Preparation of mesenchyme-free thymic epithelial rudiments. To separate the mesenchymal and epithelial components of the E12 thymus, lobes were incubated briefly in collagenase and then drawn up into mouth-controlled glass pipette (A), which shears surrounding mesenchyme, resulting in a smooth epithelial core (B) (arrowhead). *An unmanipulated E12 thymus lobe with surrounding mesenchyme still attached is shown for comparison (B). Compared with cell suspensions from whole E12 thymus lobes (C), epithelial rudiments prepared in this way are devoid of cells expressing the mesenchyme marker PDGFRα (D). Data shown are typical of 3 separate experiments.