Mature thymocytes accumulate in the thymic medulla of Il4ra^−/− mice. (A) Images of 10-wk-old WT and Il4ra^−/− mice thymus. C, cortex; M, medulla; continuous line is the CMJ; dotted lines, boundaries of mTEC-deficient areas of SP thymocyte accumulation. Scale bars in WT and top row of Il4ra^−/− images, 100 µm; bars in the bottom row of Il4ra^−/− images, 50 µm. Images representative of n = 10 mice from three experiments. (B) Quantitation of mTEC-free areas, three to four randomly chosen sections from each mouse were analyzed, n = 4 in two separate experiments. ND, not detected. Error bars indicate SEM; a Mann-Whitney nonparametric U test was performed; ****, P < 0.0001.