Histology of mouse knee joints 3 months after intra-articular injection of either 10-μm microparticles (a, b) or 1-μm microparticles (c, d). Of note, even after 3 months, both types of microparticles are present in the tissue surrounding the joint cavity. Prussian blue (PB) staining provides evidence of iron within the microparticles; see arrows in (b, d). No major signs of inflammation are evident. Original magnifications: × 20 (a, c), × 400 (b) and × 100 (d). Stains: haematoxylin and eosin (a, c) and PB (b, d).