Radiology_May04


Introduction
The existence of all multicellular

organisms on our planet is in part
attributable to the development of a
circulatory system. In vertebrates this
has evolved to a highly complex struc-
ture whose function is to supply oxy-
gen and nutrients to every living cell
within the body, making it the largest
single ‘organ’ within our bodies. Its
development and growth are stimu-
lated by a combination of genetic and
environmentally controlled factors.
The vasculature also undergoes a con-
stant process of maintenance and
repair throughout life, with the entire
vascular tree being replenished on
average every 3 years. This article
reviews some of the basic biology of
normal and abnormal vascular devel-
opment and how this relates to the
development of vascular anomalies.

Vasculogenesis
and angiogenesis 

Vasculogenesis is the first stage of
development of the vascular tree.
This involves the differentiation of

one or more mesodermally-derived
vascular precursor cell types into
angioblasts, the migratory precursors
of endothelial and vascular smooth
muscle cells.1-3 Differentiation of
angioblasts is followed by the associa-
tion and assembly of endothelial cells
to form the primitive vascular plexus.

Angiogenesis is the formation of
new vessels by sprouting or splitting of
pre-existing vessels. Sprouting (or
budding) angiogenesis occurs in vari-
ous phases. During the activation
phase there is disassembly of the vessel
wall followed by proliferation and
migration of endothelial cells into the
surrounding extracellular matrix.
During the resolution, or stabilisation,
phase, the proliferation and migration
of endothelial cells is halted and the
vessel walls, both new and old, are
then re-assembled and stabilised.
Failure to stabilise the new immature
vessel leads to it undergoing apoptosis
(cell death). Non-sprouting angiogen-
esis (also termed splitting angiogene-
sis or intussusception) is characterised
by the growth of interstitial cellular
columns into the lumen of an already
existing vessel so as to partition or
split the vessel and thus remodel the
local vascular network.

Angiogenesis is dependent upon
the complex interaction between
endothelial and smooth muscle cells
and various growth (angiogenic) fac-

tors (Table 1) and the extracellular
matrix (ECM). The ECM consists of
a complex three-dimensional net-
work of (structural) proteins, polysac-
charides and signal molecules sur-
rounding the endothelial cell layer and
contained by the basal lamina.
During the activation stage of angio-
genesis the ECM ‘softens’ and rup-
tures allowing endothelial cells to
migrate and proliferate. During and
after the resolution stage the ECM
helps to reconstitute the vessel wall
and thereafter to maintain its stability.
ECM factors are listed in Table II.

Angiogenesis during the embryon-
ic period is driven by the growing
metabolic demand within the embryo
due to the increase in organ size as
well as the increase in distance
between capillaries and cells resulting
in reduced oxygen diffusion in the
developing tissues. After birth angio-
genesis is driven by normal physiolog-
ical events such as growth, exercise,
the menstrual cycle or pregnancy, or
by pathological conditions such as
tumour, ischaemia, inflammation,
metabolic derangements, trauma, etc.

Arteriogenesis is the remodelling of
pre-existing arteriolar collateral net-
works with coalescence of these to
form larger conductance arteries
which serve to redirect blood flow
toward areas of higher metabolic
demand.

Vasculogenesis, angiogenesis and
arteriogenesis thus form a continuum
involved not only in the primary
development but also the mainte-
nance and repair of blood vessels
throughout life. The regulation of
these processes is brought about by a
genetic and environmentally deter-
mined balance between a multitude of
proangiogenic and antiangiogenic
factors and the extracellular matrix

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18 SA JOURNAL OF RADIOLOGY • May 2004

Vascular malforma-
tions part 1 — normal
and abnormal vascular

development

Ian C Duncan
FFRad (D)

Unitas Interventional Unit
PO Box 14031
Lytlelton 0410



(ECM). A full description of these
growth factors and the ECM (struc-
tural proteins) and their role in vascu-
logenesis/angiogenesis is beyond the
scope of this review, but for further
information the reader is referred to
the excellent reviews on this subject by
Kubis and Levy1,2 and Harrigan.3

The differentiation of primitive
vessels into arteries, veins or capillaries
is determined by flow patterns and is

regulated by a class of molecules
known as the ephrins. The develop-
ment of congenital arteriovenous
malformations may thus be related to
defects in the ephrin signalling sys-
tem. Local environmental stimuli
including shear stress or ischaemia in
arterio-venous malformations (AVMs)
may stimulate angiogenic factors and
produce abnormal vascular remodel-
ling. Differences in the expression of

various structural proteins and angio-
genic factors are seen in different cere-
bral vascular malformations.3,4 For
instance the endothelial and suben-
dothelial layers of AVMs express more
laminin and collagen IV than those in
low-flow cavernous malformations,
whereas cavernous malformations
express more fibronectin than AVMs.
Fibronectin and laminin are structur-
al proteins found in the basement
membranes of normal vessels, and are
thought to maintain the structural
integrity of vessel walls by anchoring
endothelial cells to the underlying
internal elastic membrane and
smooth muscle layers. Fibronectin
tends to predominate in the initial
stages of angiogenesis (activation
stage) where immature vessels with
non-adherent proliferating endo-
thelial cells exist in a fibronectin -rich
extracellular matrix. Laminin is
found mainly during the maturation
phase of angiogenesis where more
mature vessels with adherent non-
proliferating endothelium are found.
Therefore it can be concluded that
cavernous malformations are angio-
genically more immature than AVMs.
This relative immaturity of the walls
of a cavernous malformation results
in a high degree of fragility and thus a
tendency to bleed in the absence of
increased haemodynamic forces.4 In
addition to the differences in the levels
of various structural proteins between
the two malformation types there is
also a difference in the expression of
angiogenic factors. Although vascular
endothelial growth factor (VEGF), b
fibroblastic growth factor (bFGF) and
transforming growth factor α (TGFα)
are expressed in both AVMs and cav-
ernous malformations in keeping
with the active angiogenic processes
associated with their development,

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19 SA JOURNAL OF RADIOLOGY • May 2004

Table I. Angiogenetic growth factors1

1. Vascular endothelial growth factor (VEGF- A-E and PIGF)
• Stimulates angiogenesis 
• Stimulates endothelial protease production
• Causes controlled microvascular permeability to plasma proteins
• Stimulates the formation of angioblasts from precursor cells
• VEGF-C and-D are associated with lymphangiogenesis 

2. Angiopoietins: (Ang 1 to 4)
• Are ligands for the endothelium-specific receptors TIE2
• Stimulate vascular remodelling (Ang1 + TIE2)
• Stimulate endothelial maturation and stabilisation (Ang1 + TIE2 with 

VEGF)
• Ang2 with VEGF produces loss of adhesion of endothelial cells allowing 

their migration to occur

3. Fibroblast growth factor : (aFGF, bFGF)
• Are potent stimulators of endothelial cell migration, proliferation,

sprouting and tube formation 
• Are mainly involved with vascular maintenance and repair 

4. Platelet derived factor (PDGF)
• Promotes proliferation and migration of endothelial cells, vascular 

smooth muscle cells and pericytes

5. Transforming growth factor: (TGFα and β)
• TGFβ is a potent inhibitor of endothelial cell migration and prolif-

eration

6. Others, including EGF, TNFα, ephrins, etc.

Table II. Extracellular matrix factors2

1. Coagulation and fibrinolytic factors:
• Regulation of coagulation is required to avoid thrombosis or haemor-

rhage during neoangiogenesis 

2. Metalloproteinosis (MMP)
• Are proteolytic enzymes that cause degredation of the basal lamina and 

extracellular matrix before endothelial cell migration

3. Integrins:
• Promote cell-cell and cell-matrix adhesion 



bFGF is expressed in the endothelia of
CMs but not in the endothelia of
AVMs. Glial cells adjacent to AVMs
express VEGF whereas those adjacent
to CMs express bFGF and TGFα but
do not express VEGF. Venous
angiomas, by comparison, do not
express growth factors but do express
structural proteins of the mature
phase of angiogenesis.4

The above examples show distinct
biological differences between the dif-
ferent malformation types related to
the variable expression of structural
matrix proteins and angiogenic fac-
tors within each. It would be an over-
simplification to claim that a deficien-
cy or functional abnormality of one or
more specific structural proteins or
angiogenic factors will lead to the
development of a specific malforma-
tion type as the aetiological develop-
ment of vascular malformations is
undoubtedly multifactorial, depen-
ding upon a number of factors includ-
ing genetic defects, environmental
effects (biological and mechanical)
and trigger agents or events.5

Genetic factors
in vascular 

malformations 
There is primarily a basic underly-

ing genetic control of vasculogenic
and angiogenic processes, modified by
metabolic demand and other physio-
logical influences. A clear link between
the development of vascular malfor-
mations and an identifiable genetic
defect has been shown to date in sev-
eral distinct conditions.6,7

1. Hereditary haemorrhagic
telangiectasia (HHT) (Osler-Weber-
Rendu): This is a multisystemic
angiodysplasia which is inherited as
an autosomal dominant trait with

varying degrees of penetrance and
expressivity. Defects in two genes have
thus far been identified as being
responsible for the induction of the
vascular malformations seen in HHT.8

These genes are endoglin (chromo-
some 9, HHT 1) and ALK-1 (chromo-
some 12, HHT 2), both of which
encode for vascular endothelial trans-
membrane receptors of transforming
growth factor -β (TGF-β).1 TGF-β, in
turn, plays a role in endothelial cell
resolution via activation of the ALK-1
pathway, and endothelial cell activa-
tion via activation of the ALK-5 path-
way. A reduction of the levels of
endoglin in HHT-1 may lead to a
decrease in TGF-β levels, affecting
both the ALK-1 and ALK-5 pathways.
The ALK-5 pathway, however, has a
higher sensitivity to the remaining
TGF-β than the ALK-1 pathway
thereby preferentially stimulating
endothelial cell activation. In HHT-2
reduced ALK-1 proteins lead to pref-
erential relative overactivity of the
ALK-5 pathway again stimulating
endothelial cell activation. This acti-
vation then stimulates the production
of the angiogenic inducer vascular
endothelial growth factor (VEGF).
Levels of VEGF have been shown to
be significantly elevated in patients
with HHT.9 Either interruption of
TGF-β function or increased levels of
VEGF or other related angiogenic fac-
tors may thus play a role in the devel-
opment and growth of the various
vascular malformation types seen in
HHT.

2. Venous malformations: Venous
malformations result from mutations
in at least 2 genes on chromosomes
1(1p21-22) and 9 (9p21-22/TIE2).
The latter gene, TIE2, encodes an
endothelial cell surface receptor for a
group of extracellular signalling mol-

ecules called the angiopoeitins which
regulate the maturation of primitive
vessels. In a number of patients with
venous malformations the disease is
inherited as an autosomal dominant
trait. The Blue Rubber Bleb Nevus
syndrome (Bean syndrome) is an
inherited disorder where patients
develop venous malformations in the
skin, gastrointestinal tract, brain and
other organs.

3. Arterial malformations in neu-
rofibromatosis type I: NF I results
from mutations on chromosome 17.
The affected gene, neurofibromin,
encodes an intracellular signalling
protein that has a role in embryonic
vascular development.

4. Familial cavernous malforma-
tion: Cavernous malformations
(CMs) can be classified as either spo-
radic or familial. Some 20-30% of
patients with CMs in North America
may have the familial form in which
an autosomal dominant pattern of
inheritance is seen. The genetic muta-
tion responsible for the development
of these CMs has been identified on
the long arm of the 7q chromosome.10

Further point mutations have been
identified including two on the short
arm of chromasome 7(7p15-13) and
the long arm of chromosome 3(3q
25.2.27)(CCM52 +3).11,12 The first of
these genes, CCM1, has been identi-
fied more recently as encoding the
KRIT1 protein which plays a role in
intercellular signalling particularly to
endothelial cells.13

Thus the importance of the above
diseases lies in the fact that distinct
genetic mutations have now been
identified that have led directly to the
phenotypic expression of vascular
malformation development. All of the
above are inherited as autosomal
dominant traits, rendering them easy

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20 SA JOURNAL OF RADIOLOGY • May 2004



to identify as genetic abnormalities.
Others having an autosomal recessive
or polygenic pattern of inheritance
will undoubtedly be more difficult to
identify, but it is probable that as time
progresses further specific genetic
defects related to the development of
vascular malformations will be dis-
covered.6,7

Lymphangiogenesis 
Lymphatic vessels develop parallel

to blood vessels and drain tissue fluid
that results from normal leakage from
blood vessels. They arise by both
sprouting from existing embryonic
veins as well as by the in situ differen-
tiation of lymphatic endothelial cells
from lymphangioblasts.14 Certain spe-
cific proteins have been shown to
mediate the development and growth
of lymphatic endothelium. These
include vascular endothelial growth
factors C and D (VEGF-C and VEGF-
D) and the receptor VEGFR-3(or
FLT-4).15 It is thought that VEGF-C
induces lymphatic growth mediated
by VEGFR-3. Other genes that have
been shown to be relatively specifical-
ly expressed in lymphatic endothelial
cells include Prox 1, Podoplanin and
LYVE-1. Approximately 35% of
patients with primary lymphoedema
have a positive family history for the
disease. Primary congenital lym-
phoedema (Milroy's disease) results
from mutations on the VEGFR-3 (or
FLT-4) gene found on chromosome
5(5q35.3). Abnormal VEGFR-3 activ-
ity leads to reduced binding of ligands
such as angiopoeitin-1(Ang-1). The
angiopoeitins are ligands for TIE-2, an
endothelial cell receptor important in
the stabilisation and maturation of
the vascular network in embryos.
Ang-1 also seems to inhibit VEGF-
induced vessel permeability. This

would thus lead to increased lymphat-
ic permeability and the development
of lymphoedema. Late onset lym-
phoedema (lymphoedema praecox,
Meige's disease) develops at the onset
of puberty. This syndrome together
with others including lymphoedema
distichiasis (lymphoedema with
abnormal hairs from eyelid meibomi-
an glands), lymphoedema and ptosis,
and yellow nail syndrome are all
linked to inactivation mutations of
the FOXC2 (MFH-1) gene on chro-
mosome 16 (16q24).

Lymphatic malformations, in con-
trast to lymphoedemas, are true mal-
formations composed of dilated lym-
phatic channels or vesicles filled with
fluid but which do not connect with
lymphatic vessels. They are present at
birth and typically enlarge in the pres-
ence of infection.14 They demonstrate
a normal endothelial cell replication
cycle unlike lymphangiomas which
are hypercellular tumours of lym-
phatic origin.16 Because of the devel-
opmental association between veins
and lymphatics, mixed venolymphat-
ic malformations may occur.

Endothelial 
proliferation 

Endothelial cell proliferation
occurs as part of the normal angio-
genic or lymphangiogenic process.
The proliferation of endothelial cells is
induced during the activation phase
of angiogenesis by factors such as
platelet derived growth factor (PDGF)
and is inhibited during the resolution
phase by factors such as transforming
growth factor β1 (TGFβ1).1 Mulliken
and Glowacki16 classified paediatric
vascular malformations based on
endothelial cell characteristics, specifi-
cally the rate of endothelial cell

turnover. They differentiated
between haemangiomas that are
characterised by endothelial cell
hyperactivity during the proliferative
phase (rapid neonatal growth) fol-
lowed by diminishing cellularity dur-
ing the involuting phase later in life,
and vascular malformations that are
characterised by a normal rate of
endothelial cell turnover (compared
with normal blood vessel endothelial
cell turnover). Thus the malforma-
tions remain true malformations
lined with mature endothelial cells,
whereas the haemangiomas are essen-
tially endothelial cell tumours. The
lymphatic equivalents are lymphan-
gioma and lymphatic malformations,
again differentiated according to lym-
phatic endothelial cell growth charac-
teristics. The growth and develop-
ment of an infantile haemangioma
mimics the growth of the normal vas-
cular tree in that early lesions are
highly cellular with plump endothelial
cells lining vascular spaces with very
small lumina, and that as they mature
the endothelium becomes flattened
and the vascular spaces enlarge.17 Thus
haemangiomas represent a form of
‘runaway’ neoplastic endothelial cell
proliferation. Increased endothelial
proliferation is, however, also seen to
occur to a much lesser degree in vas-
cular malformations as well. The rate
of endothelial cell turnover is seven
times greater in cerebral AVMs than in
normal cerebral blood vessels.18

Increased endothelial cell turnover
has also been demonstrated in cere-
bral cavernous malformations.19  Thus
although increased, endothelial cell
growth rates are nowhere near the
scale of those seen in haemangiomas,
remaining the key differentiating fea-
ture between the two vascular abnor-
malities (Table III).

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22 SA JOURNAL OF RADIOLOGY • May 2004

The future
There is currently a tremendous

amount of research being conducted
into the role of angiogenesis and
genetic mutations in the development
of vascular malformations, tumours,
inflammatory diseases and other
pathological processes. Presently
under investigation are a number of
anti-angiogenic agents being tried for
the treatment of various tumours.3

Additionally pro-angiogenic agents
may someday play an important role
in combating the effects of tissue or
organ ischaemia. Future treatments
for vascular and lymphatic malforma-
tions will undoubtedly also involve
gene-based therapies.7

References 

1. Kubis N, Levy BI. Vasculogenesis and angiogen-
esis: molecular and cellular controls. Part 1:
Growth factors. Interventional Neuroradiology
2003; 9: 227-237.

2. Kubis N, Levy BI. Vasculogenesis and angiogen-
esis: molecular and cellular controls. Part 2:
Interactions between cell and extracellular envi-
ronment. Interventional Neuroradiology 2003; 9:
239-248.

3. Harrigan MR. Angiogenic factors in the central
nervous system. Neurosurgery 2003; 53: 639-661.

4. Kilic T, Pamir MN, Kullu S, Eren F, Ozek MM,
Black PM. Expression of structural proteins and
angiogenic factors in cerebrovascular anomalies.

Neurosurgery 2000; 46: 1179-1191.

5. Lasjaunias PL. Segmental identity and vulne-
rability in cerebral arteries. Interventional
Neuroradiology 2000; 6: 113-124.

6. Shovlin CL. Genetic aspects of cerebrovascular
diseases. Interventional Neuroradiology 2000; 6:
107-111.

7. Vikkula M, Boon LM, Mulliken JB. Molecular
genetics of vascular malformations. Matrix Biol
2001; 20: 327-335.

8. Begbie ME, Wallace GMF, Shovlin CL.
Hereditary haemorrhagic telangiectasia (Osler-
Weber-Rendu syndrome): a view from the 21st
century. Postgrad Med J 2003; 79: 18-24.

9. Cirulli A, Liso A, D'Ovido F, et al. Vascular
endothelial growth factor serum levels are ele-
vated in patients with hereditary haemorrhagic
telangiectasia. Acta Haematol 2003; 110: 29-32.

10. Dubovsky J, Zabramski JM, Kurth J, et al. A gene
responsible for cavernous malformations of the
brain maps to chromosome 7q. Hum Mol Genet
1995; 4: 453-458.

11. Craig HD, Günel M, Cepeda O, et al. Multilocus
linkage identifies two new loci for a Mendelian
form of a stroke, cerebral cavernous malforma-
tion, at 7p15-13 and 3q25.2-27. Hum Mol
Genet 1998; 12: 1851-1858.

12. Günel M, Awad IA, Finberg K, et al. Genetic het-
erogeneity of inherited cerebral cavernous mal-
formation. Neurosurgery 1996; 38: 1265-1271.

13. Couteulx SL, Jung H, Labauge P, et al.
Truncating mutations in CCM 1, encoding
KRIT 1, cause hereditary cavernous angiomas.
Nat Genet 1999; 23: 189-193.

14. Brouillard P, Vikkula M. Vascular malforma-
tions: localized defects in vascular morphogene-
sis. Clin Genet 2003; 63: 340-351.

15. Karkkainen MJ, Jussila L, Ferrell RE, Finegold
DN, Alitalo K. Molecular regulation of lym-
phangiogenesis and targets for tissue oedema.

Trends in Molecular Medicine 2001; 7: 18-22.

16. Mulliken JB, Glowacki J. Hemangiomas and vas-
cular malformations in Infants and children: A
classification based on endothelial characteris-
tics. Plast Reconstr Surg 1982; 69: 412-419.

17. Requena L, Sangueza OP. Cutaneous vascular
proliferations. Part II. Hyperplasia and benign
neoplasms. J Am Acad Dermatol 1997; 37: 887-
919.

18. Hashimoto T, Mesa-Tejada R, Quick CM, et al.
Evidence of increased endothelial cell turnover
in brain arteriovenous malformations.
Neurosurgery 2001; 49: 124-132.

19. Sure U, Butz N, Schlegel J, et al. Endothelial pro-
liferation, neoangiogenesis and potential de
novo generation of cerebrovascular malforma-
tions. J Neurosurg 2001; 94: 972-977.

Table III. Characteristics of haemangiomas and vascular malformations

Angiogenesis Endothelial cell
proliferation

Haemangiomas + ++++
Vascular malformations + +