Histological and Immunochemical Examination of Thymus (A–D) P3 normal thymus. (E–G) Mild P3 mutant thymus. (H–K) Severe P3 mutant thymus. (L–O) P13 mutant thymus. Stained with H&E for histology (A, E, H, L), BrdU (B, I, M), β-catenin (C, F, J, N), and K14 (D, G, K, O). (B) Actively dividing thymocytes are visible at the superficial edge of cortex of normal P3 thymus. Note the progression of histological abnormalities in the mutant thymus from mild P3, severe P3 to P13 (A, E, H, L). Scale bars, 20 μm.