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.