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<p>Maternal plasma histamine levels in normal pregnancy are generally similar to those in non-pregnant women during the first trimester, and usually decline gradually during the 2<sup>nd</sup> and 3<sup>rd</sup> trimesters (Brew and Sullivan, 2006). In some complications of pregnancy such as <em>hyperemesis gravidarum</em> (HG), spontaneous abortion (SA), pre-term labour (PL) and Pre-eclampsia (PE) histamine levels increase as pregnancy proceeds (Southren<em> et al.,</em> 1966; Achari, Achari and Rao, 1971; Beaven<em> et al.,</em> 1975), and the elevated levels of histamine may directly cause some of the features of these disorders (Brew and Sullivan, 2006).</p>
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<p>Placenta is a major source of histamine, but it also releases active diamine amine oxidase (DAO; EC 1.4.3.22) into the maternal circulation, which metabolises histamine (Kapeller-Adler, 1944; Gunther and Glick, 1967; Semeniuchenko, 1975; Granerus, Gillbrand and Wetterqvist, 1977; Purcell and Hanahoe, 1991; Brew, Lakasing and Sullivan, 2007).  This breaks down the histamine released from the placenta in normal pregnancy, leading to the limited changes in maternal blood histamine. Clinically, the deported placental DAO activity in maternal blood increases a 1000 fold during normal pregnancy (Southren<em> et al.,</em> 1966).</p>
<p>The DAO activity rises exponentially in the first 24 weeks of normal gestation and plateaus thereafter to form a normal histamine-DAO-axis (nHDA) (Southren<em> et al.,</em> 1966; Beaven<em> et al.,</em> 1975; Dubois<em> et al.,</em> 1977) (Ahlmark, 1944; Southren<em> et al.,</em> 1966; Gunther and Glick, 1967; Weingold and Southren, 1968; Tufvesson, 1978; Beaven<em> et al.,</em> 1975; Dubois<em> et al.,</em> 1977). In contrast, the spontaneous exponential rise of DAO activity is abrogated from gestational week 8 onwards thus, leading to a defective Histamine-DAO-Axis (dHDA) in pregnancies that present with elevated maternal blood histamine (Southren<em> et al.,</em> 1966; Achari, Achari and Rao, 1971; Beaven<em> et al.,</em> 1975; Legge and Duff, 1981). <em>Ex-vivo</em> defective Histamine-DAO-Axis also referred to Elevated Histamine Model (EHM) was developed to mimic <em>in vivo</em> dHDA to study effects of histamine in human placenta. The EHM was developed by creating <em>in vitro</em> culture model to represent normal placentae with nHDA and complicated placentae with dHDA. In the nHDA samples, the endogenous DAO activity is maintained to eliminate endogenous production of histamine, while DAO activity is blocked with aminoguanidine in the dHDA samples to allow histamine levels to elevate during the treatment period. Aminoguanidine is a specific inhibitor of DAO enzyme activity (Tamura<em> et al.,</em> 1989).</p>

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<p>Note:</p>
<p>Syncytiotrophoblast degeneration starts within 24 hours of explant culture and peaks by 48 hours. Syncytiotrophoblast regeneration is established by120 hours of culture. Therefore culture explants for 5 days and apply elevated histamine treatment thereafter for a further 24 hours.</p>
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<p>Explants culltured for 48 hours produced extremely poor quality RNA for high throughput downstream analysis.</p>
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<p>Micro explants incubated at the bottom of the well was associated with decreased explant viability and poor RNA quality.</p>
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