Thymic epithelial progenitors generate functional and organized microenvironments in the absence of PDGFRα-expressing fetal mesenchyme. Mesenchyme-free E12 thymus rudiments, placed under the kidney capsule for 3 weeks, were analyzed by immunohistochemistry for expression of the cortical epithelial marker cytokeratin 8 and the medullary epithelial marker cytokeratin 5 (A). Note that distinct cytokeratin 5⁻8⁺ cortical and K5⁺8⁻ medullary areas are present and are separated by a cortico-medullary junction (dotted line). Thymocytes harvested from whole (B,D) or mesenchyme-stripped (C,E) E12 thymus grafts were analyzed by flow cytometry for expression of CD4 and CD8, and the αβ T-cell receptor. Results are representative of least 3 independent experiments. Figures in quadrants represent the percentage of the analyzed population.