The E12 thymus anlagen has 3 distinct cellular compartments. E12 thymus lobes were disaggregated and analyzed by flow cytometry for expression of the panhemopoietic marker CD45 (A), and the panepithelial marker cytokeratin together with the mesenchyme marker PDGFRα (B). Note that most cells at this stage are CD45⁻ stromal cells, consisting of 2 dominant cytokeratin⁺PDGFRα⁻ epithelial and cytokeratin⁻PDGFRα⁺ mesenchymal subsets. Data shown are representative of 4 separate experiments. Figures in quadrants are representative of the percentage of the analyzed population.