Poor placenta development and function can lead to FGR

The main function of the placenta is the exchange of nutrients, gases, and waste products between the maternal and fetal circulations (Watson & Cross 2005). Abnormal development and function of the placental labyrinth, such as abnormal development of the blood vessels necessary for this exchange, or abnormal transport across the maternal-fetal interface, can limit the nutrients that are being delivered to the developing fetus, leading to FGR (Scifres & Nelson 2009). In addition, the adaptive changes of the spiral arteries that happen during pregnancy ensure adequate blood supply to the developing fetus. Ablation of Tpbpa-positive cells, which would normally differentiate into the invasive trophoblast, impairs trophoblast invasion of the spiral arteries, leading to defective SAR (Hu & Cross 2011). Defective remodelling of the spiral arteries results in decreased uteroplacental blood flow, increased vascular resistance, and a subsequent decrease of blood flow to the placenta, causing impaired nutrient and oxygen transport to the embryo, which prevents aerobic fetal growth (Krishna & Bhalerao 2011; Swanson & David 2015). These features are often indicators of pre-eclampsia (Granger et al. 2002). Doppler ultrasound is a non-invasive method used to assess blood flow in uteroplacental circulation (Campbell et al. 1983), by measuring the resistance index and pulsatility index of the uterine arteries as indicators of impedance. High resistance and pulsatility indices are associated with FGR (Dugoff et al. 2005; van den Elzen et al. 1995; Harrington et al. 1997; Khanduri et al. 2017; Martin et al. 2001).

Junctional zone defects that affect vascularisation can also cause FGR. For instance, a hypomorphic mutation of Ascl2 or a targeted ablation of Hrta1 lead to a smaller junctional zone with fewer spongiotrophoblast and glycogen cells, and FGR at E15.5 and E14.5,

respectively (Hasan et al. 2015; Oh-McGinnis, Bogutz & Lefebvre 2011). Overexpression of Ascl2 also leads to a smaller junctional zone with reduced spongiotrophoblast numbers, but with more glycogen cells that mislocalise in the labyrinth (Tunster et al. 2016). Targeted ablation of Esx1 in embryonic stem cells leads to enlarged junctional zone with excessive numbers of glycogen cells and mislocalisation of spongiotrophoblast cells in the labyrinth. In addition, abnormal branching of the fetal blood vessels in the labyrinth was observed. These

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placental defects were associated with FGR between E16.5 and E18.5 (Li & Behringer 1998). Phlda2 overexpression or targeted ablation led to smaller junctional zone with fewer

glycogen cells, or enlarged junctional zone with increased glycogen cell numbers,

respectively. Both overexpression and targeted ablation of Phlda2 led to FGR at E16.5 and E18.5, respectively (Tunster, Creeth & John 2016; Tunster, Tycko & John 2010). Altogether, these studies show that abnormalities in the development and function of the junctional zone can cause FGR.

Labyrinth defects can also cause FGR. Deletion of a labyrinthine trophoblast-specific transcript of Igf2 (P0) led to placental growth restriction by E11.5. Mutant fetuses exhibited growth restriction after E15.5 and had decreased birth weight. This was a result of decreased passive permeability of the placenta, with a compensatory increase of active amino acid transport at the initial stages of pregnancy. During late pregnancy, compensation fails resulting in FGR (Constância et al. 2002; Sibley et al. 2004). Additionally, Pkba-/- _mice

exhibit FGR at E14.5 and have hypotrophic placentas with decreased vascular branching and decreased labyrinth area covered by blood vessels (Yang et al. 2003). Egfl7-/- _conceptuses

have reduced placental weights and develop fetal growth restriction, as a result of reduced fetal blood space in the labyrinth and reduced chorioallantoic branching morphogenesis. Genes involved in branching morphogenesis such as Gcm1, Syna, and Synb were also downregulated in Egfl7-/- _{placentas in E8.5 and E9.5 (Lacko et al. 2017). Moreover, genetic}

ablation of Plk2 or Rgcc results in small labyrinth and defective angiogenesis, leading to FGR in E14.5-E18.5 or E16.5, respectively (Cui, Guo & Chen 2013; Ma, Charron & Erikson 2003). Finally, ablation of Synb leads to abnormal vascularisation of the labyrinth, enlarged maternal blood spaces with extremely thin walls, and FGR at E18.5 (Dupressoir et al. 2011). Placental overexpression of genes can also impair labyrinth development. For instance, overexpression of Flt1 causes altered trophoblast differentiation, small labyrinth, and loss of glycogen cells. In addition, decreased expression of fatty acid and cholesterol transporters CD36 and ABCA1, respectively, was observed. As a result, fetal weight was reduced at E18.5 (Kuhnel et al. 2017). Embryos overexpressing Pgf in T cells exhibited reduced angiogenesis of the placenta, causing placenta detachment from the uterus and pregnancy loss. However, when this defect was not severe enough to lead to pregnancy loss, the

newborn litters were growth restricted (Kang et al. 2014). Altogether, these studies show that misexpression of genes involved in the development of the different placental structures can lead to abnormal placental morphology and function, ultimately causing FGR.

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Abnormal placental development leading to abnormal expression and distribution of transporters, and prevention of direct contact of maternal and fetal blood (Brett et al. 2014) causes abnormal placental transport function, inadequate nutrient delivery to the embryo, and ultimately it impairs embryo growth and development (Bell & Ehrhardt 2002; Brett et al. 2014; Desoye, Gauster & Wadsack 2011; Winterhager & Gellhaus 2017). Placental growth factor (PlGF), VEGF receptor, and kinase insert domain receptor were reduced in placentas from pregnancies complicated with FGR, in contrast to normal, or preeclamptic pregnancies (Alahakoon et al. 2018). Reduced expression of these angiogenic factors, might be the cause of loss of vasculature and villous architecture in the human placentas (Alahakoon et al. 2018). Altogether, these studies show that FGR can be caused by abnormal placenta development leading to inadequate blood flow to the embryo, or inadequate nutrient transport through defective transport systems.