Figure 7. Plasmin is required for hard-tissue callus union.

3D μCT reconstruction of a displaced fracture at 42 DPF shows evidence of soft-tissue mineralization away from the fracture callus in Plg^–/– mice (yellow arrowheads). Coronal and sagittal 2D μCT slices of WT and Plg^–/– mouse femurs show cortical bridging by hard fracture callus (yellow pound symbols) in WT mice. In contrast, a failure to unite the proximal and distal hard-tissue callus was observed in Plg^–/– mice (red arrowheads). MSB stains demonstrate the presence of organized woven bone formation within the callus, and safranin O–stained sections confirm complete cortical bridging in WT mice (black pound symbol), whereas the fracture gap in Plg^–/– mice remains composed of chondroid soft-tissue callus (yellow asterisks) and fibrous tissue (yellow plus sign) without evidence of cortical bridging. Immunofluorescence microscopy for fibrin shows abundant fibrin deposition at the fracture site in Plg^–/– mice (red stain with DAPI blue counterstain, white arrowheads). Black boxes in safranin O sections indicate region displayed for fibrin immunofluorescence. n = 15 for each genotype. White scale bars: 1 mm (top two rows), 500 μm (bottom row); black scale bars: 1 mm.