As mentioned evt cells have been compared to tumour

As mentioned, EVT cells have been compared to tumour cells
due to their invasive properties. However, unlike tumour cells,
the behaviour of trophoblast cells within the decidual
microenvironment is controlled. Accordingly, necrosis of the
decidua is not seen as the EVT migrates deep into the tissue,
apart from in the enigmatic Nitabuch ’ s layer, a thin rim of fibrinoid
tissue subjacent to the shell at the maternal-fetal boundary (Boyd
and Hamilton, 1970). Furthermore, although placentation is often
considered as an inflammatory process (Chavan et al., 2017), the
defining classical features of inflammation (neutrophil infiltration
followed by granulation tissue, inflammatory cells, capillary
angiogenesis and fibrosis) are always absent. Our scRNA-seq
study predicts several mechanisms that could explain why
inflammatory or adaptive immune responses are less likely to
occur in this specialised environment (Vento-Tormo et al., 2018).
Abnormal placental development and pregnancy
complications
Many complications of pregnancy have their origins in abnormal
development of the placenta in the first trimester (Smith, 2010).
These include pre-eclampsia, fetal growth restriction (FGR),
unexplained stillbirth, placental abruption and preterm labour;
these complications are known collectively as the great obstetric
syndromes (GOSs) (Brosens et al., 2011). These conditions are
responsible for a high proportion of maternal and neonatal
morbidity and mortality seen in all populations, but particularly in
sub-Saharan Africa (Graham et al., 2016).
Defective trophoblast invasion is the ultimate cause of the GOSs.
Trophoblast cells invade into the decidua to gain access to the
maternal blood supply and successful EVT transformation of ∼ 30-
40 spiral arteries deep into the myometrium is essential for normal
fetal growth and development (Burton et al., 2009; Collins et al.,
2012). If the arteries are not sufficiently converted and retain their
contractile media, there is disordered perfusion of blood flow into
the intervillous space. This, together with an inadequate supply of
nutrients and oxygen, reduces the progressive branching of the
villous tree as gestation proceeds, reducing the surface area available
for exchange, with the possible outcome of FGR and stillbirth. In
addition, if the process of regression of the chorion frondiosum to
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REVIEW Development (2019) 146, dev163428. doi:10.1242/dev.163428
DEVELOPMENT

form the chorion laeve does not occur correctly, the chorionic
membranes can separate prematurely, resulting in placental
abruption or preterm labour. Pre-eclampsia results from the
release of products from the poorly perfused and stressed placenta
into the maternal circulation, triggering a systemic endothelial
disorder (Burton et al., 2019; Roberts and Redman, 1993). Thus, the
exact clinical outcome of defective trophoblast invasion depends on
the extent of arterial invasion and the number of arteries invaded.