Figure 1.

The highly fluorescent periarticular region in Col2/Tomato mice contains multipotent cells that forms the invading canal and reconstitutes subchondral bone. (A): A femur from P4 Col2/Tomato mouse was imaged by confocal microscopy and displayed at low (Aa, Ab) and high (Ac, Ad) intensity threshold. Yellow arrows point to Tomato^H cells. The periarticular region was defined as the 4–6 layers of Tomato^H cells along the epiphyseal surface (white dotted lines). Blue: DAPI. Bar = 200 μm (Aa, Ac), 50 μm (Ab, Ad). (B): High magnification images of the periarticular region (Ba), groove of Ranvier (Bb), growth plate (arrows point to the resting zone; Bc), and primary spongiosa (Bd) of the same femur showing labeling with high Tomato fluorescence. Bar = 50 μm. (C): Col2/Tomato mice were sacrificed postnatally at days indicated and the forming cartilage canal imaged by confocal microscopy. Bar = 100 μm. (D): By P18 (Da), Tomato^H cells were found to give rise to osteoblasts and osteocytes (Db), pericytes along Endomucin⁺ blood vessels (Dc), and adipocytes (Dd). (Db), (Dc), and (Dd) are high magnification images from (Da). Bar = 200 μm (Da, Db), 25 μm (Dc, Dd). (E): Monitoring of secondary ossification center expansion revealed rapid growth during the invasion and expansion phases, with a maximum epiphyseal marrow area being reached by P30. Values represent averages ± SEM (n = 3–5/age).