Size-dependent toxicity of polystyrene microplastics disrupts homeostasis and regeneration in Dugesia japonica via stem cell dysfunction and neurotoxicity

Microplastics (MPs) are ubiquitous pollutants, but their toxic effects on freshwater organisms' stem cells, nervous systems, and regeneration remain unclear. This study investigated polystyrene MPs (PS-MPs) with particle sizes of 0.5, 5 and 20 μm at concentrations of 0.1 and 1 mg/L using the planarian Dugesia japonica, a model organism with strong regenerative ability. Results showed PS-MPs entered planarians, accumulated in tissues such as the pharynx, and impaired eyespot regeneration and locomotor function. Mechanistically, PS-MPs induced apoptotic imbalance with stronger pro-apoptotic effects than proliferative impacts, caused significant DNA damage, and dysregulated neoblast-related genes including DjPcna and Djh2b. They also exerted neurotoxic effects, characterized by disrupted neural architecture observed via SYNORF1 immunofluorescence staining, downregulated neurogenes such as Djchat, DjTH and DjTPH, and motor inhibition. This neural impairment may indirectly compromise neoblast function by reducing regulatory factor secretion and interfering with neurotransmitter-mediated stem cell differentiation modulation. Notably, PS-MPs exhibited size-dependent toxicity. The 20 μm particles induced weaker neural disruption and regenerative defects than 0.5 and 5 μm counterparts. All PS-MPs disrupted homeostasis and regeneration, with 20 μm particles exerting the least severe effects overall. These findings clarify MPs toxicity mechanisms and provide valuable insights for freshwater microplastic risk assessment.