E12 thymus grafts contain host-derived mesenchyme. To determine the origin of mesenchymal cells present in thymus grafts, E12 thymus lobes from eYFP transgenic mice were transplanted into non-eYFP WT adult hosts (A). Grafts were harvested after 3 weeks, and mesenchyme cells were identified on the basis of an EpCAM1⁻CD45⁻ phenotype to exclude all hemopoietic and epithelial cells, together with the markers Ly51 and MTS15. (B-C) Host-derived eYFP⁻MTS15⁺ and eYFP⁻Ly51⁺ mesenchyme cells are readily detectable in the thymus grafts. Data shown are representative of at least 3 separate experiments.