Effects of Particles on the Placenta: studies on in vivo and in vitro models.

The relevance to investigate the effects of maternal exposure to engineered nanomaterials (ENPs) has recently emerged. The placenta represents the interface between maternal and foetal circulation, regulating the exchange of nutrients, gases, and waste material, as well as translocation of xenobiotics. Over the last years we have demonstrated the placenta as a site of accumulation of ENPs, and have studied the physic-chemical properties driving translocation across the placental barrier. We recently showed that different amorphous silica nanoparticles (SiO2NPs, I.V. administered) do not induce maternal toxicity, nor affect placental/foetal development. Biodistribution studies demonstrated that particles distributed to placentas and foetuses, although size, surface charge and gestational stage influenced biodistribution. Similarly, silver NPs accumulated in placentas and foetuses after inhalation exposure during the first 15 days of pregnancy, clearly indicating that once NPs access the circulatory system they likely arrive to the placenta, being this a highly vascularized organ. Due to ethical reasons, studies on placental translocation and toxicity of ENPs are meanly performed in rodents; however transposition to humans of results obtained in rodents should be done with caution, as species-specific differences in placental organization exist, which may result in differences in permeability and effects. Currently, the only alternative to study placental translocation in humans is the ex vivo human perfusion model, which however allows short term studies and give no information on the potential toxicity to foetal tissues. Alternative models resembling the human placental barrier are greatly needed. We are currently developing a novel in vitro model based on 2 types of stem cells derived from the pre-implantation embryo: Embryonic Stem cells (ESC), which can be induced to differentiate into all foetal tissues; Trophoblast Stem Cells (TSC) from which all trophoblast lineages can be derived. These cells, available from rodents and humans, can be easily maintained in culture. TSC, cultured on transwell (TW) inserts in the absence of growth factors, resemble the syncytiotrophoblast layer of the placenta. Administration of TiO2 NP in the upper chamber of TW impairs differentiation of ESC cultured in the lower chamber. Presence of syncytial TSC on the TW re-establishes proper differentiation, suggesting that translocation of particles is reduced. Similar results were obtained for the expression of the mesodermal differentiation marker Brachyury, which is highly expressed by ESC after 10 days of differentiation. Culture in the presence of TiO2 NP interferes with Brachyury expression in the absence of syncytial TSC, while the presence of the syncytial layer re-establishes normal expression. Our results indicate that the simulated barrier is able to counteract the adverse effect of TiO2 NP on differentiation of foetal tissues.