Mast cells stained with toluidine blue. This dye stains the nuclei dark blue and the granules dark purple. Credit: Kauczuk from Wikimedia Commons
Read about mast cells in osteoarthritis:
N. R. Gough, Beyond Allergies: Mast Cells Contribute to Osteoarthritis. BioSerendipity (1 June 2019) https://www.bioserendipity.com/mast-cells-contribute-to-osteoarthritis/