PDGFRα⁺ E12 thymic mesenchyme but not kidney capsule mesenchyme expresses insulinlike growth factors, which correlates with IGF1R expression by thymic epithelium. Semiquantitative RT-PCR was used to compare expression of FGF7 (A), FGF10 (B), IGF1 (C), and IGF2 (D) in E12 PDGFRα⁺ thymic (▪) and adult kidney capsule mesenchyme (□). In addition, IGF1-receptor expression (E) was analyzed in thymic epithelium (▪) and PDGFRα⁺ mesenchyme (▪) from E12 thymus lobes. Note that expression of IGF1 and IGF2 mRNA by fetal thymic mesenchyme correlates with IGF1R expression by thymic epithelium. Data shown are representative of 3 separate experiments and are expressed as mean ± SEM.