Thymus atrophy and loss of thymocytes in tPtch^−/− mice. (A) Thymi from vehicle-treated control animals (top row) and thymus from tPtch^−/− mice 15 and 19 days after the first tamoxifen injection (middle and bottom rows, respectively). (B) Fifteen and 19 days after tamoxifen injection, thymocyte numbers of tPtch^−/− mice were determined by flow cytometry SSC versus FSC plot (n = 4 and n = 3, respectively) and are expressed as relative numbers compared with those obtained from control animals (n = 9 for tamoxifen- and n = 7 for vehicle-treated animals of both time points). Error bars represent standard deviation of the mean. (C) Thymocytes of tPtch^−/− mice were prepared 19 days after the onset of tamoxifen treatment and analyzed by flow cytometry. Cells from vehicle-treated Ptch^flox/floxERT2^+/− and tamoxifen-treated Ptch^flox/floxERT2^−/− animals served as controls. Thymocytes within the lymphocyte fraction (SSC versus FSC, left panels) were analyzed by anti-CD4/anti-CD8 and anti-CD44/anti-CD25 stainings (middle and right panels, respectively). Relative percentages of the different thymocyte populations are indicated. Data are representative of 4 independent experiments (each experiment consisted of 1 vehicle-treated Ptch^flox/floxERT2^+/−, 1 tamoxifen-treated Ptch^flox/floxERT2^−/−, and 1 tPtch^−/− mouse).