Osteoblasts derived from mature chondrocytes in the repair callus are involved in bone fracture healing.

<p>A: Left tibia of a 2.5-month-old <i>Agc1-CreERT2; 2.3-GFP;ROSA-tdTomato</i> mouse and a <i>ROSA-tdTomato</i> control mouse were subjected to semi-stabilized fracture surgery. At 6 days after surgery, these mice were treated with tamoxifen and were scarified 8 days post tamoxifen. Panel a: fluorescence image shows no Tomato and GFP signals in the fractured tibiae of <i>ROSA-tdTomato</i> control. Panel a′: Saf-O staining of the repair callus in a. Images left to a: X-ray images of fractured tibia of control mouse. Panel b: in the fractured tibiae of <i>Agc1-CreERT2; 2.3-GFP;ROSA-tdTomato</i> mouse, Tomato⁺ cells are present specifically in growth plate (white arrow) and in the repair callus outlined by dotted white lines. Some of the Tomato⁺ cells were also positive for GFP (Tomato⁺EGFP⁺ indicated by yellow arrows). The white arrow indicates the growth plate, which was partly broken by the pin inserted into the fractured tibia. Panel b′: Saf-O staining of the callus in b. B: The left tibia of 2.5-month-old <i>Agc1-CreERT2; 2.3-GFP;ROSA-tdTomato</i> mouse was subjected to semi-stabilized fracture surgeries. At 6 days after surgery, the mouse was treated with tamoxifen and scarified 23 days post tamoxifen. Panel a: the fluorescence image revealed that there were more Tomato⁺EGFP⁺ cells in the callus compared to the callus collected 14 days after surgery (Fig. 10A). Rectangle marked panels: magnified images corresponding to the yellow rectangle in panel a. Middle right panel: blue, red and green channels; Bottom middle panel: blue and green channels; Bottom right panel: blue and red channels. Panel b: Saf-O staining shows very little red tissue, suggesting that the callus was almost all ossified.