Figure 5.

Sox9-expressing progenitors give rise to hybrid cells in the early osteophyte. (a,b,e) Sox9-CreER;Tom mice were induced with tamoxifen at 7 weeks of age. (a) Tom+ Sox9-traced cells (red) in articular cartilage (A), growth plate (G), and scattered within periosteum (P) of knee from 9-week-old uninjured mouse (n=3). Boxed region on left indicates area shown at higher magnification on the right (different tissue sections are shown). (b) Tom+ Sox9-traced cells (red) in osteophyte (outlined with dashed white line on far right) at 2 weeks post-DMM (n=3). Brightfield image of near-adjacent section stained with Safranin O and Fast Green is shown on the left. Boxed region is shown at higher magnification on the far right. (c,d) Double fluorescence in situ hybridisation in wild-type mouse knees at 1 week (c) or 2 weeks post-DMM (d) for indicated mRNA targets. Note co-expression of Col2a1 (red) with Col1a1, Ocn, Spp1 or Col10a1 (green) in the early osteophyte, and absence of Col1a1 in the tibial growth plate (G). Merged and individual channel images of the boxed osteophytes are shown to the right. n=3 for each probe combination. (e) Co-detection of Tom with Col2a1 and Col1a1 mRNA in osteophyte of Sox9-CreER;Tom mouse (n=3). Individual and merged channel images are shown. Note Tom+ Sox9-traced cells (magenta) coexpressing Col2a1 (green) and Col1a1 (red) in outlined area. Fluorescence microscopy images in all panels show nuclear counterstain in blue. Scale bars indicate 100 μm in (a,b) and 200 μm in (c–e). A, articular cartilage; DMM, destabilisation of the medial meniscus; G, growth plate; P, periosteum.