3 | TECHNICAL ISSUES
Direct localisation in the tissue presents a technical challenge that is greater with plastic NP than carbon – carbon NP can be seen by transmission electron microscopy, though it may be difficult to distinguish from other electron-dense inclusions. Density is difficult to quantify in 3D tissues, and microscopic approaches require morphometry to estimate quantity of NPs at whole organ level. Efficient internalisation of particles by Hofbauer cells – placental macrophages – has been documented in vitro . Presence of NPs in amniotic fluid or cord blood8 can provide a high standard of evidence for transfer across the placenta, but the inadvertent introduction of contaminating environmental NP during sample processing must be rigorously avoided. At present the route of transplacental transmission is unclear.5, 6 In some studies placental tissue was macerated, thus not allowing discrimination between fetal and maternal compartments or maternal blood retained in the intervillous space at delivery.9-11 Another possible approach is to isolate specific single cell subpopulations from NP-exposed placentas for examination.
Studies of particulate matter in placental cells in vivo, in vitro , and ex vivo show evidence of uptake by the syncytium.3, 5, 6 Experimental approaches are useful in building hypotheses regarding mechanisms of functional impairment. These include oxidative stress and DNA damage, altered occurrence of Hofbauer cells, and effects on transport, metabolism, endocrine function, growth, vascular development, coagulation and complement activation, barrier integrity, inflammatory status, the renin-angiotensin system, intraplacental signalling including protein kinase A and altered release of signalling substances to the fetus.2, 3 Transfer within tissue, and more specifically across the placenta to fetal compartments, is affected by particle size, particle material, weathering, charge and surface composition, dose, and gestational stage of the model.3, 12 Plastic NP of varying size and surface charge have been tested in animals and in ex-vivo dual perfusion studies in humans where particles up to 240nm were taken up by the placenta.13
Other studies suggest accumulation in the syncytium or in Hofbauer cells without further translocation.3 Thus an important component of the placenta’s defence strategy involves ingestion and retention. The presence of particles in tissue, especially when this has been reported without quantification, does not justify an assumption that damage is occurring at a level that is consequential for pregnancy outcome. However, cell culture evidence has indicated cytotoxicity in trophoblast and BeWo cells so the possibility that high levels of NP in syncytiotrophoblast could have a deleterious effect on placental function cannot be ignored.