FIGURE 1.

Effects of age on thymic development and function. Age‐related thymic involution leads to a gradual reduction in thymic cellularity and thymic stromal microenvironment disruption, including the loss of definite cortical‐medullary junctions, a reduction in cTECs and mTECs, fibroblast expansion, an increase in perivascular space (PVS), and more. The disrupted thymic stromal microenvironment disturbs thymocyte development causing decreased ETP and DP frequency, increased DN frequency, and abnormal CD3⁺ DN cell accumulation. The young thymus is able to produce functionally competent T cells expressing a broad TCR repertoire, whereas the aged thymus produces fewer naïve T cells with a restricted TCR repertoire