Migrant thymic precursors show evidence of lineage restriction prior to contact with thymic epithelium. Cell suspensions prepared from E12 fetal liver (A), isolated perithymic mesenchyme (B) and isolated thymic epithelium (C) were stained for CD45 in combination with either Flt3 or IL-7Rα. Data are expressed as an average of 3 separate experiments ± SEM. ▦ shows the proportion of CD45⁺ expressing high levels of c-kit; ▪, the proportion expressing Flt3; and □, the proportion expressing IL-7Rα. Precursors in E12 fetal liver (D) and E12 fetal thymus (E) were also analyzed by 3-color flow cytometry for expression of CD45, Flt3, and ckit. Panels D and E show c-Kit/Flt3 expression in CD45⁺ cells. Note that in contrast to fetal liver, CD45⁺ cells in the fetal thymus are enriched for ckit^loFlt3⁺ cells. Numbers in graphs indicate percentage of positive cells. Data shown are a result of 3 separate experiments.