A variety of toxicants that cause oxidative stress can cause DNA damage across a placental like trophoblast barrier1. This damage at a distance does not occur because the toxicants, including metal ions, pass through the barrier. Instead it is mediated by intercellular signalling, involving ATP release and Ca wave propagation, culminating in the secretion of as yet unidentified molecules, which cause DNA damage in the cells beneath and separate from the barrier2. Here we have examined whether this transbarrier signalling in response to Co and Cr ion exposure above the barrier could cause damage to human embryonic stem cells (hESCs). hESCs (H9 cell line) were exposed to Co and Cr6+ (50:40 ppb Co:Cr ) for 24h across a confluent barrier of BeWo cells (a trophoblast cell line). Mass spectrometry analysis confirmed that Co and Cr did not pass through the BeWo barrier. Values are means ± standard deviations, compared by students T test. DNA damage was significantly higher in exposed cells (EC) compared to control cells (CC) when assessed by comet assay (mean tail moment CC 1±0.28 Vs 2.27±0.4, P≤0.05) and staining for γ-H2AX (% cells with ≥4 foci CC 8.76±3.37 Vs 21±2.19 EC, P≤0.05). Cells with >4 γ- H2AX foci appeared in small clusters of neighbouring cells (3-10 cells per cluster) throughout exposed colonies. Addition of connexin mimetic peptide Gap26 to H9 cells prior to exposure partially rescued EC from DNA damage (% cells with ≥4 foci C 9.78±1.70 Vs EC 20.22±1.95, P≤0.01; EC 20.22±1.95 Vs EC + Gap26 15±1.20, P≤0.05) and reduced clustering (% clustered cells CC 5.06±1.38 Vs EC 23±2.07, P≤0.01; EC 23±2.07 Vs EC + Gap26 13.9±1.38, P≤0.01). Cell counts detected a significant decrease in viable, adherent cells following exposure (number of cells recorded in 12 fields of vision at X20 magnification CC 1914.67±190.18 Vs EC 1126.22±264.96, P≤0.001) concomitant to vastly increased TUNEL staining (n=9 control and 9 exposed populations). A similar loss is not seen in cells directly exposed to Co and Cr (DEC), although DEC do have increased DNA damage compared to unexposed control cells (UEC) (comet assay mean tail moment UEC 1±0.29 Vs DEC 5.44±0.13, P≤0.001). This suggests that the loss of EC is not secondary to DNA damage but is caused by the release of a signalling molecule from the barrier in response to Co and Cr exposure. Investigation of barrier conditioned media by ELISA has established TNF α as a potential candidate for cell death effects (absorbance at 450nm CC 0.009±0.008 Vs EC 0.023±0.003, P≤0.05 in one tailed test). This investigation has shown that hESCs may be damaged by exposure to metal ions through a trophoblast barrier. In addition, it has highlighted an important potential biological mechanism, whereby the trophoblast barrier between mother and fetus may secrete an apoptosis inducing signal to the embryo in response to oxidative stress.