Compartmentalization of Notch ligand expression in the E12 thymus. PDGFRα⁺ mesenchyme (□) and EpCAM1⁺ epithelial cells (▪) were obtained from E12 thymus and analyzed for expression of Notch ligands by RT-PCR (A-D). Note that in comparison to thymic epithelial cells, with the possible exception of Jagged-1 (C), thymic mesenchymal cells do not express appreciable levels of the Notch ligands analyzed. Data shown are representative of 3 separate experiments and are expressed as mean ± SEM.