T-lymphoid progenitor cells in CCR7/CCR9-deficient embryos. (A) BrdU was pulsed for 2 hours to pregnant mice carrying E14.5 embryos, and the frequency of BrdU-incorporated cells in CD45⁺ embryonic thymocytes was measured by flow cytometry. Bars indicate means plus or minus standard errors (n = 3 to 8), and the results show no significant difference. (B) Ex vivo E14.5 fetal thymocytes of indicated mice were stained with annexin V. Bars indicate means plus or minus standard errors (n = 4 to 10), and the results show no significant difference. (C-F) Fetal liver cells (FL, panels C and E) and fetal blood cells (FB, panels D and F) from E12.5 embryos of indicated mice were 3-color stained for Lin (TER119, B220, Gr-1, Thy1.2 and NK1.1), CD117, and PIR. Plotted in panels C and D are the means plus or minus standard errors (n = 3 to 7) of the number (FL, panel C) and the frequency (FB, panel D) of Lin^-CD117⁺PIR⁺ cells, and no significant difference was noted among the 4 groups. Representative flow cytometry profiles are shown in panels E and F. Histograms shown on the left indicate Lin expression profiles and the gates for Lin^- cells. Numbers with dot profiles of CD117 versus PIR in Lin^- cells indicate frequency within the boxes. (G) Fetal liver cells from E12.5 embryos of B6 mice were 4-color stained for Lin, PIR, CCR9, and either CD117 or CCR7. Representative flow cytometry profiles of Lin versus PIR (left, bold), CCR9 versus CD117 within Lin^-PIR⁺ population (left, top), and CCR9 versus CCR7 within indicated Lin/PIR populations (right) from 3 independent measurements are indicated. Numbers indicate frequency within the boxes.