a, Quantification of mSSC/million events in normal and MF joints of adult (9wks) and aged (1 yr) mice. Graph shows mean + /- s.e.m. Two-tailed Student’s t-test. Exact P values to 2 significant figures. (n = 10 mice per group). b, Representative pentachrome stains of 2-weeks post MF surgery in aged joints with (from left-right) PBS, BMP2, and BMP2 + sVEGFR1. Scale bar 500μm. (n = 3 per group). c, Schematic of the experimental outline. Microfracture surgery is performed on the joints of GFP + adult mice (Ad/MF). GFP + mSSCs are sorted by flow cytometry and cotransplanted with BMP2 + sVEGFR1 into MF defects of irradiated mice. Analysis is performed by brightfield microscopy (BF), immunofluorescence (IF), and pentachrome staining. d, Top, Representative BF (left) and fluorescence microscopy (right) of GFP + Ad/MF mSSC 4 weeks after they were transplanted into the MF defect of an irradiated joint. Scale bars 500μm. (n = 5 mice). Middle, Representative pentachrome stain (scale bar 500μm) shown with higher magnification (scale bar 100μm) and respective IF of GFP + Ad/MF mSSC 4 weeks after they were transplanted into the MF defect of an irradiated joint. Positive staining marked by white arrows. Representatives of three separate experiments. Bottom right, a negative control IF stain was performed for each experiment. Scale bars 500μm.