Thymic epithelial cells undergo a temporally regulated phase of cellular proliferation. Freshly dissected thymus lobes were disaggregated and analyzed by flow cytometry to detect expression of cytokeratin and the proliferation marker Ki67. (A) A typical example of an E14 thymus preparation, with double-positive cytokeratin⁺Ki67⁺ cells representing proliferating thymic epithelial cells. Figures in quadrants represent the percentage of the analyzed population. (B) The percentages of cytokeratin⁺Ki67⁺ cells present in the thymi of the indicated ages, together with the percentage of PDGFRα⁺ mesenchyme within the nonhemopoeitic nonepithelial (CD45⁻EpCAM1⁻) thymic fraction. Results are averaged from at least 3 independent experiments, and are presented with standard deviations (error bars).