Uptake of different-sized microplastic particles in skin and corneal epithelial cells from Atlantic salmon

Abstract Background Given the growing concern over ocean-bound microplastics and their impact on wild and aquacultured aquatic species, understanding the cellular response to such pollutants is crucial. Previous research has shown that salmon keratocyte cells, which form the key physical barrier, can internalize plastic polymers in the 400–1000 nm size range. However, whether there is any size limitation of particles is not yet known. As such, the current study aims to evaluate an approximate cut-off size of particles that could be internalized by fish skin and corneal keratocytes, together with viability upon exposure. Results In the current study, we exposed Atlantic salmon keratocytes to relatively large polystyrene (PS) particles for 24 h and analyzed ingestion or cell-association of particles using deconvolution fluorescence microscopy accompanied by cell viability tests. The present study showed that individual keratocytes internalized particles, where the size limit was approximately 15 µm, while sheaths of cells collectively contained 15–30 µm particles. A single cell could accommodate 1–3 particles in the 7–10 µm range. Cells in cell-sheaths, acting collectively, can accommodate larger particles (up to 30 µm) that individual cells cannot. A cell viability assay confirmed that the cells’ membranes mostly remained intact after exposure to particles up to 30 µm. Larger particles caused nuclear deformation, though nuclear internalization remains unconfirmed and requires further study. Despite particle uptake, keratocytes maintain membrane integrity, and most cells remained viable even after prolonged exposure. Conclusion This study shows that keratocytes can internalize polystyrene particles approx. up to 15 µm in size, with smaller particles being ingested in greater numbers. Although the cells were exposed for a prolonged period, only a minor fraction of cells showed nuclear membrane ruptures. These findings shed light on keratocyte particle internalization, offering insights into cellular responses to foreign particles like nanomaterials and microplastics.