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As mentioned evt cells have been compared to tumour

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As mentioned, EVT cells have been compared to tumour cellsdue to their invasive properties. However, unlike tumour cells,thebehaviouroftrophoblastcellswithinthedecidualmicroenvironmentiscontrolled.Accordingly,necrosisof thedecidua is not seen as the EVT migrates deep into the tissue,apart from in the enigmatic Nitabuchs layer, a thin rim of fibrinoidtissue subjacent to the shell at the maternal-fetal boundary (Boydand Hamilton, 1970). Furthermore, although placentation is oftenconsidered as an inflammatory process (Chavan et al., 2017), thedefining classical features of inflammation (neutrophil infiltrationfollowedbygranulationtissue,inflammatorycells,capillaryangiogenesis and fibrosis) are always absent. Our scRNA-seqstudypredictsseveralmechanismsthatcouldexplainwhyinflammatory or adaptive immune responses are less likely tooccur in this specialised environment (Vento-Tormo et al., 2018).Abnormal placental development and pregnancycomplicationsMany complications of pregnancy have their origins in abnormaldevelopment of the placenta in the first trimester (Smith, 2010).Theseincludepre-eclampsia,fetalgrowthrestriction(FGR),unexplained stillbirth, placental abruption and preterm labour;these complications are known collectively as the great obstetricsyndromes (GOSs) (Brosens et al., 2011). These conditions areresponsibleforahighproportionofmaternalandneonatalmorbidity and mortality seen in all populations, but particularly insub-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 thematernal blood supply and successful EVT transformation of30-40 spiral arteries deep into the myometrium is essential for normalfetal growth and development (Burton et al., 2009; Collins et al.,2012). If the arteries are not sufficiently converted and retain theircontractile media, there is disordered perfusion of blood flow intothe intervillous space. This, together with an inadequate supply ofnutrients and oxygen, reduces the progressive branching of thevillous tree as gestation proceeds, reducing the surface area availablefor exchange, with the possible outcome of FGR and stillbirth. Inaddition, if the process of regression of the chorion frondiosum to8REVIEWDevelopment (2019) 146, dev163428. doi:10.1242/dev.163428DEVELOPMENT
form the chorion laeve does not occur correctly, the chorionicmembranescanseparateprematurely,resultinginplacentalabruptionorpretermlabour.Pre-eclampsiaresultsfromtherelease of products from the poorly perfused and stressed placentainto the maternal circulation, triggering a systemic endothelialdisorder (Burton et al., 2019; Roberts and Redman, 1993). Thus, theexact clinical outcome of defective trophoblast invasion depends onthe extent of arterial invasion and the number of arteries invaded.

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