Microscope with 488, 568 and 633 nm filters. Images were prepared using Zeiss LSM image browsing software. Transmission Electron Microscopy For transmission electron microscopy (TEM), pre-cartilaginous tissue-like aggregates were collected after differentiation, and then washed in PBS before fixation with 4% glutaraldehyde in Millonig’s phosphate buffer for 1 hour at room temperature. Post-fixation was carried out in 2% OsO4 for 20 minutes. The tissues were then dehydrated in ethanol, and subsequently infiltrated with Polybed 812 resin (Polysciences). Polymerization was performed at 37uC for 24 hours. Silver-gray sections were cut with an ultramicrotome equipped with a diamond knife, and sections were stained with uranyl acetate and lead citrate, then examined in an H-7000 Hitachi electron microscope. References 1. McGonagle D, Jones E (2008) A potential role for synovial fluid mesenchymal stem cells in ligament regeneration. Rheumatology (Oxford) 47:1114–6. 2. 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