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To compare paired human corneas after storage at 4°C in Chen medium (CM) and Optisol-GS medium (OM) for 7, 10, 14, and 21 days.One cornea of each pair from nine human donors was randomly stored in either CM or OM, with its mate cornea stored in the other medium. Three pairs of corneas were stored for 7 days and two pairs each were stored for 10, 14, and 21 days at 4°C. Baseline corneal thickness measurements and endothelial photographs were obtained with a specular microscope. Corneal thickness measurements were also taken on days 7, 10, 14, and 21 of storage. At the end of storage, the corneas were warmed 2 hours before endothelial photographs were taken and were then placed in fixative. A corneal endothelial analysis system was used to compare changes in endothelial size and shape after storage. After fixation, the corneal endothelium was examined by scanning electron microscopy (SEM), and TdT-dUTP terminal nick-end labeling (TUNEL) assays with 4`6-diamidino-2-phenylindole (DAPI) counterstaining were performed on tissue sections of each cornea. A laser scanning confocal microscope and an automated digital analysis system were used to detect the presence of TUNEL-positive apoptotic cells in each cell layer and to determine keratocyte densities.Mean corneal thickness at 0, 7, 10, 14, 21 days of storage was 0.69 ± 0.05 mm, 0.69 ± 0.06 mm, 0.73 ± 0.08 mm, 0.87 ± 0.04 mm, and 0.87 ± 0.03 mm, respectively, for CM and 0.65 ± 0.06 mm, 0.59 ± 0.07 mm, 0.63 ± 0.03 mm, 0.60 ± 0.03 mm, and 0.69 ± 0.02 mm, respectively, for OM (p < 0.0001). The mean decrease in endothelial cell density at the end of the 7-, 10-, and 14-day storage periods was 11 ± 10% for the CM corneas and 5 ± 5% for the OM corneas (p = 0.18). SEM showed an intact endothelial monolayer in all corneas. The mean percentages of TUNEL-positive cells in epithelium, stroma, and endothelium of CM-stored corneas were 4 ± 4%, 2 ± 3%, and 0.1 ± 0.3%, respectively, and did not differ from the OM-stored corneal values of 4 ± 3%, 2 ± 4%, and 0.9 ± 1.5%. The percentage of TUNEL-positive cells did not increase with storage time. Keratocyte density was 368 ± 130 cells/mm2for CM-stored corneas and 447 ± 96 cells/mm2for OM-stored corneas (p = 0.13).Corneas stored in CM were thicker during storage than those stored in OM. The two storage media did not differ with respect to endothelial cell loss during storage or to the percentage of TUNEL-positive cells or keratocyte density at the end of the storage period.