Fetal thymus colonization in CCR7/CCR9-deficient mice between E11.5 and E14.5. (A) Single-cell suspensions isolated from the fetal thymus of indicated mice (WT, wild-type; R9KO, CCR9-knockout; R7KO, CCR7-knockout; R7/R9KO, CCR7/CCR9-double-knockout) were stained for CD45. The frequency of CD45⁺ cells was measured by flow cytometry. Thymocyte numbers were calculated from the frequency of CD45⁺ cells and the total viable cell numbers identified by the trypan blue dye exclusion method. Symbols indicate thymocyte numbers in individual embryos, and bars indicate means plus or minus standard errors. Numbers indicate means. (B, C) Sagittal sections of frozen embryos from indicated mice were 2-color-stained for CD45 (Alexa 633; green in panel C) and keratin (FITC; red in panel C). Plotted in panel B are the means plus or minus standard errors of the numbers of CD45⁺ cells in and attached to the third pharyngeal pouch that contains the thymic primordium. Representative images are shown in panel C, and the numbers of sections analyzed are indicated in panel B. (D) Number (means ± standard errors) of CD45⁺TER119^-FcR^- E14.5 fetal liver cells from indicated mice that were attracted to the deoxyguanosine-treated E14.5 B6 fetal thymus lobe in 1-day culture was determined as previously described.^6 *P < .05; ^***P < .001; NS, not significant.