In the classic two-stage model of the syndrome, deficient spiral arterial conversion is thought to lead to placental oxidative stress through malperfusion, which induces the placenta to release factors into the maternal circulation that cause endothelial cell activation

[2] and [3]. There is a wealth of data indicating that placental oxidative high throughput screening compounds stress occurs in the early-onset form of the syndrome [4] and [5], and experiments conducted on term villous explants in vitro have confirmed that oxidative stress is a sufficient stimulus for the release of an array of cytokines and pro-inflammatory factors from the trophoblast [6]. The explant model system has enabled the intermediary signalling pathways activated to be identified [7], and IOX1 the

relevance of these to the in vivo situation is confirmed by the fact that the same changes are seen following labour, when placental oxidative stress is induced through ischaemia–reperfusion secondary to uterine contractions [8]. Oxidative stress can cause widespread disruption of cell function however, and rarely occurs in isolation to other cell stress responses. Over the last decade, close links have been identified between oxidative stress and endoplasmic reticulum (ER) stress, with each being able to induce the other [9], [10] and [11]. The ER is most commonly recognised for its role in the post-translational modification of proteins, but recently only it has emerged that the organelle is also a central co-ordinator of diverse signalling pathways

regulating cell metabolism, proliferation and death. This role is perhaps not surprising given that protein synthesis is central to cellular integrity and function, and is a heavily energy dependent process requiring an adequate supply of nutrients and oxygen. Disturbances of ER function lead to a state known as ER stress, and activate a series of evolutionarily conserved signalling pathways collectively referred to as the Unfolded Protein Response (UPR). Initially, the UPR aims to restore ER homeostasis, but if these attempts fail then the apoptotic cascade is activated. These pathways are now recognised as playing a central role in the pathophysiology of chronic diseases, such as neurodegenerative diseases and diabetes [12]. Here, we consider evidence that they also contribute to the placental pathology in cases of early-onset pre-eclampsia. The ER consists of a series of interconnecting flattened membranous sacs with an intraluminal space of 20–30 nm located in the perinuclear region of a cell, being continuous with the outer membrane of the nucleus. It is responsible for the synthesis and post-translational folding and assembly of all secretory and membrane-bound proteins, including hormones, growth factors and receptors.