Thymic atrophy in APEX2-null mice. (A) Organ weight. The body weight and weight of each organ of 32- or 33-day-old F₇ male mice were measured, and the weight of each organ was normalized to the body weight of each mouse. The weight of each organ of APEX2-null mice (KO) was expressed relative to the weight of the corresponding organ of wild-type mice (WT) and shown as a percentage ± SD of the WT value. Open columns indicate WT (n = 11); solid columns, KO mice (n = 13). ^**P < .01. No asterisk indicates not statistically significant (P > .05) (B) Total number of thymocytes. Means ± SD of the total number of thymocytes in thymus from 4-week-old male mice are shown. Open column indicates WT (n = 7); solid column, KO mice (n = 7). ^**P < .01. (C) Histology. Thin sections of thymi of 4-week-old male mice were fixed with formaldehyde, embedded in paraffin, and stained with hematoxylin and eosin. Left panels are WT; right panels are KO mouse. (D) Flow cytometric analysis of thymocyte sizes. Thymocytes from WT (upper panel) and KO (lower panel) mice were analyzed by flow cytometry. Size distributions of thymocytes represented by forward scatter heights (FSC-Hs) are shown in histograms. Fractions of small and large thymocytes are indicated by S and L, respectively. (E) APEX2-null thymocytes show abnormal progression of the cell cycle. Isolated nuclei were stained with propidium iodide and cell cycle distribution was determined by flow cytometry. Left panels indicate representative histograms of DNA contents in isolated nuclei of thymocytes from WT (n = 7) and KO mice (n = 8). The distribution of isolated nuclei in each cell cycle phase was determined with the use of ModFit software and is shown with SD in the right panel. ^**P < .01.