Download /- - - Elastic fibers

Volgograd state medical university
Department of histology, embryology, cytology
Lecture:
for the 2nd course
English medium students
Volgograd, 2015
The objectives:
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.
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.
5. Find common and distinctive features in the structure of heart
and large vessel wall.
Scheme of the CVS
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).
Microvasculatory network: arterioles + precapillary
arterioles + capillaries + postcapillary venules +
venules.
Blood and Extracellular Fluid Volumes
Peripheral
arteries 10 %
Heart and
lungs 20 %
Capillaries
5%
Peripheral
veins 65 %
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.
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 the
Microvasculature
Endothelial Cell Arrangement in a Capillary
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.
The Capillaries in the Connective
Tissue, Methylene Blue.
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).
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.
Capillary of the
Cerebellum,
Monkey, H & E, x540.
The Fenestrated
Capillary,
TEM, x 10,000
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
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.
Types of the
Capillaries
There are several
different types of
capillaries based
on the integrity
of their endothelium: continuous,
fenestrated and
sinusoidal.
The Capillary of
Continuous Type
Continuous capillaries (somatic type) are
those in which the
endothelial cells form a
complete internal lining
without any intercellular
or intracytoplasmic defects. They are not
interrupted by pores or
fenestrae. It is the most
common type in which
substances are transported across the wall by
pinocytosis.
They are present
in muscle, nervous and
CT. The play significant
role
in
blood-brain
barrier.
The Capillary of the
Fenestrated 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, endocrine glands.
Fenestrated Capillary,
TEM & SEM
The Sinusoidal Capillary
Sinusoids (discontinuous) capillaries 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 abundant in hematopoetic tissues and
liver.
Comparison of Capillaries, Sinusoids and Sinuses
Diagnostic Continuous
features
capillaries
Fenestrated Lymph
capillaries
capillaries
sinusoids Venous
sinuses
Lymph
sinuses
Typical
location
muscle
Most
viscera
Lymph
nodes
Liver,
spleen,
bone
marrow
spleen
Lymph
nodes
Endothelium
continuous
continuous
Contiguous
but not
continuous
Disconti-nuous,
many
macrophages
Disconti
-nuous,
many
macrophages
Discontinuous,
many
macrophages
Fenestrations in
endothelium
none
Many small Only in
(0.07lacteals
0.1mcm)
Variable, none
larger
(0.1-0.2
mcm)
none
Phagocytic
endothelium
none
none
active
Vary
active
none
Limited
activity
Comparison of Capillaries, Sinusoids and Sinuses (cont.)
Diagnostic
features
Continuous
capillaries
Fenestrated
capillaries
Lymph
capillaries
sinusoids Venous
sinuses
Lymph
sinuses
Diameter
of lumen
Small (610mcm),
regular
Small (610mcm),
regular
Larger
(10-50
mcm),
irregular
Variable Largest, Large,
(5-30
irregular irregular
mcm),
irregular
Basement
membrane
Well
developed,
continuous
Well
developed,
continuous
Scanty or
absent
Scanty
Scanty,
or absent discontinuous
absent
Intercellular spaces
none
none
none
Present
0.10.5mcm
variable
present
Pericytes
present
present
absent
Maybe
present
in liver
absent
absent
Junctional present
complexes
present
Usually
absent
Absent,
except in
spleen
absent
No data
CLINICAL CORRELATIONS:
 Permeability of microvessels may increase under
certain conditions (inflammation, liberation of the
biologically active substances such as histamine and
bradykinin).
 This may result in edema of perivascular space and
increased infiltration by blood cells which migrate by
diapedesis through the intercellular junctions.
Transport across Capillary Endothelium
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 bloodbrain 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.
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:
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.
3. Antithrombogenic
function
nonthrombogenic container for blood.
they
serve
as
The are 4 types of
Types of Microcirculation
microcirculation
in
Precapilthe human body:
Capillary
lary
1) Usual sequence of the Arteriole
sphincter
arterioleprecapillary
1
Postarteriole (metarteriole) –
capillary
capillary- postcapillaryMetartevenule
venule- venule- vein.
rioles
2) Arteriovenous anastomosis – absence of
capillaries when exchange is not essential
and most important is to
provide fast blood-flow.
3) Arterial portal system (in
kidney).
4) Venous portal system in
liver
and
adenohypophysis.
2
Arteriovenous
Anastomosis
Capillary
3
Glomerular
Capillaries
4
Vein
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, 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
medium
(elastic) (muscular)
arterioles
precapillary capilarterioles
llaris
>1 cm
2.5 сm
0.5 mm1 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
-
-
-
-
+
diameter of lumen,
range
average
Distinguishing Features of Human Blood Vessels
(classification is based on the diameter and cellular wall composition)
arteries
large
(elastic)
medium
(muscular)
arteriole
s
precapillary
arterioles
capilllaris
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
distriregulate capillary
bution
blood
sphincters
of blood pressure
by
changes
in
diameter
exchange
of О2 и
С02,
nutrients,
waste
products,
etc.
Venules are the last
segment of the microcirculatory network. Three types
of them are distinguished:
postcapillary venules, collecting 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
Venule, Venule.
Distinguishing Features of Human Blood Vessels
Capillaries
Postcapillary
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(relative
amount)
-
-
+/-
+
+ (largely
in adventitia)
Elastic
fibers
-
-
+/-
+/-
+
++
Pericytes
+
++(incom ++++(com- plete
plete layer)
layer)
-
-
Vasa
vasorum
-
-
++
++++
-
-
Distinguishing Features of Human Blood Vessels (cont.)
Capillaries
Postcapillary
venules
Collecting
(pericytic
venules)
Muscular
venules
Veins
medium
Veins large
Nerve
supply
-
-
-
+
++
+++
Lymphatics
-
-
-
-
+/-
+++
Blood
pressure
adult
average
Hg mm
22
No data
No data
12
5
3 (maybe
negative
near
heart)
Blood flow 0.1
velocity
sm/sec
No data
No data
0.5
5
15
Highly
permeable,
important
in bloodtissue
exchange
Transport
venous
blood
Collect
venous
blood
Carry
venous
blood to
heart
Function
Exchange
Similar to
of O2, CO2, capillarie
nutrients,
s
waste
products
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 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),
 small arteries (arterioles have already been covered
by us). Some authors distinguish also hybrid and
mixed types.
Aorta,
Human,
Weigert's
elastic
tissue stain
and
phloxine,
162 x.
Elastic Artery
Elastic Artery, Orcein, Low Magnification
Intima
Elastica
intema
Media
Adventitia
Adventitia
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.
Elastic Artery,
Monkey,
H & E, x540.
AORTA
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.
Elastic Artery, Orcein, x 132
Aorta,
Tunica Media
At the
histological
section of aorta
and the laminae
will appear as
undulating lines
due to
postmortem
arterial collapse.
Axillary 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 ARTERY
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.
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.
The Muscular Artery,
H & E, x 132.
MUSCULAR ARTERY,
x 132
Muscular
Artery,
High
Magnification,
H&E
Muscular
Artery,
Orcein
VEINS
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
Companion Vein
vein
artery
Muscular Artery and
Companion Vein
vein
artery
Medium-Sized 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.
SCHEME OF THE MUSCULAR ARTERY AND VEIN
Arteries do not contain valves!
1.
The lumen of an artery is smaller than in its companion vein.
2.
3.
The wall of the artery is thicker and more rigid than its companion vein.
Arteries are generally better supplied with elastic fibers and smooth
muscle cells than veins while collagen fibers are more widely used in
veins.
The tunica media of the artery is the thickest coat, while the tunica
adventitia of veins is the thickest coat.
Veins tend to be more loosely constructed than arteries.
The internal elastic lamina is better developed in arteries than in veins.
4.
5.
6.
The
MediumSized Vein,
H & E.
The
MediumSized
Vein,
H & E.
The vein is probably maximally stretched, accommodating a
large volume of blood – veins have high capacitance.
Large 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.
Vena Cava Inferior, H & E.
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 Human,
Weigert's* stain and
phloxine, 162 x.
Comparison of Elastic Artery and Muscular Artery
Elastic Artery
Muscular Artery
Tunica intima: width~one-fifth of
total wall; less elastin than in media
Tunica intima thinner in muscular
artery (in many areas, endothelium
lies directly on internal elastic
lamina)
Tunica media comprises bulk of wall
Occasionally a split internal elastic
lamina
Mainly elastic fibers in media; some
smooth muscle cells
Chiefly smooth muscle in media;
relatively few collagenic, reticular,
and elastic fibers
Tunica adventitia relatively thin;
both collagenic and elastic fibers
Adventitia thick;
approximately one-third to twothirds of thickness of media; both
collagenic and elastic fibers
Characteristics of Veins
TYPE
TUNICA INTIMA
TUNICA MEDIA
TUNICA ADVENTITIA
Large veins
Endothelium;
basal lamina,
valves in some;
subendothelial
CT
CT; smooth
muscle cells
Smooth muscle
cells oriented in
longitudinal
bundles; cardiac
muscle cells near
their entry Into the
heart; collagen
layers with Fbl.
Medium and
small veins
Endothelium;
basal lamina,
valves in some;
subendo-thelial
CT
Reticular and
elastic fibers,
some smooth
muscle cells
Collagen layers
with fibroblasts
Venules
Endothelium;
Sparse CT and a Some collagen
basal lamina
few smooth
and a few
(pericytes,postmuscle cells
fibroblasts
capillary venules)
The HEART
The HEART
LAYERS OF THE HEART
Endocardium myocardium, epicardium
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,
 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.
HEART
PURKINJE FIBERS, periodic acidSchiff 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;
 noncontractile modified muscle fibers called Purkinje fibers.
Note subendocardial location of the Purkinje fibers.
CARDIAC MUSCLE: Purkinje fibers, cross section,
H. & E., 162 x.
Heart Wall
(Atrium),
H&E
HEART, Ventricular wall, Rhesus monkey,
H. & E., 5 x.
CARDIAC MUSCLE, Human, Mallory's stain, 612 x.
CARDIAC MUSCLE
Longitudinal section,
Human,
phosphotungstic
acid - hematoxylin
stain, 162 x.
CARDIAC MUSCLE: Relaxed and contracted
muscle fibers Human, Mallory-azan stain, 1416 x.
CARDIAC MUSCLE
Lipochrome pigment Human, Mallory's stain, 1416 x.
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
I band
Diagrammatic Representation of the Sarcoplasmatic Reticulum
and System of T-tubules in Skeletal and Cardiac Muscle
skeletal
cardiac
T-tubule
T-tubule
Z line
Sarcotubules
A band
Sarcoplasmic
tubules
Terminal
cistemae
Z line
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 Ttubules are generally associated with only one sarcoplasmatic cisterna.
Epicardium 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:

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.
HEART
HEART, Ventricular wall, Human,
phosphotungstic acid, hematoxylin stain, 7x
IMPULSE-CONDUCTING SYSTEM
Aorta
Superior
vena cava
Sinoatrial
node
Left bundle branch
Anterior fascicle
Atrioventricular node
Bundle of His
Right bundle
branch
Posterior fascicle
Purkinje system
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,
4)
Purkinje fibers. Impulse-conducting system is isolated from surrounding
myocardium by a CT sheet and consists of the three types of cells.
Purkinje
Fibers,
H&E
Purkinje
Cells, Large
Magnificati
on, 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
Impulse-Conducting System
Purkinje Cell
Pace-maker Cell
Transitional Cell
Feature
Pace-Maker Cells
Transitional
Purkinje Cells
Location
Constitute sino-atrial and
a atrioventricular nodes
Size
10x25 mc
Sino-atrial and
atrioventricular nodes
+ sites of connection
between Purkinje cells
and cardiomyocytes
Elongated, longer
than pace-makers
Nucleus
Round
Elongated
Elongated, often 2
Cytoplasm
Very clear
Higher density
Lower density (compared to the transitional
cells)
Mitochondria
Several large
Numerous small
Numerous small
Subendocardial layer
from AV-bundle of His
to the apex of the heart
50x100 mc
Golgi Complex
+
+
++
Cisternae of RER
+
++
+
Myofibrils
+
+++
++
Vesicles
++
+
+
Glycogen particles
++
+
Basal Lamina
+
+
+++
+
around the whole fiber
Desmosomes, nexuses,
fasciae adherentes
Cell junctions
Function
Zonulae adherentes
Zonulae adherentes
Generate contraction impulse, conduct it to the cardiac Conduct impulse to
the cardiac myocytes
myocytes and transitional
cells
Conduct impulse to the
transitional cells
Thank you!