Figure 1.

The normal structure of the thymus gland and the changes that occur with atrophy. The thymus parenchyma is composed of cortex and medulla, which are separated by cortico-medullary junction (CMJ). The cortex is constructed from thymic epithelial cells (TEC) as its framework and thymocytes filling in the interspace. The medulla is composed of TECs, naïve T cells and macrophages. The process of T cell maturation begins with the transformation of double negative (DN) thymocytes to double positive (DP) thymocytes in the outer cortex of the thymus gland. DP thymocytes then undergo positive and negative selection in the inner cortex and medulla, respectively, to become self-tolerant single positive (SP) thymocytes. Finally, SP thymocytes mature and are transported to peripheral lymphoid tissues. Thymic atrophy leads to the destruction of thymus structure and abnormal maturation of thymocytes. In thymic atrophy, the medullary structure is destroyed and the CMJ is disappeared, so both positive and negative selection are interrupted. Without positive selection, premature exiting of DP thymocytes may occur. Without negative selection, the autoreactive T cells may have the opportunity to migrate out of the thymus. In addition, cortical structural disorders not only cause the depletion of a large number of DP thymocytes, but also delay the maturation of SP thymocytes, leading to the accumulation of SP thymocytes. Besides, the figure also shows the main affected sites of thymus corresponding to different pathogens.