the outer elastic lamina 0)>1 demarcating the end of the tunica

media is also easily visible. The tunica adventitia 0TA1 is a mi&ture of green collagen

fibers and blac" elastic fibers interwoven with each other to provide strength and

resilience.

Slide F demonstrates the brachial artery and vein. The artery has the typical

structure of a distributing artery, and it has a very nice tunica intima and tunica

media. The vein shows the typical structure of tunica intima, scanty tunica media,

and a thic" CT tunica adventitia. There are also valves present.

Arteries 'n >ongitudinal Section

Since arteries are pretty common in almost any microscope slide you5ll loo" at, it5s

important to be able to recogni#e them when they aren5t cut in cross section, as all

the previous e&amples have been.


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Here are two e&amples from tissue sections. The left section is from slide 8B the

right from slide /BF. Since the smooth muscle in the wall of an artery is oriented

with the long a&is of the smooth muscle cells aroundthe long a&is of the vessel

proper, in cross sections, the muscle cells are cut in longitudinal view, but in long

sections of the vessels, as here, the cells are cut in crosssection. Compare these

profiles 0both at about B66&1 to the sections of smooth muscle in &ercise /6, andyou5ll be able to ma"e out the boundaries of the smooth muscle easily.

Aneurysms

Sometimes the wall of an arteryespecially a big one li"e the aorta, which is

sub!ected to all or nearly all the pressure the heart can generateis wea"ened by

disease, malnutrition, age, or some other affliction. +nder a good head of pressure,


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blood begins to dissect the layers of reinforcing material, separating them and

causing the side of the artery to bulge, e&actly as a garden hose does when its wall is

wea"ened. This bulge is an aneurysm. Chronic high blood pressure ma"es this more

li"ely, though an aneurysm can occur even when systemic blood pressure is lower

than normal. (eedless to say, an aneurysm that bursts will really spoil your plans

for the wee"end. 'f it5s a big one that blows out, such as the aorta, death will berapid.

3eins

3eins are

those

vessels

leading

blood bac"

towards

the heart.As a rule,

they have

much

thinner

walls than

arteries

do, though

in cross

sectional

area

they5reusually

larger

than the

corresponding artery, because they have to carry the same volume of blood at a

lower pressure. Since veins are on the post2capillary side of the circulatory loop,

operating at much lower pressures, there5s less need for burst resistance. Thin walls

are also important because much of the pressure that drives blood through veins is

generated notby the heart, but by contraction of the muscles in the region of the

vein. This %suishes% the blood bac" through the vein. -ecause they have low

pressures, some veins have venous valves in them to prevent bac" flow. This is

especially true of medium si#ed veins in the e&tremities, as they have to lift bloodagainst gravity.

0'f an animal is held totally immobile for a long period of time, the return of the

blood to the heart is diminished, because contraction of the muscles of the limbs no

longer pushes blood bac" to the heart. Since the heart can only put out what it ta"es

in, the net result of decreased venous return is decreased arterial output. At some

point the output declines to the point where the brain is no longer receiving


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sufficient blood, and the animal loses consciousness. This happens surprisingly

often. Soldiers on parade, when forced to remain at the position of %attention% for

long periods, will from time to time "eel over and uite literally %drop out% of the

ran"s as they

faint.1

3enous 3alves

3eins of a

certain si#e

usually have

valves to

prevent bac"

flow. 3eins of

this type are

usually found

in thee&tremities, the

valves allowing

blood to move

bac" towards

the heart with

less effort.

3enous

pressures are

so low, and the

propelling

force has tofight against

not only the resistance of the vessels but the relentless pull of gravity, that valves in

the vein allow the blood to be pushed up above and when the valve closes without

running bac" down when pressure slac"s off.

The venous valve has a great deal of structural similarity to the valves of the heart.

There5s a CT core with epithelium on both sides. Two valve flaps meet in the center

of the vessel. $ou can find such valves on slide F, and when you do, note especially

the direction of the valve: it5s designed to permit blood to flow in one direction only.

?ooling of blood occurs on the superior side of the valve flaps. 'n animals with long

lives and vertical postures, the continual stress that the weight of this blood imposes

sometimes causes outpouching of the vein. 'f there is a wea"ening of the wall on the

downstream side of the valve, the blood will push it out into a small bubble

analogous to the aneurysm in an artery. The result is what we call %varicose veins%

that many peopleespecially those who wor" on their feetdevelop in later life.

This image from slide F shows a fairly large vein, cut more or less in cross section.

The flaps on both sides of one of its valves are visible.


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Capillaries

Capillaries are small vessels, whose walls are thin enough to allow the diffusion of

nutrients, o&ygen and carbon dio&ide across them.They are %where the action is% in

gas and nutrient e&change: with a few e&ceptions, no cell of the body is very far

from one, because access to the blood is an absolute reuirement: cell death wouldresult from ano&ia or the loss of nutrient and waste transport.

Capillaries are by far the most numerous class of vessels, though they5re so small

they can only be appreciated in microscopic sections. There are two types: closed or

continuous capillaries, and fenestrated capillaries.

The closed type is found in locations where rapid %bul"% transfer of materials

between the blood and the tissue it served isn5t needed, such as in muscles. Closed

capillaries can move material in and out using a process of seuential endocytosisand e&ocytosis, as well as simple diffusion for small molecules.

enestrated capillaries, which have actual pores in their walls, are located wherever

immediate movement of materials is a functional necessity, such as in endocrine

organs and in the "idney. They don5t demonstrate the %transcytosis% activity of the

closed type, because bul" movement of ions, hormones, nutrients, etc. through the

pores is ample


24/28

Here5s a scanning

electron micrograph of

a capillary running

alongside muscle

fibers. This is a closed

capillary. This stri"ingimage shows the

appro&imate si#e of a

capillary very well:

they5re !ust about the

same si#e as the

erythrocytes that flow

through them, maybe

/6 7m in luminal

diameter. 'n this

picture you can see an

erythrocyte pee"ingout at the bro"en end,

and the anchoring

fibrils of connective

tissue that attach it to

the surrounding muscle mass 0*1.

Compare this to the diagram above: the wall is made of very thin suamous cells

which are sealed together at the edges by desmosomes in other words, there5s only a

tunica intima, and not much of that beyond the lining epithelium and the basement

membrane it rests on. 'n a section it5s normal to see parts of several of the mural

cells in the same plane, since there5s uite a bit of interdigitation between theirregular borders of ad!acent cells.

$ou may also see a second cell type, the pericyte. Technically this isn5t part of the

capillary wall. ?ericytes %embrace% the wall but never ma"e contact with the blood,

as the mural cells do they5re believed to have some sort of contractile function.

They5re so closely associated with the mural epithelium they share a basement

membrane with it.


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Despite

their small

si#e,

capillaries

are so

numerousthat

they5re

easy to

find. They

often will

have blood

cells in

them,

which also

ma"es

them easyto spot.

This one is

from

s"eletal

muscle on slide /B. S"eletal muscle has an abundant blood supply, and this capillary

is lying between two of the large muscle cells. A couple of erythrocytes are visible,

and the bore diameter of the capillary is !ust large enough to accommodate them

0double arrow1. A white blood cell 0probably a neutrophil1 is visible in the left end of

the field. As it happens there are no capillary cell nuclei nor pericyte nuclei visible,

they5re out of the plane of the section but the wall itself can be see. The cytoplasm

of the mural cells is very scanty, and hence the precise limits of the cell and the CT

around it are hard to ma"e out.

The capillary at right is

from white fat. 't5s cut in

cross section. The plane of

section has passed through

the nucleus of the mural

cell, and the lumen is filled

with the cytoplasm of an

erythrocyte.

All blood vessels, of

whatever type, are derived

from embryonic

mesoderm, the only one of

the three layers of the

embryo that has angiogenic

potential. 't shouldn5t


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therefore be too surprising to find that connective tissues, which are also of

mesodermal origin, are usually well supplied with blood vessels. That5s the case

here: white fat is a connective tissue, and the fibrous CT that separates each fat cell

0the clear spaces in this image1 from the others is a second type of CT. This

capillary5s function is to move the components of the fat stored in it into and out of

the fat cell, and to serve the needs of this tissue for nutrient and waste transport. 'nyoung animals, which haven5t had time to accumulate the %wear and tear% pigment

lipofuscin in their fat cells 0see &ercises @ and 8 for a discussion of lipofuscin1 the

presence of so many capillaries give the fat a pin"ish2white color. 'n albinos, animals

that lac" melanin, it5s the blood circulating in capillaries of the eye that5s the source

of its pin" color.

A fenestrated capillary has %holes%

in it the word comes fromfenestra,

the >atin for %window.% These holes

are actually pores, places where the

plasma membrane is perforated, toallow for the movement of bul"

materials from the capillary lumen

to the e&terior, and vice versa. This

type of capillary is found in places

where the rapid movement of

materials is vital to function, such as

in endocrine organs, where

movement of hormones into the

blood is carried out. enestrated

capillaries are also found in the

"idney, again a place where uic"transit between two compartments is

the design criterion.

't5s not possible to see the

fenestrations in a light microscope

preparation. To appreciate the

nature of these pores, an electron microscopic image is reuired, because they5re

below the level of the light microscope5s resolution. Such an image is provided here,

at about F6,666 diameters. This e&ample is from the "idney, but similar capillary

profiles could be found anywhere fast movement of large molecules is important to

normal function. The capillaries of the glomerulus 0the tuft of blood vessels that fills

the renal corpuscle1 use hydrostatic pressure from the arterial supply to filter blood

plasma through the pores in the formation of urine 0for details, see &ercise @81. As

this image ma"es clear, the fenestrations are actual openings in the capillary wall.

The arrows show the direction of flow of materials, driven by the higher pressure in

the capillary lumen than in -owman5s space. *emember, the imageshown here is

two2dimensional, but the cellisn5t. The %brea"s% indicated are really more or less


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circular perforations in the plasma membrane, and they have depth they lie on both

sides of the plane of the section.

Sinusoids

There5s onemore category

of blood vessels

to be dealt

with. These are

somewhat li"e

capillaries, and

might be

considered as a

sub2set.

Sinusoidsare a

form of large,irregular

capillary2type

vessel they

have the thin,

intima2only

wall

construction of

a capillary and

may be

fenestrated.

Sinusoids arefound where

slow flow,

intimate contact between blood and tissue, and rapid e&change of materials are

reuired. $ou5ll find them on slide /@8 in the liver, between plates of hepatic cells.

Sinusoids are

flattened and

irregular in shape,

as the image at

right ma"es clear.

This scanning M

picture is a plastic

cast of the

sinusoids in the

placenta of a goat,

another place

where slow flow

and efficient


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transfer of materials is important to normal function. 'n this method the blood

spaces are filled with plastic the plastic is allowed to harden, and the tissue digested

away to leave an accurate impression of the shape of the filled space.

Cardiovascular System From Veterinary Histology PR

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