CVS HISTOLOGY

Dr. Nabil Khouri

http://anatomy.kmu.edu.tw/BlockHis/Block3/slides/block4_24.html

The Heart Wall

Cardiac Muscle • Contract as a single unit

• Simultaneous contraction

due to depolarizing at the

same time

• Intercalated disk to speed

depolarization automaticity

M -myocardium;

E - endocardium;

En -endothelium;

S -ubendothelial

layer

Cardiac Muscle Longitudinal Section

• Cardiac muscle consists of muscle cells mononucleated with centrally placed nucleus. Nuclei are oval, rather pale and which is 10 - 15 µm wide.

• Cardiac muscle is innervated by the autonomic nervous system.

• Cardiac muscle exhibits cross-striations.

• Cardiac muscle is for these reasons also called involuntary striated muscle.

X40 Magnification

cell nucleus

Intercalated Discs

One cell

The Cardiac

Muscle Cells

Adherens Junction Desmosome Gap junction

Fascia adherens – major portion

of transverse component.

Anchoring sites for actin, and

connect to the closest sarcomere.

Macula adherens –

(desmosomes) transverse and

lateral components. Bind

individual myocytes to one

another. stop separation during

contraction by binding

intermediate filaments, joining

the cells together. Macula

adherens junctions are also

called desmosomes.

Gap junctions - lateral

component. Allow action

potentials to spread between

cardiac cells by passage of ions

between cells, producing

depolarization of the heart

muscle. Allows muscle to act as

syncytium.

– Cardiac cells are connected by intercalated discs

– Intercalated discs house desmosomes and gap junction.

• Desmosomes provide strength so that the cell do not get ripped

apart during contraction

• Gap junctions are made of the connexin proteins and form a pore

through which the cells can communicate.

Cardiac Muscle Tissue

Cardiac Muscle “Cross section”

X40 Magnification

• The fibrous skeleton of the heart consists of masses of dense connective tissue in the

endocardium which anchors the valves and surrounds the two atrioventricular canals,

maintaining their proper shape. Section through a leaflet of the left atrioventricular

valve (arrows) shows that valves are largely dense connective tissue (C) covered with a

thin layer of endothelium. The collagen-rich connective tissue of the valves is stained

pale green here and is continuous with the fibrous ring of connective tissue at the base of

the valves, which fills the endocardium (En) of this area between the atrium (A) and

ventricle (V). The chordae tendinae (CT), small strands of connective tissue which bind

distal parts of valve leaflets, can also be seen here. The interwoven nature of the cardiac

muscle fibers, with many small fascicles, in the myocardium (M) is also shown.

Purkinje fibers 40X

Are modified cardiac muscle cells.

Compared to ordinary cardiac muscle

thicker cells: Contain large amounts of

glycogen fewer myofibrils.

Blood Vessels histology

• Blood is carried in a closed system of vessels that begins

and ends at the heart

• The three major types of vessels are arteries, capillaries, and

veins

• Arteries carry blood away from the heart, veins carry blood

toward the heart

• Capillaries contact tissue cells and directly serve cellular

needs

General Structure of Blood Vessels

Structure of blood vessel (Tunics)

• Tunica interna (tunica intima)

– Endothelial layer that lines the lumen of all vessels

– In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present

• Tunica media

– Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system

– Controls vasoconstriction/vasodilation of vessels

• Tunica externa (tunica adventitia)

– Collagen fibers that protect and reinforce vessels

– Larger vessels contain vasa vasorum

General Histology Structure of Blood

Vessels

A Comparison of a Typical Artery and a

Typical Vein

Histological Structure of Blood

Vessels

• Thick-walled arteries near the heart; the aorta and its major branches

– Large lumen (2.5-1 cm diameter) allow low-resistance conduction of blood

and act as conduits

– Contain elastin in all three tunics

– Withstand and smooth out large blood pressure fluctuations

– Allow blood to flow fairly continuously through the body

Elastic (Conducting) Arteries

Large (Elastic) artery.

• Elastic Arteries are classified by:

• The tunica intimae consists of a lining of endothelial cells

that rest on a thin layer of connective tissue.

• The tunica media arranged as lamellae, interspersed with the

smooth muscle cells of the tunica media and collagen fibers

are found between the layers of elastic fibers

• There are no elastic lamellae in the adventitia, but elastic

fibers are present, though relatively few in number and can

not be observed by H&E stain.

• Brown adipose tissue is one of the two types of adipose

tissue. Its primary purpose is to generate body heat. In

contrast to white adipocytes (fat cells) which contain a

single, large fat vacuole, brown adipocytes contain several

smaller vacuoles and centrally located nuclei.

Elastic (Conducting) Arteries

Muscular arteries

• The tunica intimae consists of an endothelial lining and a small

amount of connective tissue.

• The muscular arteries are characterized by a layer of internal

elastic lamina separating the tunica intima from the tunica media.

• The artery has a thicker tunica media, a narrower lumen than the

similarly sized vein, and thickened elastic laminae that are not

present in the vein.

• Muscular arteries have more smooth muscle and less elastin in the

tunica media than elastic arteries.

• The less prominent and more variable external elastic lamina lies

between the tunica media and the adventitia.

• The tunica adventitia is composed of collagen fibers (pink), elastic

fibers (black) and vasa vasorum.

30

Muscular arteries Are called distributing

arteries

– Middle sized

.3mm-1cm

– Changes diameter

to differentially

regulate flow to

organs as needed

– Internal as well as

external elastic

lamina

– Most of what we

see as “arteries”

Tunica media larger in proportion to the

lumen, thus “muscular”

Muscular artery

This slide is stained with Verhoeff's stain to visualize the elastic fibers, and with eosin to

show the cellular structures.

36

Tunica media has only a few layers of

smooth muscle cells

Arterioles – Smallest: .3mm-

10um

– Only larger ones have all 3 layers

– Regulated 2 ways:

• Locally in the tissues

• Sympathetic control

– Systemic blood pressure can be regulated through them

– Deliver blood into capillaries

• Arterioles – smallest arteries; lead to capillary beds

– Control flow into capillary beds via vasodilation and

constriction

“muscular” middle sized artery

Endothelial

cell

Smallest ARTERIOLE

Smallest arteriole, in essence, is a capillary with smooth muscle

cells wrapped around it, with modifications to the endothelial

cells - less transport, more interaction with SMCs.

Smooth muscle

cell SMC/

VSMC Reticular fibers

Contraction regulates flow

by need Vasoconstriction

For fast flow &

non-stick, until

clotting is needed

Controls passage

through the wall

Helps control

blood flow

Mechanical support

Capillaries

Heart to arteries to capillaries to veins to heart

• Capillaries are smallest – 8-10um

– Just big enough for single file erythrocytes

– Composed of: single layer of endothelial cells surrounded by basement membrane

• Universal function – Oxygen and nutrient delivery to tissues

– CO2 and nitrogenous waste (protein break-down product) removal

• Some also have tissue specific functions

The Organization of a Capillary Bed

Capillary Beds

• A microcirculation of interwoven networks of

capillaries, consisting of:

– Vascular shunts – metarteriole–thoroughfare

channel connecting an arteriole directly with a

postcapillary venule

– True capillaries – 10 to 100 per capillary bed,

capillaries branch off the metarteriole and return

to the thoroughfare channel at the distal end of the

bed

44

Capillary Structure

Figure 21.4

Continuous Capillaries

• Continuous capillaries are

abundant in the skin and muscles,

and have:

– Endothelial cells that provide

an uninterrupted lining

– Adjacent cells that are held

together with tight junctions

– Intercellular clefts of unjoined

membranes that allow the

passage of fluids

• Continuous capillaries of the

brain:

– Have tight junctions

completely around the

endothelium

– Constitute the blood-brain

barrier

Fenestrated Capillaries

• Found wherever

active capillary

absorption or filtrate

formation occurs

(e.g., small intestines,

endocrine glands,

and kidneys)

• Characterized by:

– An endothelium

riddled with pores

(fenestrations)

– Greater

permeability to

solutes and fluids

than other

capillaries

Sinusoids • Highly modified, leaky,

fenestrated capillaries

with large lumens

• Found in the liver, bone

marrow, lymphoid

tissue, and in some

endocrine organs

• Allow large molecules

(proteins and blood

cells) to pass between

the blood and

surrounding tissues

• Blood flows sluggishly,

allowing for modification

in various ways

• Collect blood from all tissues and organs

and return it to the heart

• Are classified according to size

– Venules

– Medium-sized veins

– Large veins

Veins

The transition from capillaries to venules

occurs gradually

• The immediate postcapillary venules are similar structurally

to capillaries, with pericytes, but range in diameter from 15 to

20 m.

• A. Postcapillary venules participate in the exchanges between

the blood and the tissues and, are the primary site at which

white blood cells leave the circulation at sites of infection or

tissue damage.

• B. Venules converge into larger collecting venules which have

more contractile cells. With greater size the venules become

surrounded by recognizable tunica media with two or three

smooth muscle layers and are called C. Muscular venules.

• A characteristic feature of all venules is the large diameter of

the lumen compared to the overall thinness of the wall

• Venules collect blood from capillary networks

and gradually merge to form veins. Venules

PCV: Postcapillary venules. CV: Capillary

venules. MV Musculsr venules

Veins

• Blood entering veins is under very low pressure and moves

toward the heart by contraction of the tunica media and external

compressions from surrounding muscles and other organs.

Valves project from the tunica intima to prevent back-flow of

blood.

• Most veins are small or medium veins with diameters less than

one centimeter.

• Located in parallel with corresponding muscular arteries.

• The intima usually has a thin subendothelial layer

• The media consists of small bundles of smooth muscle cells

intermixed with reticular fibers and a delicate network of elastic

fibers.

• The collagenous adventitial layer is well-developed.

Small Veins.

• (a): Micrograph of small vein (V) shows a relatively large

lumen compared to the small muscular artery (A) with its

thick media (M) and adventitia (Ad). The wall of a small vein

is very thin, containing only two or three layers of smooth

muscle. X200. H&E.

Medium sized vines • Vein with a much less

compact muscle layer than you saw in the preceding arteries. unica media and adventitia, which is at least as wide as the media, and often even wider.

• There is no evident inner elastic membrane

• (b): Micrograph of a convergence between two small veins showing valves (arrow). Valves are thin folds of tunica intima projecting well into the lumen which act to prevent backflow of blood. X200. H&E.

(c): Micrograph of a

medium vein (MV)

showing a thicker

wall, but still less

prominent than that

of the accompanying

muscular artery

(MA). Both the

media and

adventitia are better

developed, but the

wall is often folded

around the

relatively large

lumen. X100. H&E.

• (d): Micrograp

h of a medium

vein containing

blood and

showing valve

folds (arrows).

X200. Masson

trichrome.

{ Adventitia

{

Intima

Bundles of longitudinal smooth muscle

Occasional circular SMC Numerous elastic fibers

LARGE VEIN Details

• The big venous

trunks,

Paired with elastic arteries close to

the heart, are large veins

Large veins have a well-developed

tunica intima, but the tunica

media is relatively thin, with

few layers of smooth muscle and

abundant connective tissue. The

adventitial layer is thick in large

veins and frequently contains

longitudinal bundles of smooth

muscle. Both the media and

adventitia contain elastic fibers,

but elastic laminae like those of

arteries are not present.

• Most veins have valves, but

these are most prominent in

large veins.

Special features of

veins

• veins contain valves

– Prevent backflow of blood

• Valves

– Prevent backflow

– Most abundant in legs (where

blood has to travel against

gravity)

• Muscular contraction

– Aids the return of blood to heart

in conjunction with valves

Mechanical issues…

(really good to know)

http://medsci.indiana.edu/histo/toc.htm