STOWAWAYS

In recent years, plastic use has increased significantly, resulting in more plastic waste entering the environment. Plastic consists of polymers and various additives that provide strength, protection, flexibility, and color. In the environment, plastic is broken down into microplastics (<5 mm) and nanoplastics (≤1 µm) through processes such as photodegradation and microbial degradation. These degraded micro- and nanoplastics (MNPs) are classified as secondary microplastics (MPs). In contrast, primary MPs are intentionally added to products such as cosmetics and can also end up in the environment through their use. MNPs can subsequently enter food products and drinking water, making human exposure possible through ingestion, inhalation, and dermal contact. MNPs have already been detected in various human samples, including blood, the placenta, meconium, and amniotic fluid. These findings indicate that both pregnant women and their unborn children are exposed to MNPs. However, the potential effects of these MNPs on human health remain largely unclear. The placenta is a temporary organ formed during pregnancy that has critical functions, such as nutrient transport to the fetus, acting as a barrier, and the synthesis of steroid hormones (steroidogenesis). During pregnancy, steroidogenesis involves the mother, placenta, and fetus to produce essential steroid hormones. These hormones, such as progesterone and estrogens, are vital for maintaining pregnancy and supporting fetal development. Although the potential health effects of MNPs are still unknown, plastic additives and nanoparticles, such as bisphenol A and diesel exhaust particles, have already been shown to disrupt placental function through endocrine interference. While the endocrine disrupting properties of plastic additives are well known, the potential for MNPs themselves to act as endocrine disruptor remains largely unexplored. Given the critical role of the endocrine system during pregnancy and fetal development, it is essential to investigate how MNPs may affect hormonal function. Animal studies, particularly those involving rodents, are frequently used to determine developmental effects in toxicological research. While these models can provide valuable insights, the rodent placenta differs significantly from the human placenta. For instance, the rodent placenta has more tissue layers separating maternal and fetal circulation, a much shorter gestation period (21 days versus nine months in humans), and continued reliance on the corpus luteum for hormone production, whereas in humans, the placenta has a prominent role in steroid hormone synthesis during pregnancy. Currently, several in vitro models are available for placental research, however, many lack the structural complexity of the human placenta and are therefore less representative for the human situation. In this thesis, the potential endocrine disruption potential of MNPs is assessed, with special focus on the human placenta. Chapter 1 introduces the topic and outlines the objectives of this thesis. It provides background on MNPs, the placenta, and endocrine-disrupting substances, and discusses existing in vitro models relevant to MNP research. The types of polymers commonly used in MNP studies are described. We then outline the scope of the thesis and the aims of the thesis. The objectives of this thesis were: • Assess endocrine disrupting properties of MNPs, particularly on ERand AR-activity, and steroidogenesis; • Develop a human-relevant in vitro placental model to study placental barrier and endocrine function of MNPs; • Examine the effects of MNPs on placental function in human-relevant in vitro models, specifically on steroidogenesis; • Explore the developmental effects of MNPs using new approach methodologies.