Cells were trypsinized and adjusted to a concentration of 2.5 × 104 cells/cm2 seeded in 96‐well plates (Costar) and incubated for 16 hrs with basal growth medium. Then medium was replaced with basal growth medium in control cultures and with 50% basal growth medium mixed 1:1 with OEC‐CM. Normoxic cultures were placed in both normal‐oxygen and in 1% oxygen conditions. Cellular viability was evaluated at time 0, 3, 8 and 24 hrs. Tests were performed adding a solution of 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (MTT; Sigma‐Aldrich, Saint Louis, Missouri, USA) at concentration of 5 mg/ml and placed for 2.5 hrs in a CO2 incubator. Media were gently removed, MTT solvent (DMSO, Sigma‐Aldrich, Saint Louis, Missouri, USA) was added, and cells were agitated on an orbital shaker for 5 min at RT. The absorbance was measured using a Varioskan Flash spectrophotometer (Thermo Scientific, Wilmington, USA) at 550 nm. Results were expressed as the percentage MTT reduction of control cells. The experiment was performed three times with six replicates per condition each time. Data are shown via standard box‐and‐whiskers plots in which the central‐line represents the median, the upper‐ and lower bounds of the boxes are min and max value and points represent all values expressed as percentage of control, assumed as 100%.