Introduction:
Placenta is a very important
organ for the growth and development of the embryo and foetus. It forms a
connection between the blood supply of the embryo and the blood supply of
mother; though the connection is being separated by thin membranes. Placenta is
an essential site for the gaseous, waste and nutrient exchange between the
foetus and the mother. Apart from this it synthesizes hormones that maintain
and regulate pregnancy. Development of placenta is a highly regulated process
that is initiated after the implantation of blastocyst in the endometrium. The
outer layer of blastocyst becomes the trophoblast that has an important role in
the implantation and development of the embryo and foetus. It develops
gradually into the placenta and provides nutrition to the foetus. Invasion of
the placenta by the trophoblast and spiral artery re-modelling is essential for
further development of placenta. Therefore, it is very important to know about
the whole complex process of placental development as any deviation from the
normal process of placentation may lead to serious consequences. [Dr.Hills
2010]
Development
of placenta:
Differentiation of trophoblast:
The development of placenta
begins when the blastocyst attaches itself to the maternal endometrium. The
outer layer of the cells that form the blastocyst is termed as trophoblast and
this is the first extra-embryonic cell lineage differentiation. By the 6-7 days
after fertilization, this trophoblast invades the endometrium and gets
differentiated into two layers viz. an inner cytotrophoblast and outer
syncytiotrophoblast [Fig.1]. Other
cytotrophoblast cells differentiate into extravascular trophoblast (EVT) and
this EVT plays a very crucial role in re-modelling of spiral arteries. Cytotrophoblast
is mononuclear and is active mitotically whereas syncytiotrophoblast is
multinucleated and is derived from cytotrophoblast. The cytotrophoblast keep on
making new trophoblast cells. Various factors are responsible for regulating
this trophoblast differentiation in humans like oct-4, Hash-2, FGF-4, Id-2, Stra
13, H1F1a, Gcm-1, BMP, EGF, PPARγ, DIx-4, SDF-1, E-factor, TNFα etc. There must
be synchronization between the implantation and placentation by the blastocyst.
The endometrial extracellular matrix consists of proteins to which trophoblast
attaches and with the help of proteases causes the degradation of these
proteins. [Varney et al. 2004]
Fig.1 Differentiation of trophoblast into cytotrophoblast &
syncytiotrophoblast [cited in
Hills 2010]
Invasion of trophoblast and development of villous structure:
The trophoblast starts to
invade the endometrium after 6-7 days of fertilization by protruding the
endometrial stroma with finger-like projections. From syncytiotrophoblast
finger-like projections arise which penetrate into the endometrial stroma through
the endometrial epithelium [Fig.2]. Endometrial
stroma consists of glands and capillaries. When syncytiotrophoblast invades the
stroma around 8th day, lacunae or hollow spaces start forming
leading to the erosion of endometrial glands and rupture of capillaries. These
capillaries fill the lacunae with embryotropin which is a mixture of secretions
from the glands and maternal blood. This provides nutrition to the embryo.
Around 12th day post fertilization the lacunae in the
syncytiotrophoblast join together to communicate with each other and the fused
lacunae develops into intervillous space (IVS). The IVS is not filled with
highly oxygenated maternal blood until the establishment of foetal vessels in
villi. Therefore, the placenta remains in a hypoxic environment in initial
months. The endometrium undergoes decidual reaction at the same time. The cells
of the stroma become plump by accumulating glycogen and lipids and are known as
decidual cells. The capillaries become dilated to form sinusoids which undergo
erosion by the invading trophoblast to fill the lacunar network with blood. [Blackburn
2007]
Fig.2 showing invasion of trophoblast [cited in Reproductive biology
2010]
Decidualization of the
endometrium plays an important role in protection of the foetus and regulation
of placentation. Decidua consists of three areas viz. decidua basalis (part
that lies below the site of implantation of embryo), Decidua capsularis (part
overlying implanted embryo) and Decidua Patietalis (remaining part of the
decidua) [Fig.3]
Fig.3 showing various parts of decidua [cited in Nursing
crib.com 2010]
Cytortophoblast differentiates
into vascular cytotrophoblast and the extravascular trophoblast. The vascular
cytotrophoblast fuses to form chorionic villi and extravascular cytotrophoblast
is responsible for the remodelling of spiral arteries. Chorionic villi start to
form 14 days post fertilization when cytotrophoblast arranges itself into
column of cells that extend into the syncytiotrophoblast and thus form the
primary villi. The primary villi start branching and are then termed as
secondary villi. The mesenchymal cells that are present in the secondary villi
start to form blood vessels in the villi and these blood vessels are connected
with the blood vessels of chorionic sac and to the embryo through the umbilical
cord. These villi are termed as tertiary villi now [Fig.4]. The villi that are attached to the
decidua basilis are termed as anchoring villi. These villi consist of arteries,
veins, arterioles and venules. The villi that grow from the sides of the
anchoring villi and project into intervillous space are termed as intermediate
villi. Intermediate villi branch and form terminal villi which contain dilated
capillaries.
Fig.4 showing primary, secondary & tertiary villi [cited
in Anatomy, University of Michigan
Medical School 2000]
The cytotrophoblast cells at
the end of the villi penetrate deep into the endometrium and around 8th
week post fertilization these villi cover the whole of the area of the
chorionic sac. The villi present in the decidua basilis keep on proliferating
forming a tree-like structure called as chorionic frondosum. Development of
villous structure and angiogenesis is stimulated by VEGF (vascular endothelial
growth factor), placental-like growth factor and by hypoxic environment. The
villi that are present near the decidua capsularis degenerate as blood flow to
these villi decrease due to the compression of decidua capsularis. This result
in an avascular area called as chorion leave. It fuses with the decidua vera
and forms chorion (outer membrane) and amnioblasts of the cytotrophoblast forms
amnion (inner membrane). The villi near the decidua basilis rapidly increase
their number and have more blood and mesenchymal area. The remnants of the
decidua form the placental septa; these septa separate incompletely forming
placental cotyledons. In between 15-30 cotyledons are present and the septa
restrict the exchange of blood between these cotyledons. [Varney et al. 2004]
Spiral artery remodelling:
During pregnancy the uterine
spiral arteries are remodelled into dilated uteroplacental vessels by the
mechanism classified as physiological changes [Fig.5]. Spiral arteries develop under the
influence of progesterone. After 30-40 days of ovulation the trophoblast starts
to cause the erosion of maternal spiral arteries. The remodelling of spiral
arteries is very essential for foetal growth and development and supply
nutrients to the foetus therefore; they are highly remodelled by invasive
trophoblast for this purpose. Spiral arteries originate from the radial
arteries that are present at the endometrial or myometrial borders.
Fig.5 showing unmodified & Trophoblast modified artery
[cited in Hills 2010]
The trophoblast invades the
spiral arteries and leads to the remodelling of these arteries into dilated and
inelastic vessels. For this process to occur, these arteries undergo to replace
their endothelium and smooth muscle cells, they lose their elasticity, become
dilated and there is loss of vasomotor control [Fig.6]. Trophoblast-dependent apoptopic
mechanism and cell-cell interactions contribute to the smooth muscle cell loss
during remodelling of arteries. Spiral artery remodelling decrease the
resistance of maternal blood flow and increase the blood flow towards the
placenta to maintain the foetal requirement. The changes that occur during
remodelling of arteries can be divided into three steps: - [Kaufmann et al 2003]
1. Vascular
changes that involve trophoblast invasion independent mechanisms that are
thought to be mediated by decidual artery renin-angiotensin systems. Initially
these arteries undergo alterations in their properties, muscular hypertrophy,
vacuolation and dilatation of their lumen.
2. Remodelling
of arteries involving removal of vascular smooth cells and endothelial cells by
invasive extravillous trophoblast (EVT).
3. Infiltration
of the artery walls by the endovascular trophoblast by which endothelial cells
are replaced with endovascular EVT and extracellular fibrinoid is deposited.
Fig.6 showing various steps in uterine spiral artery
remodelling, starting from the non-pregnant stage [cited in Pijnenborg et al.
2006]
Some vascular changes occur
before trophoblast invasion but, invasion by trophoblasts increases the
remodelling of spiral arteries. The failure of vessels to remodel can cause
IUGR, pre-eclampsia or miscarriage. [Pijnenborg et al. 2006]
Pathways of endovascular trophoblast invasion:
The mechanism of trophoblast
invasion of uteroplacental arteries is not understood fully. Invasion by
endovascular trophoblasts does not occur uniformly at placental bed as invasion
being more distinct in the central region. Anatomically endovascular
trophoblast has been thought to follow two different types of pathways viz. extravasation or intravasation pathway [Fig.7] [Kaufmann et al 2003]. The theory of
extravasation says that endovascular trophoblast cells enter the arterial lumen
from the intervillous space by migrating against the blood flow. The
trophoblast cells adhere to the endothelium and infiltrate the vessel walls and
cause changes in the vascular media leading to the loss of smooth muscle cells
and elastic fibres by forming intraluminal trophoblastic plugs. The theory of
extravasation is much supported by studies done on rhesus monkey [Blankenship
et al 1993]. According to intravasation model there is increased movement into
the vessel of interstitial trophoblast from outside and endovascular
trophoblasts represent an end stage of differentiation of interstitial
trophoblasts. Later, the cells of extravillous trophoblast invade the arterial
walls and enter the lumen of spiral arteries. According to Craven et al the
peripheral villi are directed by the uteroplacental flow into marginal veins.
These peripheral villi adhere to the endothelial surfaces cell columns and the
cells of this column extravasate venous walls.
Fig.7 showing A- extravasation & B- intravasation [cited in Kaufmann et al. 2003]
Factors influencing trophoblast invasion & spiral artery
remodelling:
Various factors [Fig.8] regulate
trophblast invasion and remodelling of spiral arteries like HGF (Hepatocyte
growth factor) which promotes trophoblast migration & invasion and effects
cell mobility of extravillous trophoblast by stimulating an increase in nitric
oxide production through the MAPK (Mitogen activated protein kinase) pathway. Members
of IGF (Insulin like growth factor) family like IGF-II bind directly to IGF-R2
(Type-2 IGF receptor) and acts through MAPK pathway to induce trophoblast
invasion. IGFBP-1 (Insulin like growth factor binding protein-1) is also
expressed along with IGF-II. Both IGFBP and IGF-II affect trophoblast invasion
independent of each other. TGF-β (Transforming growth factor β) family which consists
of TGF-β 1, 2 and 3 are thought to restrict extravillous trophoblast
differentiation along the invasive pathway by their anti-proliferative and
anti-invasive properties. TGF-β 2 causes up regulation of α1, α2,
α3, α4 integrin expression which leads to increased
adhesiveness.. The effects of TGF-β are mediated through the factor H1F1α
(Hypoxia inducible factor 1) [Lala & Chakraborty 2003]. LIF (leukaemia
inducible factor) which is a member of interleukin-6 is important for spiral
artery remodelling.
Apart from this, cell adhesion
molecules like integrins, cadherins especially E-cadherin and VE (Vascular
endothelial) cadherin, nectin, connexins, trophinin, PECAM-1 (Platelet
endothelial cell adhesion molecule) also play an important role in trophoblast
differentiation and cell-cell and cell-extracellular matrix interactions [Aplin
et al. 2009]. TNF-α (Tumour necrosis factor) can induce trophoblast apoptosis
alone or with IFN-γ (Interferon γ). MMP’s (Matrix metalloproteinases) are
essential for spiral artery remodelling and angiogenesis. MMP-2 and MMP-9 or
gelatinases A and B play a crucial role by causing trophoblast degradation and
remodelling of extracellular matrix. MMP-26 which is recently identified is
also thought to be involved in tissue remodelling [Cohen et al. 2005]. Many
Growth factors and cytokines such as inhibins and activins e.g. activin-A also
play pivotal role in decidualization, angiogenesis, implantation and apoptosis
[Jones et al 2002]. uNK (uterine natural killer) cells secrete soluble factors
that lead to the disruption of smooth muscle cell disruption and PPARγ
(Peroxisome proliferator activated receptor-γ) expressed in villous and
extravillous cytotrophoblast and syncytiotrophoblast is also one of factors
effecting trophoblast differentiation and remodelling of arteries [Fournier et
al. 2007].
Fig.8 showing summary of important factors regulating
trophoblast invasion & spiral artery remodelling [cited in Hills 2010]
Conclusion:
Differentiation of
trophoblast, invasion of extravillous cytotrophoblast into maternal endometrium
and spiral artery remodelling during placental development is helpful in
establishing an adequate balance between maternal and foetal systems. uNK
cells, macrophages and apoptosis and whole range of signalling cascade is also
involved which ensure normal pregnancy any deviation can cause early pregnancy
loss, pre-eclampsia and IUGR. There are various processes describing about trophoblast
invasion and remodelling like trophoblast independent changes, flow
interruption by endovascular plugging, the two-wave hypothesis of decidual and
myometrial invasion, Intramural incorporation, intravasation & extravasation,
combination of both, endovascular mimicry controversy & maternal vascular
repair and the role of uNK cells in spiral artery remodelling but, which one
holds best is doubtful as each model is deficient in explaining every aspect of
invasion and modelling in one or the
other way. I am in favour of a combination of intravasation and extravasation
model proposed by Kam et al. along with a role of uNK cells as it justifies
infiltration and replacement of arterial media and adventitia by the
interstitial trophoblast cells and replacement of endothelium by invading
trophoblast afterwards. Furthermore, most of these studies are carried out on
human samples so this model sounds more accurate to me as far as its relevance
in humans is considered but, it is difficult to draw a perfect conclusion from
the data available as research work has its own limitations like poor
availability of research tools as the easily available placenta does not cover
the required field which is placental bed and the hysterctomized uteri that are
available are mostly diseased so it becomes very difficult to get the desired
results. Moreover, human samples involve an ethical issue also. The easily
available samples are obviously the animal samples but, their comparability to
humans is an important question. But, as human placenta is heamochorial which
is also found in rats, mice etc. therefore, animal samples can be useful. The
development of human trophoblast cell culture has been very useful in
discovering information on trophoblast differentiation. Though lot of work is
done in this field but, how molecular factors play a role and what mechanism
controls trophoblast invasion exactly is not understood fully. Though immense
research is done in understanding placental development as a whole still lot of
work is required in this area as process of trophoblast invasion and spiral
artery remodelling remains quite controversial. So, I think more of research
work is required in coming times to understand all of this properly.
Dr.Bharati Sood
Nice
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