in detail

1. BIOLOGICAL ACTIONS OF
ENDOTHELIUM
Presenter : Dr. AnuPriya J

2. SCHEME
• Introduction
• History
• Structure
• Biological actions of endothelium
-Transport functions
-Vascular tone
-Host defence
-Growth factors
-Haemostasis
-Angiogenesis
• Role of endothelium in disease

3. INTRODUCTION
• Endothelial cells are mesodermal in origin
• Forms an interface between circulating blood or
lymph in the lumen and the rest of the vessel wall.
• Also has various important biological functions.
• Key determinants of health and disease in blood
vessels.

4. HISTORY
• 17th century – William Harvey – circulation of blood
• 19th century – new view of the circulatory system – tissues and
cells
• Friedrich von Recklinghausen – Virchow’s assistant – credited with
establishing a method for staining lines of cell junctions with silver
• Endothelium – first described by Virchow in capillaries as a simple
membrane with flattened nuclei
• Swiss Anatomist Wilhelm His – introduced the term endothelium.

5. HISTORY
• Waldeyer – suggested restricting the term to those cells that
make up the innermost layer of blood and lymph vessels &
the posterior lining of the cornea.
• 20th century textbooks – Gray’s anatomy, A.A. Bohm and
Colleagues’ textbook of Histology and other books.
• Newer advances – implications of technology – microscopy
and cell culture.
• ENDOTHELIUM IN ACTION – NOT JUST A COVERING.

6. STRUCTURE

7. • The cells that form the endothelium are called endothelial
cells.
• Endothelial cells in direct contact with blood are called
vascular endothelial cells, whereas those in direct contact
with lymph are known as lymphatic endothelial cells.
• The epithelial lining of the vascular system
• Almost always simple squamous epithelium with some
exceptions
STRUCTURE

9. • Endothelial cells are very flat, have a central nucleus, are
about 1-2 μm thick and 10-20 μm in diameter.
• The cells are typically flat and elongate, with their long axes
oriented parallel to the direction of blood flow in the artery.
• Nuclei of endothelial cells are also elongated in the direction
of blood flow.

11. STRUCTURE
• The cytoplasm is relatively simple with few organelles, mostly
concentrated in the perinuclear zone.
• The most obvious feature is the concentration of small
vesicles (pinocytotic vesicles)
• Weibel-Palade bodies : Rod like cytoplasmic inclusions –
electron dense structures which contain Von Willebrand
Factor(vWf)

13. • Endothelial cells are connected by adherens, tight and gap
junctions.
• Fenestrated – glomeruli, capillaries of endocrine glands
• Sinusoidal – liver sinusoids.

14. Biological actions
• Maintenance of extracellular matrix
• Transport function
• Pro and antithrombotic
• Defense mechanism
• Vascular tone
• Regulation of cell growth
• Angiogenesis

15. Maintenance of extracellular matrix
Synthesis of
• Basal lamina – type IV collagen, laminin
• Glycocalyx – proteoglycans

16. Transport function
• Maintenance of selective permeability barrier
• Simple diffusion – oxygen, carbon dioxide.
• Active transport – glucose, amino acids, electrolytes.
• Pinocytosis – water, small molecules, soluble proteins.
• Receptor mediated endocytosis (clathrin dependent process)
– LDL, transferrin, growth factors, antibodies, MHC complexes.

17. Regulation of immune responses
• Secretion of interleukins (IL-1,IL-6,IL-8)
• Expression of MHC II molecules
• Cell adhesion molecules and their receptors
expressed on the endothelial surface.
• Leucocyte rolling

19. Pro and antithrombotic
• Smooth surface of endothelium - glycocalyx, a proteoglycan
coat
• Negative charge on the surface of endothelial cells –
glycosaminoglycans - mainly heparan sulfate
• Anticoagulants and antithrombotic substances present on
intact endothelial surface.
• Prothrombotic substances released from damaged
endothelium.

20. Pro and antithrombotic
Secretion of
• Anticoagulants – thrombomodulin, tissue factor pathway
inhibitor & others
• Antithrombogenic agents – prostacyclin, tissue plasminogen
activator, antithrombin III, heparin
• Prothrombogenic agents (released after damage to the cells)
– tissue thromboplastin, von willebrand factor, plasminogen
activator inhibitor.

22. Regulation of cell growth
Secretion of
• Growth stimulating factors – PDGF, GM-CSF,
G-CSF, M-CSF,FGF
• Growth inhibiting factors – Heparin, TGF-β

23. Vascular tone
Secretion of
• Vasoconstrictors – endothelin, angiotensin
converting enzyme.
• Vasodilators – endothelium derived relaxing
factor/nitric oxide, prostacyclin

24. Nitric oxide

25. Nitric oxide
o Nitric oxide synthase – 3 isoforms (NOS 1, NOS 2, NOS 3).
o NOS 3 – present in endothelial cells.
Functions
• Vasodilation
• Inhibits platelet aggregation
• Inhibits transcription of adhesion molecules
• Inhibits vascular smooth muscle proliferation
Stimulus
• Shear stress, Inflammation ,acetylcholine, substance P,
bradykinin, serotonin, endothelin, thrombin
Inactivated by hemoglobin
Action lasts for 3-6 s

26. Prostacyclin - actions synergistic to NO
Carbon monoxide – acts in the same manner
as NO

28. Endothelin
• Endothelial cells produce endothelin-1 (ET1)
• Potent vasoconstrictor
• Secretion – regulated via transcription of ET1 gene
Preprohormone
Big endothelin 1
ECE
Endothelin 1
• ECE – Endothelin converting enzyme

29. Endothelin

30. Endothelin
• ETA receptor – Specific to ET-1
– Found in many tissues
– Mediates vasoconstriction produced by ET-1
– Acts via cAMP
• ETB receptor – responds to all three endothelins
– coupled to Gi
– Mediates vasodilation

31. Angiogenesis
• Vasculogenesis – formation of new blood vessels
denovo – embryonic life – angioblasts from
mesoderm
• Angiogenesis – formation of new blood vessels from
pre-existing blood vessels - Stimulants : pregnancy,
hypoxia, inflammation, trauma, tumors

32. Angiogenesis
• Facilitated by vascular endothelial growth factor(VEGF).
• Also by Angiogenin, FGF,α5β1 integrin
• VEGF is produced by many cell types including tumor
cells, macrophages, platelets, keratinocytes and renal
mesangial cells.
• Release stimulated by HIF produced by hypoxic cells.
• Regulation of vascular cell growth in the placenta, wound
healing, tissue repair and tumor growth.

34. APPLIED ASPECTS
• Animal experiment – inhibit NOS – prompt rise in BP
This suggests – tonic release of NO – necessary to
maintain BP
• Nitroglycerin & other vasodilators – stimulate
guanylyl cyclase - treat angina

35. APPLIED ASPECTS
ASPIRIN
- Irreversible inhibition of cyclooxygenase
- Reduction of both TXA2 and PGI2
- PGI2 – produced by endothelial cells
- TXA2 – produced by platelets
- Endothelial cells produce new COX in a matter of hours
- Platelets cannot manufacture the enzyme COX – the level
rises only after new platelets enter the circulation (4-8 days)
- Therefore, aspirin – reduces clot formation
- Prevent MI, Unstable angina, TIA, Stroke

36. APPLIED ASPECTS
• Topical VEGF, PDGF and FGF help in wound healing.
• Inhibitors of VEGF, PDGF and FGF → treat cancer -
prevent angiogenesis - stop or slow the growth or
spread of tumors.
• Colony stimulating factors injected - hemopoiesis

37. Atherosclerosis
• Damaged endothelium – free radicals
• Free radicals + LDL – Oxidized LDL
• Oxidized LDL + macrophages – foam cell
• Nitric oxide – reduced

38. Endothelium in cardiovascular disease
• Secretion of endothelin is increased by a variety of stimulants
including hypoxia, catecholamines and angiotensin II.
• Activity of nitric oxide is blunted.
• Plasma concentration of endothelin is elevated and the levels
correlate with hemodynamic disturbance.
• The major source of circulating endothelin in heart failure is
the pulmonary vascular bed.

39. Endothelium in cardiovascular disease

40. Endothelium in cardiovascular disease
• Endothelin-1 is a potent vasoconstrictor and mitogen that
binds to endothelin A and B receptors in the pulmonary
vasculature.
• Acute intravenous administration of endothelin antagonists
improved hemodynamics in patients with heart failure.
• Oral endothelin antagonists are being developed.

41. Smoking
• Nicotine - opens up the intercellular junctions and
allow large molecules to pass through the wall.
• Such toxins can potentiate degenerative changes in
blood vessels and lead to vascular disease.

42. Clinical Assessment of Endothelial Function
• Endothelial function can be assessed invasively using
acetylcholine, which induces endothelium-dependent dilation
and smooth muscle–mediated constriction.
• In healthy coronary arteries, endothelium-dependent dilation
predominates. In the presence of endothelial damage,
vasoconstriction predominates.
• The coronary artery diameter is compared by quantitative
angiography before and after infusion of acetylcholine.

43. Clinical Assessment of Endothelial Function
• Non invasive method - High-resolution ultrasound to measure
the brachial artery diameter in response to reactive
hyperemia.
• Close correlation between endothelial dysfunction in the
forearm and coronary endothelial dysfunction.
• Endothelial function correlates inversely with serum
C-reactive protein (CRP).
• Endothelial cell activation leads to increased expression of
inflammatory cytokines and adhesion molecules
• E-selectin, vascular cell adhesion molecule 1, intercellular
adhesion molecule 1, and P-selectin.

44. • VENDY’S ENDOTHELIAL FUNCTION
MEASUREMENT VIDEO

45. References
• Ganong's Review of Medical Physiology, 24th Edition
• Guyton and Hall Textbook of Medical Physiology, 12th Edition
• Histology: A Text and Atlas Michael H. Ross, 4th Edition
• Kumar and Clark's Clinical Medicine, 8th Edition
• Robbins and Cotran Pathologic basis of disease, 7th Edition
• Best & Taylor's Physiological Basis Of Medical Practice, 13/ E.
• Internet references

46. You are only as old as your endothelium
-- Paul VanHoutte, Mayo Clinic (1983 )

47. • http://circ.ahajournals.org/content/1
15/10/1285.full
•http://circ.ahajournals.org/content/1
09/23_suppl_1/III-27.full

48. Atherosclerosis

51. • An active role in supplying nutrients to the
subendothelial structures.
(note : tunica intima and media – blood supply
from blood in vessel lumen
tunica adventitia – has its own blood vessels –
k/a vasa vasorum in artery)

52. • Nitric oxide prevents oxidative modification of low-density
lipoprotein (LDL) cholesterol.9 Oxidation of LDL has been proposed
as a major mechanism of the atherosclerotic
process;10furthermore, plasma and macrophage content of oxidized
LDL in coronary plaques correlate with severity of acute coronary
syndrome.11 Conversely, impaired production or activity of NO leads
to events or actions that promote atherosclerosis, such as
vasoconstriction, platelet aggregation, smooth muscle cell
proliferation and migration, leukocyte adhesion, and oxidative
stress.12 Oxidized LDL cholesterol increases synthesis of caveolin-1,
which inhibits production of NO by inactivating eNOS.2 Oxidative
stress can also interfere with the production and activity of NO by a
number of mechanisms that are independent of LDL. For example,
the free radical superoxide anion rapidly inactivates NO and
destroys tetrahydrobiopterin, a cofactor required for NO
synthesis.13

53. • The blood–brain barrier (BBB) is a highly selective permeability barrier that separates the
circulating blood from the brainextracellular fluid (BECF) in the central nervous system (CNS).
The blood–brain barrier is formed by capillary endothelial cells, which are connected
by tight junctions with an extremely high electrical resistivity of at least 0.1 Ω⋅m. The
blood–brain barrier allows the passage of water, some gases, and lipid soluble molecules by
passive diffusion, as well as the selective transport of molecules such as glucose and amino
acids that are crucial to neural function. On the other hand, the blood–brain barrier may
prevent the entry of lipophilic, potential neurotoxins by way of an active transport
mechanism mediated by P-glycoprotein. Astrocytes are necessary to create the blood–brain
barrier. A small number of regions in the brain, including the circumventricular organs (CVOs),
do not have a blood–brain barrier.
• The blood–brain barrier occurs along all capillaries and consists of tight junctions around the
capillaries that do not exist in normal circulation.[1] Endothelial cells restrict the diffusion of
microscopic objects (e.g., bacteria) and large or hydrophilic molecules into thecerebrospinal
fluid (CSF), while allowing the diffusion of small hydrophobic molecules (O2, CO2,
hormones).[2] Cells of the barrier actively transport metabolic products such as glucose across
the barrier with specific proteins.[citation needed] This barrier also includes a thick basement
membrane and astrocytic endfeet.[3]

55. Guyton
• in Formation of New Blood Vessels .Three of those that have been best characterized are
vascular endothelial growth factor (VEGF), fibroblast growth factor, and angiogenin, each of
which has been isolated from tissues that have inadequate blood supply.
• Presumably, it is deficiency of tissue oxygen or other nutrients, or both, that leads to
formation of the vascular growth factors (also called "angiogenic factors").
• Essentially all the angiogenic factors promote new vessel growth in the same way. They cause
new vessels to sprout from other small vessels. The first step is dissolution of the basement
membrane of the endothelial cells at the point of sprouting. This is followed by rapid
reproduction of new endothelial cells that stream outward through the vessel wall in
extended cords directed toward the source of the angiogenic factor. The cells in each cord
continue to divide and rapidly fold over into a tube. Next, the tube connects with another
tube budding from another donor vessel (another arteriole or venule) and forms a capillary
loop through which blood begins to flow. If the flow is great enough, smooth muscle cells
eventually invade the wall, so some of the new vessels eventually grow to be new arterioles
or venules or perhaps even larger vessels. Thus, angiogenesis explains the manner in which
metabolic factors in local tissues can cause growth of new vessels.

56. Guyton
• Certain other substances, such as some steroid hormones, have exactly the opposite effect on small
blood vessels, occasionally even causing dissolution of vascular cells and disappearance of vessels.
Therefore, blood vessels can also be made to disappear when not needed. Peptides produced in the
tissues can also block the growth of new blood vessels. For example, angiostatin, a fragment of the
protein plasminogen, is a naturally occurring inhibitor of angiogenesis. Endostatin is another
antiangiogenic peptide that is derived from the breakdown of collagen type XVII. Although the precise
physiological functions of these antiangiogenic substances are still unknown, there is great interest in their
potential use in arresting blood vessel growth in cancerous tumors and therefore preventing the large
increases in blood flow needed to sustain the nutrient supply of rapidly growing tumors. Vascularity Is
Determined by Maximum Blood Flow Need, Not by Average Need An especially valuable characteristic of
long-term vascular control is that vascularity is determined mainly by the maximum level of blood flow
need rather than by average need. For instance, during heavy exercise the need for whole body blood
flow often increases to six to eight times the resting blood flow. This great excess of flow may not be
required for more than a few minutes each day. Nevertheless, even this short need can cause enough
VEGF to be formed by the muscles to increase their vascularity as required. Were it not for this
capability, every time that a person attempted heavy exercise, the muscles would fail to receive the
required nutrients, especially the required oxygen, so that the muscles simply would fail to contract.
However, after extra vascularity does develop, the extra blood vessels normally remain mainly
vasoconstricted, opening to allow extra flow only when appropriate local stimuli such as oxygen lack,
nerve vasodilatory stimuli, or other stimuli call forth the required extra flow.

57. Angiogenic Cascade
• Tm interaction with vasculature switch to angiogenic
phenotype, enabling tumor progression
- Endothelial receptor binding /
activation
- Formation of angiogenic mother
vessels
- Morphogenesis of mother vessels
- Basement membrane dissolution
- Endothelial cell proliferation
- Endothelial cell migration
- Vascular tube formation
- Arterial-venous differentiation
- Vascular stabilization

59. Guyton
• When blood flows through the arteries and arterioles, this causes shear
stress on the endothelial cells because of viscous drag of the blood against
the vascular walls. This stress contorts the endothelial cells in the
direction of flow and causes significant increase in the release of NO. The
NO then relaxes the blood vessels. This is fortunate because the local
metabolic mechanisms for controlling tissue blood flow dilate mainly the
very small arteries and arterioles in each tissue. Yet, when blood flow
through a microvascular portion of the circulation increases, this
secondarily stimulates the release of NO from larger vessels due to
increased flow and shear stress in these vessels. The released NO
increases the diameters of the larger upstream blood vessels whenever
microvascular blood flow increases downstream. Without such a
response, the effectiveness of local blood flow control would be decreased
because a significant part of the resistance to blood flow is in the
upstream small arteries. NO synthesis and release from endothelial cells
are also stimulated by some vasoconstrictors, such as angiotensin II, which
bind to specific receptors on endothelial cells. The increased NO release
protects against excessive vasoconstriction.

60. CORNEAL ENDOTHELIUM
• Misnomer here
• a simple squamous or low cuboidal monolayer of mitochondria-rich cells
responsible for regulating fluid and solute transport between the aqueous
and corneal stromal compartments. (The term endothelium is a misnomer
here. The corneal endothelium is bathed by aqueous humour, not by
blood or lymph, and has a very different origin, function, and appearance
from vascular endothelia.) Unlike the corneal epithelium the cells of the
endothelium do not regenerate. Instead, they stretch to compensate for
dead cells which reduces the overall cell density of the endothelium and
has an impact on fluid regulation. If the endothelium can no longer
maintain a proper fluid balance, stromal swelling due to excess fluids and
subsequent loss of transparency will occur.
• Not derived from mesoderm
• Derived from neural crest

61. Pro and antithrombotic
Why blood doesn’t clot in an intact blood vessel?
• ADPase enzyme on the membrane degrades ADP
• Tissue Factor Pathway Inhibitor on the membrane
• Thrombomodulin functions as a cofactor in the thrombin-induced
activation of protein C in the anticoagulant pathway
• Prostacyclin synthesis
• Inactivate vasoactive substances

64. • The foundational model of anatomy makes a
distinction between endothelial cells and
epithelial cells on the basis of which tissues
they develop from, and states that the
presence of vimentin rather than keratin
filaments separate these from epithelial
cells.[4] Many considered the endothelium a
specialized epithelial tissue.[5]