Volgograd state medical universityVolgograd state medical university

Department of histology, embryology, cytologyDepartment of histology, embryology, cytology

VolgogradVolgograd, 20, 201515

for the 2for the 2ndnd course courseEnglish medium studentsEnglish medium students

for the 2for the 2ndnd course courseEnglish medium studentsEnglish medium students

Lecture:Lecture:Lecture:Lecture:

5. Find common and distinctive features in the structure of heart and large vessel wall.

The objectives:The objectives:

1. Learn the structure of different blood vessels: arteries, veins, vessels of the microcirculatory network.

1. Learn the structure of different blood vessels: arteries, veins, vessels of the microcirculatory network.

2. Reveal structural and functional correlations in all the parts of the circulatory system.

2. Reveal structural and functional correlations in all the parts of the circulatory system.

3. Compare the structure and ultrastructure of cardiac muscle with those of the other types of muscle tissue.

4. Distinguish typical and atypical cardiomyocytes referring to their structure and function.

Scheme of the CVS

Microvasculatory network: arterioles + precapillary arterioles + capillaries + postcapillary venules + venules.

GENERAL DEFINITIONS.GENERAL DEFINITIONS.

Circulatory system = cardiovascular (blood vascular) system + lymph vascular system.

Cardivascular system = heart + arteries + capillaries + veins.

Layers of blood vessels: tunica intima, tunica media, tunica adventitia.

Circulatory system = macrovasculature (vessels more than 0.1 mm in a diameter) + microvasculature (vessels visible only under light microscope).

Blood and Extracellular Fluid Volumes

The capillaries, although associated with only 5% of blood volume, present by far the largest surface area for substance exchange, estimated to be about 600 m2.

Capillaries5 %

Peripheralarteries 10 %

Heart andlungs 20 %

Peripheralveins 65 %

The capillaries are the smallest functional unit of the blood vascular system and are inserted between arterial and venous limbs of the circulation. They branch extensively to form elaborate networks, the extent of which reflects the activity of an organ or tissue. Highly branched, closely packed networks of capillaries are present in the lungs, liver, kidneys, glands and mucous membranes. Capillaries together with arterioles and venules comprise microcirculatory bed (diameter of the vessels does not exceed 100 mc).

The Diagram of theMicrovasculatureThe Diagram of theMicrovasculature

The blood vascular system has a continuous lining which consists of a single layer of endothelial cells. The cell borders are serrated or wavy. Thereafter the addition of the accessory coats can be traced progressively in larger vessels.

Endothelial Cell Arrangement in a CapillaryEndothelial Cell Arrangement in a CapillaryEndothelial Cell Arrangement in a CapillaryEndothelial Cell Arrangement in a Capillary

The Capillaries in the Connective Tissue, Methylene Blue.

The Capillaries in the Connective Tissue, Methylene Blue.

4. The length of the capillaries usually varies from 0.25 to 1 mm (the latter being characteristic of the muscle tissue). In adrenal cortex, renal medulla they may be up to 50 mm long. The total length of all the capillaries of the human body has been estimated at 96,000 km.

Some figures regarding capillaries for consideration:

1. Most of the cells in the human body are no farther than 50 micrometers from a capillary.

2. In humans surface area of capillaries is some 600 square meters.

3. The total cross-sectional area of capillaries is approximately 800 times greater than that of the aorta (compare the rate of flow through capillaries and large arteries).

Capillary of the Cerebellum, Monkey, H & E, x540.

Capillary of the Cerebellum, Monkey, H & E, x540.

Basic structure of the capillaries reveals tunica intima which consists of endothelium and a basal lamina, while the tunica media and tunica adventitia are greatly reduced. The wall of the capillary consists of a single layer of flat endothelial cells. Each endothelial cell is a curving thin plate with an ovoid or elongated nucleus. Usually the cells are stretched along the axis of the capillary and have tapering ends. The nucleus causes the cell to bulge into the capillary lumen. Cells are bound together by junctional complexes and have many pinocytotic vesicles.

The Fenestrated Capillary,TEM, x 10,000

The Fenestrated Capillary,TEM, x 10,000

The Fenestrated Capillary, TEM, x 10,000 External to their endothelium lies a discontinuous layer of intimal pericytes (arrow) that are enclosed by the endothelial basement membrane. Some authors consider the layer of pericytes to be a reduced tunica media of the capillaries. Pericytes are pluripotential cells which may give rise to some other types of cells such as fibroblasts for instance. Following tissue injuries pericytes proliferate and differentiate to form new blood vessels and CT cells, thus participating in the repair processes. Tunica adventitia is thin and contains some collagen and elastic fibers embedded in a small amount of the ground substance. Adventitial cells (less differentiated), fibroblasts, macrophages and mast cell may also be present here.

There are several different types of capillaries based on the integrity of their endothe-lium: continuous, fenestrated and sinusoidal.

Types of the CapillariesTypes of the Capillaries

Continuous capil-laries (somatic type) are those in which the endothelial cells form a complete internal lining without any intercellular or intracytoplasmic de-fects. They are not interrupted by pores or fenestrae. It is the most common type in which substances are trans-ported across the wall by pinocytosis.

They are present in muscle, nervous and CT. The play significant role in blood-brain barrier.

The Capillary of Continuous TypeThe Capillary of Continuous Type

Fenestrated or perforated capillaries contain a number of pores 60-70 nm in diameter that permit more rapid transcapillary transport than can occur by micropinocytosis. Fenestrae may be traversed by a thin diaphragm-like structure. Diffusion provided by the fenestrations is the most important mechanism by which materials pass between blood plasma and interstitial fluid. These capillaries are found in the pancreas, intestines, endo-crine glands.

The Capillary of the Fenestrated TypeThe Capillary of the Fenestrated Type

Fenestrated Capillary, TEM & SEMFenestrated Capillary, TEM & SEM

Sinusoids (discontinuous) capil-laries have a greatly enlarged diameter (up to 40 mc).Separations may occur between the endothelial cells and the basal lamina surrounding the sinusoid may also be incomplete, and phagocytic cells (as Kupfer’s cells in the liver) are sometimes closely associated with the outer wall of the sinusoid.Discontinuous endothelial cells with large fenestrae without diaphragms and discontinuous basal lamina provide enhances exchange between blood and tissue. Sinusoids are especially abun-dant in hematopoetic tissues and liver.

The Sinusoidal Capillary

Comparison of Capillaries, Sinusoids and Sinuses

Diagnostic features

Continuous capillaries

Fenestrated capillaries

Lymph capillaries

sinusoids Venous sinuses

Lymph sinuses

Typical location

muscle Most viscera

Lymph nodes

Liver,

spleen, bone marrow

spleen Lymph nodes

Endothe-lium

continuous continuous Contiguous but not continuous

Disconti--nuous, many macro-phages

Disconti-nuous, many macro-phages

Disconti-nuous, many macro-phages

Fenestra-tions in endothe-lium

none Many small (0.07-0.1mcm)

Only in lacteals

Variable, larger (0.1-0.2 mcm)

none none

Phagocy-tic endotheli-um

none none none active Limited activity

Vary active

Comparison of Capillaries, Sinusoids and Sinuses (cont.)

Diagnostic features

Continuous capillaries

Fenestrated capillaries

Lymph capillaries

sinusoids Venous sinuses

Lymph sinuses

Diameter of lumen

Small (6-10mcm), regular

Small (6-10mcm), regular

Larger (10-50 mcm), irregular

Variable (5-30 mcm), irregular

Largest, irregular

Large, irregular

Basement mem-brane

Well developed, continuous

Well developed, continuous

Scanty or absent

Scanty or absent

Scanty, disconti-nuous

absent

Intercellu-lar spaces

none none none Present 0.1-0.5mcm

variable present

Pericytes present present absent Maybe present in liver

absent absent

Junctional complexes

present present Usually absent

Absent, except in spleen

absent No data

This may result in edema of perivascular space and increased infiltration by blood cells which migrate by diapedesis through the intercellular junctions.

CLINICAL CORRELATIONS:CLINICAL CORRELATIONS:

Permeability of microvessels may increase under certain conditions (inflammation, liberation of the biologically active substances such as histamine and bradykinin).

Transport across Capillary Endothelium

b) small hydrophilic and hydrophobic molecules (oxygen, glucose) can diffuse or be actively transported across the plasmalemma of capillary endothelial cell. Fenestrae and pinocytotic vesicles represent large pores through which even larger molecules can permeate.

Functions of the capillaries:

1. Permeability – the capillaries serve as a selective permeability barrier. They are referred to as exchange vessels since at this site oxygen, carbon dioxide, substrates and metabolites are transferred from blood to the tissues and from the tissues to blood. Permeability depends on the size and charge of the permeating molecules and the structure of the endothelial cells (in brain they lack micropinocytotic vesicles and form blood-brain barrier impermeable for many macromolecules and thus protecting brain).

a) water and smallest hydrophilic molecules can cross the capillary wall by diffusing through the intercellular junctions (paracellular pathway) which are considered to be small pores from physiological point of view.

3. Antithrombogenic function - they serve as nonthrombogenic container for blood.

Functions of the capillaries:Functions of the capillaries:

2. Metabolic functions

a) activation (conversion of angiotensin I to angiotensin II

b) inactivation – conversion of norepinephrine, serotonin, bradykinin to biologically inert compounds

c) lipolysis – breakdown of lipoproteins

d) production of vasoactive factors – endothelins, VCAM etc.

4) Venous portal system in liver and adeno-hypophysis.

Types of MicrocirculationTypes of MicrocirculationThe are 4 types of microcirculation in the human body:

1) Usual sequence of the arteriole- precapillary arteriole (metarteriole) – capillary- postcapillary venule- venule- vein.

2) Arteriovenous anasto-mosis – absence of capillaries when ex-change is not essential and most important is to provide fast blood-flow.

3) Arterial portal system (in kidney).

Vein

Precapil-lary sphincter

Capillary

Post-capillaryvenule

Arterio-venousAnasto-mosisCapillary

Arteriole

Metarte-rioles

Glome-rular Capil-laries

1

2

3

4

Arteriole and Venule, Containing Blood Cells. H & E., x540.

Arterioles (A) are terminal arterial vessels that regulate blood flow into the capillary beds. In the sections the width of the wall is approximately equal to the diameter of its lumen. The endothelium of the tunica intima is supported by a thin endothelial CT layer consisting of type III collagen and a few elastic fibers embedded in ground substance. A thin fenestrated internal elastic lamina is absent in the small and terminal arterioles but present in larger ones. In small arterioles the tunica media is composed of a single layer of SM cells, that completely encircles the endothelial cells. In larger ones two or three layers of SM cells may be present. The external elastic lamina is absent. The adventitia is scant and is represented by fibroelastic CT housing a few fibroblasts.

A

BLOOD VESSELS Mesentery, Rabbit, BLOOD VESSELS Mesentery, Rabbit, hematoxylin stain, A. 162 x., B. 612 x.hematoxylin stain, A. 162 x., B. 612 x.BLOOD VESSELS Mesentery, Rabbit, BLOOD VESSELS Mesentery, Rabbit,

hematoxylin stain, A. 162 x., B. 612 x.hematoxylin stain, A. 162 x., B. 612 x.

Distinguishing Features of Human Blood Vessels(classification is based on the diameter and cellular wall composition)

arteries large

(elastic)

medium (mus-cular)

arterioles precapillary arterioles

capil-llaris

diameter of lumen, range

average>1 cm

2.5 сm

0.5 mm-

1 сm

0.4 сm

<0.5mm

30mcmм

10-40 mcmм

25 mcmм

5-10 mcmм

8 mcmм

wall thickness (average)

2 mm 1mm 20 mcm no data 1 mcm

smooth muscle (relative amount)

++ +++ ++++ + -

elastic fibers ++++ ++ + +/- -

vasa vasorum ++ + - - -

pericytes - - - - +

Distinguishing Features of Human Blood Vessels(classification is based on the diameter and cellular wall composition)

arteries large

(elastic)

medium (mus-cular)

arterioles

precapil-lary arterioles

capil-llaris

innervation +++, esp. sensory

++ +++,esp. motor

+++ esp. motor

-

lymph vessels ++ + - - -

blood pressure (adult), mm Hg

100 95 35 no data 22

blood flow velocity (average, сm/sec

45 12 1.0 no data 0.1

functions elastic recoils maintain flow in diastole

distri-bution of blood

regulate blood pressure by changes in diameter

capillary sphincters

exchange of О2 и С02, nutrients, waste products, etc.

Venules are the last segment of the microcir-culatory network. Three types of them are distinguished: postcapillary venules, collec-ting venules and (muscle) venules. They differ in size, wall thickness, presence of the smooth muscle cells.

The smallest of them (postcapillary venules and collecting venules) are very similar to capillaries: they have comparable diameter, thin wall, pericytes. They lack smooth muscle cells and nerve supply. Their permeability is also comparable to that of the capillaries: they allow passage of blood cells through its wall.

Arteriole, Postcapillary Arteriole, Postcapillary Venule, Venule.Venule, Venule.

Arteriole, Postcapillary Arteriole, Postcapillary Venule, Venule.Venule, Venule.

Distinguishing Features of Human Blood Vessels

. Capillaries Post-

capillary venules

Collecting (pericytic venules)

Muscular venules

Veins medium

Veins large

Diameter of lumen (range, average)

5-12 mcm

8 mcm

12-30 mcm

20 mcm

30-50mcm

40mcm

50 mcm-3 mm

1mm

3 mm-1 cm

0.5cm

>1cm

3 cm

Wall thickness

1mcm 2mcm No data 0.1mm 0.5mm 1.5mm

Smooth muscle(re-lative amount)

- - - +/- + + (largely in adven-titia)

Elastic fibers

- - +/- +/- + ++

Pericytes + ++(incomplete layer)

++++(com-plete layer)

- - -

Vasa vasorum

- - - - ++ ++++

Distinguishing Features of Human Blood Vessels (cont.)

. Capillaries Post-

capillary venules

Collecting (pericytic venules)

Muscular venules

Veins medium

Veins large

Nerve supply

- - - + ++ +++

Lym-phatics

- - - - +/- +++

Blood pressure adult average Hg mm

22 No data No data 12 5 3 (maybe negative near heart)

Blood flow velocity

sm/sec

0.1 No data No data 0.5 5 15

Function Exchange of O2, CO2, nutrients, waste products

Similar to capillaries

Highly permeable, important in blood-tissue exchange

Transport venous blood

Collect venous blood

Carry venous blood to heart

The Venule in the Connective Tissue, Methylene Blue.

The Venule in the Connective Tissue, Methylene Blue.

Postcapillary venules are important sites for blood - interstitial substance exchange.

The Large Venule in the Connective Tissue, Methylene Blue.

The Large Venule in the Connective Tissue, Methylene Blue.

small arteries (arterioles have already been covered by us). Some authors distinguish also hybrid and mixed types.

THE MAIN FEATURES OF THE ARTERIESTHE MAIN FEATURES OF THE ARTERIES

1. Arteries are blood vessels conveying blood away from the heart.

2. With the exception of the pulmonary artery and umbilical artery all of them carry oxygenated blood.

3. Arteries decrease in size and increase in number as they proceed distally from the heart.

4. Arteries are classified according to the size or predominant tissue component into:

large elastic (or conducting) arteries (aorta, the common carotid and sublavian arteries, the common iliac arteries and the pulmonary trunk).

medium-sized muscular (distributing) arteries (brachial, ulnar, renal etc),

Aorta,Human, Weigert's elastic tissue stain and phloxine, 162 x.

Aorta,Human, Weigert's elastic tissue stain and phloxine, 162 x.

Elastic ArteryElastic Artery

Elastic Artery, Orcein, Low Magnification

The wall is relatively thin as compared to their wide lumen (1/10 of the vessel diameter). The intima of aorta is rather thick (150 mc), it is composed of the endothelium with the basal lamina and subendothelial layer rich in collagen and elastic fibrils and longitudinally arranged bundles of smooth muscle cells. Tunica media is the thickest layer (2 mm thick) and consists largely of elastic tissue that forms 50 to 70 concentric sheets or elastic membranes. The adventitia is relatively thin and contains bundles of CF, a few EF, nerve fibers, lymphatics and blood vessels (so-called vasa vasorum - a small system of vessels to nourish the heavy arterial wall.

Intima

Media

Adventitia

Adventitia

Elasticaintema

Elastic Artery, Monkey, H & E, x540.

The elastic membranes of the tunica media are called fenestrated as they have holes (O) in them to facilitate diffusion of nutrients and waste materials. Successive lamina are connected by elastic fibers (EF). The abundant elastic tissue in the wall of the aorta helps to make the wall easily distensible and helps maintain a constant blood flow.

AORTAAORTAAORTAAORTA

Elastic Artery, Orcein, x 132Elastic Artery, Orcein, x 132

Aorta, Tunica MediaAorta, Tunica Media

At the histological section of aorta and the laminae will appear as undulating lines due to postmortem arterial collapse.

Axillary Artery, Gomori TrichromeAxillary Artery, Gomori Trichrome

Additional Types of the Large Arteries:- Mixed arteries: carotid external, axillary (they have both elastic and smooth muscle components mixed in the media).- Hybrid arteries: visceral branches of the abdominal aorta (smooth muscle dominates in the internal part of media while elastic – in the external part of it.)

Functional Correlations:

Large arteries are called conducting arteries since their major function is to transport blood away from the heart.

These arteries also serve to smooth out the large fluctuations in pressure created by the heart beat.

During ventricular contraction (systole), the elastic laminae of conducting arteries are stretched and reduce the pressure change.

During ventricular relaxation (diastole) ventricular pressure drops to a low level but the elastic rebound of conducting arteries helps to maintain arterial pressure.

As a consequence, arterial pressure and blood flow decrease and become less variable as the distance from the heart increases.

Muscular arteries include most of the named arteries such as femoral, renal, brachial, ulnar and radial. They may be as large as the femoral or branchial arteris or as small as unnamed arteries just visible to the unaided eye. While the main function of the elastic arteries is to conduct blood the main function of the muscle arteries is to distribute blood to specific organs.Upon demand they have the capacity to increase greatly in size. For example, an occlusion of the principal arteries to a region the smaller collateral, muscular arteries enlarge sufficiently to effectively carry the needed blood to the ishemic area.

MUSCULAR ARTERY

The Muscular Artery, H & E, x 132.

The tunica intima consists of a layer of endothelium and a flattened subendothelial layer of collagenous and elastic fibers (the latter may be absent in the smaller divisions of muscular arteries). To these two layers is added a fenestrated internal elastic lamina which delimits the tunica intima from tunica media. The tunica media ™ is very thick and mainly muscular, consisting of 5-30 helical layers of smooth muscle that are concentrically arranged. Among the smooth muscle cells may be fine reticular, CF & EF, as well as some amorphous intercellular substance. An external elastic lamina is located between the medial and adventitial layers and consists of several elastic fibers. The adventitia is rather thick and is about ½ of the width of the media. It consists of collagen and elastic fibers, a few fibroblasts and adipose cells. Lymphatics, vasa vasorum and nerves are also found in the adventitia, and these structures may penetrate to the outer part of media. There are interrupted elastic lamellae (E) in the tunica media.

MUSCULAR ARTERY, x 132

MUSCULAR ARTERY, x 132

Muscular Artery, High Magnification, H & E

Muscular Artery, High Magnification, H & E

Muscular Artery, Orcein

Muscular Artery, Orcein

VEINSVEINS

1. Veins carry blood from the capillary bed to the heart.

2. With the exception of the umbilical and pulmonary veins they carry deoxygenated blood.

3. They may be referred to as capacitance vessels because more than 70% of the total blood volume is in this portion of the CVS at any one time.

Muscular Artery and Vein.

Muscular Artery and Vein.

Muscular Artery and Companion VeinMuscular Artery and Companion Vein

vein artery

vein artery

Muscular Artery and Muscular Artery and Companion VeinCompanion VeinMuscular Artery and Muscular Artery and Companion VeinCompanion Vein

Valves occur first in the post-capillary venules and are particularly numerous in the veins of the legs which conduct blood against gravity.

Cerebral veins, veins within organs and bone marrow, jugular veins, vena cava superior and inferior have no valves.

Medium-Sized VeinMedium-Sized VeinMedium-Sized VeinMedium-Sized Vein

 

SCHEME OF THE MUSCULAR ARTERY AND VEINSCHEME OF THE MUSCULAR ARTERY AND VEINSCHEME OF THE MUSCULAR ARTERY AND VEINSCHEME OF THE MUSCULAR ARTERY AND VEIN

6. The internal elastic lamina is better developed in arteries than in veins.

Arteries do not contain valves! Arteries do not contain valves!

1. The lumen of an artery is smaller than in its companion vein.

2. The wall of the artery is thicker and more rigid than its companion vein.3. Arteries are generally better supplied with elastic fibers and smooth

muscle cells than veins while collagen fibers are more widely used in veins.

4. The tunica media of the artery is the thickest coat, while the tunica adventitia of veins is the thickest coat.

5. Veins tend to be more loosely constructed than arteries.

The Medium-Sized Vein, H & E.

The Medium-Sized Vein, H & E.

The Medium-Sized Vein, H & E.

The Medium-Sized Vein, H & E.

The vein is probably maximally stretched, accommodating a large volume of blood – veins have high capacitance.

Large Vein - Vena Cava InferiorLarge Vein - Vena Cava Inferior

Large veins have a diameter greater than 10 mm.

  Adventitia is very prominent and makes up most of the wall.

At the junction with heart the adventitia of the pulmonary veins and venae cavae (VCS, VCI) are provided with a coat of cardiac muscle.

A system of vasa vasorum is best developed in large veins, they may even extend into the media and even intima.

The innermost layer of the adventitia (1) displays thick collagen bundles (arrows) arrayed in a spiral configuration, which permits it to become elongated or shortened with respiratory excursion of the diaphragm. The middle layer presents smooth (or cardiac) muscle cells, longitudinally disposed.

The outer layer (3) is characterized by thick bundles of CF interspersed with elastic fibers. The region contains vasa vasorum which supply nourishment to the wall of the vena cava.

Vena Cava Inferior, H & E.Vena Cava Inferior, H & E.

VENA CAVA Human, Weigert's* stain and phloxine, 162 x.

VENA CAVA Human, Weigert's* stain and phloxine, 162 x.

Comparison of Elastic Artery and Muscular ArteryComparison of Elastic Artery and Muscular Artery

Elastic Artery Muscular Artery

Tunica intima: width~one-fifth of Tunica intima: width~one-fifth of total wall; less elastin than in mediatotal wall; less elastin than in media

Tunica intima thinner in muscularTunica intima thinner in muscular

artery (in many areas, endotheliumartery (in many areas, endothelium

lies directly on internal elasticlies directly on internal elastic

lamina)lamina)

Tunica media comprises bulk of wallTunica media comprises bulk of wall Occasionally a split internal elasticOccasionally a split internal elastic

laminalamina

Mainly elastic fibers in media; someMainly elastic fibers in media; some

smooth muscle cellssmooth muscle cells

Chiefly smooth muscle in media;Chiefly smooth muscle in media;

relatively few collagenic, reticular,relatively few collagenic, reticular,

and elastic fibersand elastic fibers

Tunica adventitia relatively thin;Tunica adventitia relatively thin;

both collagenic and elastic fibersboth collagenic and elastic fibers

Adventitia thick;Adventitia thick;

approximately one-third to two-approximately one-third to two-

thirds of thickness of media; boththirds of thickness of media; both

collagenic and elastic fiberscollagenic and elastic fibers

Characteristics of VeinsCharacteristics of VeinsTYPE TUNICA INTIMA TUNICA MEDIA TUNICA ADVENTITIA

Large veinsLarge veins Endothelium; Endothelium; basal lamina, basal lamina, valves in valves in some; some; subendothelisubendothelial CTal CT

CT; smooth CT; smooth muscle cellsmuscle cells

Smooth Smooth muscle cells muscle cells oriented in oriented in longitudinal longitudinal bundles; bundles; cardiac cardiac muscle cells muscle cells near their near their entry Into the entry Into the heart; heart; collagen collagen layers with layers with Fbl.Fbl.

Medium and Medium and small veinssmall veins

Endothelium; Endothelium; basal lamina, basal lamina, valves in valves in some; some; subendo-subendo-thelial CTthelial CT

Reticular Reticular and elastic and elastic fibers, some fibers, some smooth smooth muscle cellsmuscle cells

Collagen Collagen layers with layers with fibroblastsfibroblasts

VenulesVenules Endothelium; Endothelium; basal lamina basal lamina (pericytes,po(pericytes,post-capillary st-capillary venules)venules)

Sparse CT Sparse CT and a few and a few smooth smooth muscle cellsmuscle cells

Some Some collagen and collagen and a few a few fibroblastsfibroblasts

The HEARTThe HEARTThe HEARTThe HEART

The HEARTThe HEARTThe HEARTThe HEART

subendocardial layer (SeL), a loose CT continuous with the endomysium of the myocardium (My). In subencardial layer of the ventricles and their common septum Purkinje fibers are scattered.

HEARTHEARTHEARTHEART

Endocardium (En) has the following stratification: a continuous layer of simple

squamous epithelium (End) lying on the BL,

subendothelial layer (SL), a thin layer of loose CT with a few fibroblasts and delicate collagen fibers (C),

myoelastic layer (ML), consisting of relatively dense CT with thick CF & EF (El) and scattered predominantly vertically oriented SMC,

LAYERS OF THE HEARTEndocardium myocardium, epicardium

noncontractile modified muscle fibers called Purkinje fibers. Note subendocardial location of the Purkinje fibers.

PURKINJE FIBERS, periodic acid-Schiff and hematoxylin stains, 162 x.

PURKINJE FIBERS, periodic acid-Schiff and hematoxylin stains, 162 x.

muscle fibers

Myocardium is the thickest of the tunics of the heart and consists of :

contractile fascicles of cardiac muscle fibers arranged in sheets in a complex, spiral manner;

CARDIAC MUSCLE: Purkinje fibers, cross section, H. & E., 162 x.

CARDIAC MUSCLE: Purkinje fibers, cross section, H. & E., 162 x.

Heart Wall (Atrium), H & E

Heart Wall (Atrium), H & E

HEART, Ventricular wall, Rhesus monkey, HEART, Ventricular wall, Rhesus monkey, H. & E., 5 x.H. & E., 5 x.

HEART, Ventricular wall, Rhesus monkey, HEART, Ventricular wall, Rhesus monkey, H. & E., 5 x.H. & E., 5 x.

CARDIAC MUSCLE, Human, Mallory's stain, 612 x.CARDIAC MUSCLE, Human, Mallory's stain, 612 x.

Longitudinal section, Human, phosphotungstic acid - hematoxylin stain, 162 x.

Longitudinal section, Human, phosphotungstic acid - hematoxylin stain, 162 x.

CARDIAC MUSCLECARDIAC MUSCLE

CARDIAC MUSCLE: Relaxed and contracted muscle fibers Human, Mallory-azan stain, 1416

x.

CARDIAC MUSCLE: Relaxed and contracted muscle fibers Human, Mallory-azan stain, 1416

x.

CARDIAC MUSCLELipochrome pigment Human, Mallory's stain, 1416

x.

CARDIAC MUSCLELipochrome pigment Human, Mallory's stain, 1416

x.

Scheme of Cardiomyocyte

Scheme of Cardiomyocyte

Intercalated disks

Junctional Specialization of the Intercalated Disc of the Cardiac Muscle

Relationship of Cross-Banding to the Arrangement of Thick and Thin Filaments

Diagrammatic Representation of the Sarcoplasmatic Reticulum and System of T-tubules in Skeletal and Cardiac Muscle

skeletal

I ba

nd

A

ba

nd

T-tubuleSarcotubules

Terminalcistemae

cardiac

Z line

Z line

T-tubule

Sarcoplasmic

tubules

Diad

The T-tubules are found at the level of the Z-band in cardiac muscle rather than at the A-I junction as in skeletal muscle; the sarcoplasmatic reticulum is not as well developed and wanders irregularly through the myofilaments. Terminal cisterna (lateral expansions of the sarcoplasmatic reticulum) are flattened and discontinuous leading to formation diads rather than triads of the skeletal muscle since the T-tubules are generally associated with only one sarcoplasmatic cisterna.

mesothelium (Mes), with its BL, both on the inner aspect of the pericardium facing the epicardium fibrous layer (FL), consisting of dense CT with blood and lymphatic vessels and nerve fibers.

HEARTHEARTHEARTHEARTEpicardium is composed of

mesothelium (Mes), a single layer of squamous epithelial cells lying on the basal lamina (BL);

subepicardial layer (SpL), consisting of a loose CT, rich in EF fibers, blood and lymphatic vessels, and nerve fibers and with a variable number of adipose cells (Ad), predominantly along the coronary vessels. 

The heart is enveloped by a fibroserous sac – the pericardium (P) built up of the following layers: 

HEART, Ventricular wall, Human, phosphotungstic acid, hematoxylin stain, 7x

HEART, Ventricular wall, Human, phosphotungstic acid, hematoxylin stain, 7x

4) Purkinje fibers. Impulse-conducting system is isolated from surrounding myocardium by a CT sheet and consists of the three types of cells.

IMPULSE-CONDUCTING SYSTEMIMPULSE-CONDUCTING SYSTEMIMPULSE-CONDUCTING SYSTEMIMPULSE-CONDUCTING SYSTEM

Right bundle branch

Superior vena cava

Sinoatrial node

Atrioventri-cular node

Bundle of His

Purkinje system

Posterior fascicle

Anterior fascicleLeft bundle branch

Aorta

This is a system of specially modified cardiac muscle cells with function of generating and conducting impulses of heart contraction to various parts of myocardium, as well as to assure proper succession of beat of atria and ventricles. It is composed of:1) sinoatrial node or node of Keith and Flack,

2) atrioventricular node or node of Ascoff and Tawara,

3) atrioventricular bunch or bundle of His with its left and right branches,

Purkinje Fibers, H & E

Purkinje Fibers, H & E

Purkinje Cells, Large Magnification, H & E

Purkinje Cells, Large Magnification, H & E

These cells conduct action potentials rapidly (3-4 ms compared to 0.5 ms for cardiac muscle) to all regions of both ventricles, causing ventricular depolarization and then contraction.

Ultrastructure of the Pace-Maker Cell of the Ultrastructure of the Pace-Maker Cell of the Impulse-Conducting SystemImpulse-Conducting System

Ultrastructure of the Pace-Maker Cell of the Ultrastructure of the Pace-Maker Cell of the Impulse-Conducting SystemImpulse-Conducting System

Pace-maker Cell Transitional Cell

Purkinje Cell

Feature Pace-Maker Cells Transitional Purkinje Cells

LocationConstitute sino-atrial and a atrioventricular nodes

Sino-atrial and atrioventricular nodes + sites of connection between Purkinje cells and cardiomyocytes

Subendocardial layer from AV-bundle of His to the apex of the heart

Size 10x25 mc Elongated, longer than pace-makers 50x100 mc

Nucleus Round Elongated Elongated, often 2

Cytoplasm Very clear Higher densityLower density (compar-ed to the transitional cells)

Mitochondria Several large Numerous small Numerous small

Golgi Complex + + ++

Cisternae of RER + ++ +

Myofibrils + +++ ++

Vesicles ++ + +

Glycogen particles ++ + +++

Basal Lamina + ++

around the whole fiber

Cell junctions Zonulae adherentes Zonulae adherentes Desmosomes, nexuses, fasciae adherentes

Function

Generate contraction impul-se, conduct it to the cardiac myocytes and transitionalcells

Conduct impulse to the cardiac myocytes

Conduct impulse to the transitional cells

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