Equine explants were divided into control and irradiated groups, cultured in explant form for 7 to 10 days, digested, and cultured in monolayer for an additional 10 to 12 days in 10% FBS for maturation of the senescence phenotype. (A) Morphology of control and irradiated chondrocytes from Equine Donor 3 after 12 days of monolayer culture; scale bar = 200 μm. (B) Representative SA-β-Gal flow cytometry plot with the region two standard deviations above control MFI marked (Equine Donor 3). (C) SA-β-Gal mean fluorescence intensity (MFI) fold increase above control quantified; (*) indicates significance by one-sample t-test compared to a hypothetical value of 1, p=0.0272. (D) SA-β-Gal flow quantified as percentage of cells SA-β-Gal high. SA-β-Gal high cells delineated as those expressing SA-β-Gal fluorescence above 2 standard deviations from the control MFI; (#) indicates significance by paired t-test, p=0.0031. (E) Information from equine donors included in the analysis (n=7 thoroughbred horses), no horses excluded from analysis.