Model for Fgf7 actions in the involuted thymus. (Left) The number and quality of thymic epithelial cells are reduced in the involuted thymus. As a result, the production of epithelial-derived thymopoietic factors is attenuated. In this environment, ETPs and their downstream progenitors display an aging phenotype characterized by increased Ink4a expression and reduced proliferation. (Right) Fgf7 binds to thymic epithelial cells and induces them to secrete various cytokines. These in turn act on thymocytes and mediate down-regulation of Ink4a expression. As a result, the proliferative potential of ETPs and DN cells is partially restored and the number of T-cell progenitors is increased.