Histological presentation of the cardiovascular system

1. Cardiovascular system
(Histology)
Residents in Surgery - MUHAS
Presenters 1. Dr. Charles nhungo
2. Dr. Albert Lazaro
3. Dr. Meshack Brighton
4. Dr. Tusal mavji patel
5. Dr. Daudi Katwana
Coordinator : Prof. DAVID NGASSAPA

2. Outline
• Objectives
• Introduction to Cardiovascular system
• Histological features of the heart
• Histological features of the blood vessels
• Differences between arteries and vein
• Clinical application of cardiovascular system
• References

3. Objectives
At the end of this session each student should be able to:
1. Describe the histological features of the heart
2. Describe the histological features of blood vessels
3. Differentiate the histological features of arteries, veins and
capillaries.
4. Explain the clinical application of the histology of the
cardiovascular system.

4. Introduction to CVS
• Composed of the heart and a
continuous system of blood vessels
including arteries, arterioles, capillaries,
venules, and veins.
• Together the lymphatic vessels and the
cardiovascular system form the
circulatory system.

5. A. The heart
• The heart is a muscular organ that contracts
rhythmically to pump blood through the
circulatory system.
• A muscular pump of four chambers; two atria
and two ventricles.
• Atria (R&L) receive blood from the systemic
and pulmonary venous circulations
respectively.
• Ventricles (R&L) pump blood into the
pulmonary and systemic circulations
respectively.
• Heart valves are present at the two outflow
portals to prevent backflow.

6. The heart Cont…
The heart wall is composed of three layers:-
I. The Epicardium: Outer protective layer of
the heart (visceral pericardium)
II. The Myocardium: Muscular middle layer
of the heart
III. The Endocardium: Inner layer of the heart.

7. I. The Epicardium
• A smooth thin layer covering the outer surface of
the heart and its superficial vasculature, composed
of flat mesothelial cells lying on a stroma of
fibrocollagenous support tissue that contains elastin
fibers.
• Also contain adipose tissues, blood vessels and
nerves which supplies the heart.
• It is continuous with a similar layer (parietal
pericardium) lining the inner aspect of the
fibrocollagenous and elastic pericardial sac that
encloses the heart.
• The space between the two pericardia (pericardial
space) contains serous fluid to give a friction free
environment during contractions.

8. I. The Epicardium - histology
The constant layer of fibrocollagenous tissue F. Flat monolayer of mesothelial cells Me (secretion of lubricating fluid)
Micrograph (a) shows epicardium with a large branch of a coronary artery CA, Myocardium M, Adipose tissue A.
Micrograph (b) shows epicardium over most of the heart surface where the fibrocollagenous layer lies directly on the
myocardium without adipose tissue

9. II. The Myocardium
• It’s the thickest layer consisting of :
• Contractile cardiocytes, which contract to pump.
• Myoendocrine cardiocytes, producing atrial natriuretic
factor.
• Nodal cardiocytes, specialized to control the rhythmic
contraction of the heart.
• Histologically comprised of central nuclei and are
linked to each other by intercalated disks.
• The transverse component of the intercalated disk
connects adjacent cardiac muscle cells.

10. II. The Myocardium – histology
Cardiac muscle fibers connected by Intercalated discs ID, and having central nuclei and regular cytoplasmic striations
The extensive and intimate capillary network C

11. III. The Endocardium
It’s the inner most layer with three sublayers:
a. Endothelium: Thin inner layer (squamous cell epithelium and
supporting connective tissue)
b. Myoelastic layer: Middle layer - Smooth muscle fibers and
connective tissue.
c. Subendocardial layer: (Deep layer of connective tissue) Contains
small blood vessels, nerves, and bundles of the conduction system
(Purkinje fibers).
• The subendocardial layer is not present in papillary muscles and chordae
tendineae inserted at the free edges of the mitral and tricuspid valves.

12. III. The Endocardium
Histological slide showing the layers of the endocardium

13. Other structures
Cardiac skeleton
• Dense fibrous connective tissue
• forms part of the interventricular and interatrial septa,
surrounds all valves of the heart extends into the valve
cusps and the chordae tendineae to which they are
attached
Heart Valves
• Located between the atria and ventricles and at the
opening to the aorta and pulmonary vessels.
• Attached to the annuli fibrosi, which forms its core
• Covered on both sides by endocardium. On the atrial
sides smooth muscles are present while on the ventricular
side elastic fibers are prominent
• Arranged in a manner that allow a unidirectional flow of
blood

14. Other structures
Conducting system of the heart
• Located within the subendocardial layer
and adjacent myocardium
• Generates and propagates waves of
depolarization that spread through the
myocardium to stimulate rhythmic
contractions
• Consists of two nodes of specialized
myocardial tissue in the right atrium (the
sinoatrial (SA) node “the pacemaker” and
the atrioventricular (AV) node

15. Clinical application
• Myocardial infarction (MI) - death of heart tissue usually due to
coronary artery occlusion
• Cardiac tamponade - compression of the heart due to an increase of
fluid in the pericardium
• Cardiomyopathy - Disease of the heart muscles
• Rheumatic valve disease – healing results in progressive scaring of
the valve leaflets and their elastic content is replaced by irregular
masses of collagen scar leaving the valves more rigid, the leaflets may
also fuse limiting their ability to open (stenosis) or close
(incompetence).
• Calcific valve disease – mainly affects the aortic valve, particularly if it
is congenitally abnormal with 2 cusps. The valves become more
thickened and distorted by fibrous scarring and calcium nodules
rendering the valves leaflets immobile and impairs supply from the
left ventricle during systole leading to heart failure

16. B. Blood Vessels
• Blood vessels are the part of the circulatory
system and microcirculation that transports
blood throughout the human body
• The arterial system provides a distribution
network to the tiny peripheral
microcirculation, the capillaries and
postcapillary venules, which are the main sites
of interchange of gas and metabolite
molecules between the tissues and the blood
• The venous system carries blood from the
capillary system back to the heart.

17. B. Blood Vessels
• Walls of blood vessels contain
I. Endothelium
II. Smooth muscle
III. Connective tissue
• With exception to capillaries and postcapillary
venules
• The only structural elements represented are the
endothelium, its basal lamina, and pericytes
• The amount and arrangement of these tissues
in vessels are influenced by mechanical factors,
primarily blood pressure, and metabolic factors
reflecting the local needs of tissues.
Pictorial representation of an artery showing the
structural arrangement of endothelium, smooth
muscles and connective tissue into their respective
Tunica (intima, media and externa)

18. I. Endothelium
• A specialized epithelium that acts as a
semipermeable barrier between two major
internal compartments; the blood and the
interstitial tissue fluid.
• Vascular endothelial cells are squamous,
polygonal, and elongated with the long axis in
the direction of blood flow
• Endothelium with its basal lamina is highly
differentiated to mediate and actively monitor
the bidirectional exchange of molecules by
simple and active diffusion, receptor-
mediated endocytosis, transcytosis, and other
mechanisms
Venule

19. Functions of the endothelium
• Non-thrombogenic surface- prevents clot formation by actively
secreting substances such as heparin, tissue plasminogen activator
and von Willebrand factor.
• Regulate local vascular tone and blood flow by secreting various
factors that stimulate smooth muscle contraction (E.g. Endothelin 1
and angiotensin converting enzyme [ACE]) or relaxation (including
nitric oxide [NO] and prostacyclin).
• Roles in inflammation and local immune responses by secreting
several factors such as P-selectin and interleukins.
• Secrete various growth factors, including proteins promoting
proliferation of specific white blood cell lineages and cells that make
up the vascular wall

20. II. Smooth muscle
• Fibers occur in the walls of all vessels
larger than capillaries and are arranged
helically in layers.
• In arterioles and small arteries, the
smooth muscle cells are connected by
many more gap junctions and permit
vasoconstriction and vasodilation.

21. III. Connective tissue
• Present in the walls of blood vessels in amounts and
proportions that vary based on local functional requirements.
• Has three components
I. Collagen fiber - present in the basement membranes, tunica
media, and adventitia, respectively.
II. Elastic Fiber - Guarantee the resilient shrinkage of the expanded
vascular wall
III. Ground substance - forms a heterogeneous gel in the
extracellular spaces of the vessel wall. Affects the diffusion and
permeability across the wall

22. Structural Plan of Blood Vessels
• Blood vessels are usually composed of the
following layers, or tunics (L. tunica, coat).
i. Tunica Interna - (Tunica Intima)
ii. Tunica Media - middle layer of
smooth muscle, collagen & elastic
fibers.
iii. Tunica Externa - (Tunica Adventitia) -
outermost layer of loose connective
tissue.
Pictorial representation of a transverse section of an artery
showing its structural plan represented by the tunica intima, tunica
media and tunica externa

23. i. Tunica Interna /intima
• The intima (the innermost layer), consists of:-
• Single layer of endothelial cells (simple squamous
cells) supported by a subendothelial layer of
loose connective tissue(Basement membrane)
• Sometimes has smooth muscle fiber
• In arteries the intima includes a thin layer,
the internal elastic lamina, composed of
elastin, with holes allowing better diffusion
of substances from blood deeper into
the wall.
• The smooth inner layer repels blood
cells and platelets preventing thrombosis.
Pictorial representation of the Structural
layers of an Artery(Lt) and a Veins(Rt)

24. ii. Tunica media
• The media consists primarily of
concentric layers of helically arranged
smooth muscle cells.
• Interposed among these cells are
variable amounts of elastic fibers and
lamellae, reticular fibers (collagen type
III), proteoglycans, and glycoproteins.
• This layer is much thicker in arteries
than veins.
• In arteries, the media has a thinner
external elastica lamina, which
separates it from the tunica adventitia.
Arterial cross sectional diagram showing structural layers
(with vasa vasorum in the tunica adventitia)

25. iii. Tunica Externa/Adventitia
• The adventitia consists principally of collagen
(type I) and elastic fibers.
• In larger arteries there is the external elastic
membrane separating it from tunica media.
• The adventitial layer gradually becomes
continuous with the connective tissue of the
organ through which the vessel runs.
• Large vessels usually have arterioles,
capillaries, and venules that branch profusely
in the adventitia and the outer part of the
media.
• Theses are called vasa vasorum ("vessels of the
vessel"), and function to provide metabolites to
the adventitia and the media.

26. Blood vessels
1. Arteries
a. Elastic/conducting Arteries
b. Muscular/Distributing Arteries
c. Arterioles
2. Veins
3. Capillaries
a. Fenestrated
b. Continuous capillaries
c. Sinusoids (Discontinuous capillaries

27. 1. Arteries
a) Elastic/conducting Arteries
• These large vessels are also called conducting arteries because
their major role is to carry blood to smaller arteries.
• Are the aorta, the pulmonary artery, and their largest branches;
• The prominent feature of elastic arteries is the thick media in
which elastic lamellae, each about 10 μm thick, alternate with
layers of smooth muscle fibers.
• The adult aorta has about 50 elastic lamellae (more if the
individual is hypertensive).

28. a) Elastic/conducting Arteries – histology
. In micrograph (a), the three basic layers of the wall can be seen: the tunica intima I, the broad tunica media M and the tunica
adventitia A.
At high magnification in (b), it is seen to consist of concentric fenestrated sheets of elastin (stained black) separated by
collagenous tissue (stained reddish-brown) and smooth muscle fibres (stained yellow). As seen in micrograph (a), the
collagenous tunica adventitia (stained reddish-brown) contains small vasa vasorum V

29. b) Muscular/Distributing Arteries
• Distribute blood to the organs and help
regulate blood pressure by contracting or
relaxing the smooth muscle in the media.
• Larger amounts of smooth muscle cells in
Tunica media
• Have a lesser amount of elastin in their tunica
media
• They have a clear layer of the internal elastic
layer between tunica intima and tunica media
• They have few well defined external elastic
layer between tunica media and tunica
adventitia
• Tunica adventitia is really broad and it
contains elastin and collagen.
-Internal elastic lamina IEL elastic layer between the tunica
intima and the tunica media.
-External elastic lamina EEL lies between the tunica
media M and the adventitia
-The tunica adventitia A is of variable thickness

30. Elastic Arteries Vs Muscular Arteries
• Largest blood vessel in
the body
• Larger-diameter
• Tunica media contain
high proportion of
elastic fiber
• Eg: aorta & pulmonary
trunk
• Most numerous
vessels in the body.
• Medium size
• Tunica media contain
high proportion of
smooth muscle fiber
• Eg: brachial artery or
popliteal artery

31. C. Arterioles
• Arterioles less than 50 μm diameter are
called terminal arterioles.
• The smallest terminal arteriole is < 12 μm
• Internal elastic lamina is poorly developed.
• Tunica media. 1-5 layers of smooth muscle
cells
• Tunica adventitia- very thin connective tissue
layer
• Metarterioles- terminal branches

32. Clinical application
• Aneurysm formation - In atheroma and
with age, the elastic fibers and smooth
muscle of arteries undergo degeneration
and are replaced by non-elastic collagen
leading to dilation of the elastic artery
(usually aorta), i.e – aneurysm
• Atherosclerosis thickening or hardening
of the arteries. It is caused by a buildup
of plaque in the inner lining of an artery.

33. 2. VEINS
• Veins carry blood back to the heart from microvasculature all over the
body.
• Blood entering veins is under very low pressure and moves toward
the heart by contraction of smooth muscle fibers in the media and by
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 of 10 mm or
less.

34. VEINS HISTOLOGY
• All the 3 tunics are present but
not well defined.
• Tunica intima – endothelium +
thin subendothelial layer
• Tunica media – very thin layer of
smooth muscle cells &
connective tissue
• Tunica adventitia – thickest layer
of loose connective tissue &
longitudinally arranged bundles
of smooth muscle cells & nerves
& vasa vasorum

35. ARTERY VEIN
i. Thick and elastic
muscular walls
ii. No valves
iii. carriers of
oxygenated blood
(exception:
pulmonary arteries
which carry de-
oxygenated blood
from the heart to
the lungs).
i. Thin and slightly
muscular walls.
ii. internal valves (to
prevent back flow
of blood)
iii. veins carry de-
oxygenated blood
(exception:
pulmonary veins
which carry
oxygenated blood
from the lungs to
the heart)

36. Structural Plan of Blood Vessels Cont…

37. Clinical application
• Deep Venous Thrombosis (DVT)- blood clot formation in a deep vein,
usually of the legs or pelvic region
• Phlebitis - inflammation of a vein
• Thrombophlebitis - inflammation of a vein with formation of a clot
• varicose vein - swollen and/or twisted veins, usually of the legs

38. 3. CAPILLARIES
• Microscopic vessels that connect
arteriole to venule.
• Capillary wall composed of ONLY a
single layer of endothelial cell & a
basement membrane.
• NO tunica media & NO tunica
externa
• Peculiar- protruding nucleus in the
lumen.
• Flow of blood through capillary is
called Microcirculation.

39. TYPES OF CAPILLARIES
According to histologic types, continuity
of endothelial cells and their basement
membrane capillaries are classified into:
i. Continuous capillaries
ii. Fenestrated capillaries
iii. Sinusoids (Discontinuous capillaries)

40. i. CONTINUOUS CAPILLARY
• The most common type
• Have tight, occluding junctions sealing
the intercellular clefts between all the
endothelial cells to produce minimal fluid
leakage.
• All molecules exchanged across the
endothelium must cross the cells by
diffusion or transcytosis.
• Found in muscle, connective tissue, lungs,
exocrine glands, and nervous tissue.

41. ii. Fenestrated capillaries
• Have tight junctions, but perforations
(fenestrations) through the endothelial
cells allow greater exchange across the
endothelium.
• The basement membrane is continuous
in both these capillary types.
• Are found in organs where molecular
exchange with the blood is important,
such as endocrine organs, intestinal
walls, and choroid plexus.

42. iii. SINUSOIDS CAPILLARIES
• Usually have a wider diameter than the
other types
• Have discontinuities between the
endothelial cells.
• Large fenestrations through the cells, and a
partial (discontinuous) basement
membrane.
• Found in organs where exchange of
macromolecules and cells occurs readily
between tissue and blood, such as in bone
marrow, liver, and spleen.

43. Clinical application
• Systemic capillary leak syndrome(Clarkson’s disease) - Rare disorder
characterized by repeated flares of massive leakage of plasma from
blood vessels into neighboring body cavities and muscles.
• Can result in sharp drop in BPs and can lead to organ failure if not
treated.

44. References
1. Junqueira’s Basic Histology Text and Atlas, 13th edition
2. Wheater’s Functional Histology A Text and Colour Atlas, 6th edition
3. Textbook of Human Histology. Inderbir Singh, 1st Edition
4. Krauses Essesntial Human Histology, 3rd edition