Introduction: Thymus structure and function through the lifespan 2.1. What is the thymic microenvironment? 2.2. Overview of thymus development and decline 2.3. Cellular components of the thymic microenvironment 2.4. Phenotypic hallmarks of the thymic microenvironment during involution 3. Formation of the microenvironment depends on crosstalk between TECs and multiple cell types 3.1. Thymocyte and TEC differentiation are interdependent 3.2. Epithelial-mesenchymal interactions and the role of NCCs in thymus development 3.3. Thymus compartment formation and elaboration of the vascular network. 4. Evidence for thymic epithelial stem/progenitor cells in the fetal and postnatal thymus 4.1. Defining stem/progenitor cells

4.2. Evidence for fetal thymic epithelial stem/progenitor cells 4.3. Evidence for TESC/TEPC in the postnatal thymus 5. Regulation of the thymic microenvironment: proliferation, differentiation, or both? 5.1. Foxn1 is a key regulator of TEC differentiation and proliferation throughout the thymus lifespan 5.2. The role of cell cycle regulation in maintaining the size of the thymic microenvironment 6. Clinical and translational relevance of understanding the thymic microenvironment 7. Acknowledgements