Inhibition of TPA-induced hyperplasia by phytosphingosine (H&E staining, ×400). Hairless mice were topically treated with TPA (10 nmoles in acetone) and then divided into 3 groups (n = 7). Each group of mice was treated with (A) vehicle (ethanol:propylene glycol, 7:3), (B) phytosphingosine (PS, 20 µmoles), or (C) troglitazone (20 µmoles) 30 min after TPA treatment. Three days after a single application, 4-mm punch biopsies were taken and stained with H&E. (D) Control group. (E) The epidermal thickness was determined using a computerassisted digital morphometric analyzer, ImagePro-Plus software. Data are presented as means ± SEM (n = 7). *Significant at P < 0.05.