Figure 4. Liver. Cells expressing p16 (A) or p21 (B) were identified by IHC staining in the liver of Young, Middle-aged, and Old donors. Graphs represent the quantification (%) of p16-positive (A) and p21-positive (B) cells from 5 tissue cores from independent donors per organ and age group; data represent the means ±SD from 5 different donors. p values were determined by one-way ANOVA with Tukey adjustments for multiple comparisons where appropriate. **, p < 0.01; *, p < 0.05.