PDGFRα-expressing fetal mesenchyme regulates thymus growth and the availability of intrathymic niches. (A) Kidneys excised from mice that 3 weeks earlier received either whole unmanipulated (arrow) or mesenchyme-stripped (arrowhead) E12 thymus grafts under the renal capsule. Note the increased growth achieved in unmanipulated versus stripped thymus grafts (B). Importantly, grafts formed from epithelial cores reassociated with mesenchyme prior to transplantation were found to grow and contain similar thymocyte numbers in a manner comparable with unmanipulated thymus lobes. Grafts of unmanipulated of mesenchyme-stripped thymus lobes were analyzed for either thymocyte cellularity (C) or by immunohistochemistry using ERTR7 to identify host-derived mesenchyme that had invaginated the graft (D-E). Data shown are typical of 3 separate experiments. Results are averaged from at least 3 independent experiments, and are presented with standard deviations (error bars).