Unimpaired migration of CD4⁺ cells in the medulla of CCR7-deficient mice. Cryosections of the thymi from adult (6- to 8-week-old) wild-type (+/+) and CCR7^−/− (−/−) mice were stained with antibodies to CD4 (green) and CD8 (red) to identify DP and SP cells. DP cells appear in yellow. Nuclear staining was performed with 4,6-diamidino-2-phenylindole (DAPI; white). Cortex and medulla topography is reflected by the DAPI stain (left panels). CD4 SP cells are normally found in the medulla of CCR7-deficent animals. (A) A representative region (objective magnification, 4×; numeric aperture [NA], 0.16) is shown of the thymus of CCR7-deficient animals containing several medullary areas, all of them filled with CD4 SP cells (green). Note that even the medullary areas that are misplaced at the outer rim of the organ contain CD4 SP cells (arrows). (B) High magnification picture (objective magnification, 20×/0.75 NA) showing a representative medullary area of the thymus of wild-type (+/+) and CCR7-deficient mice (−/−). Whereas CD4 SP cells are normally found in the medulla of both mouse strains, CD8 SP cells are less abundant in CCR7^−/− mice. Images were acquired using an Olympus BX61 microscope. See “Immunohistology” for more image acquisition information. (C) Mean (± SD) of the numbers of CD4 and CD8 SP cells per medullary area. Quantitative data were obtained from analysis of 3 mice from each genotype.