Thymus

 

 

      The thymus is a lymphopoietic organ located in the upper part of the anterior mediastinum. It is a bilobular organ demarcated into an outer cortex and central medulla. The cortex is densely packed with small lymphocytes (thymocytes), cortical epithelial cells, and a few macrophages. The medulla is less cellular and contains more mature thymocytes mixed with medullary epithelial cells, dendritic cells, and macrophages. The primary purpose of the thymus is to serve as a compartment for maturation of T lymphocytes. Precursor T cells leave the bone marrow and enter the thymus through arterioles in the cortex. As they travel through the cortex and the medulla, they interact with epithelial cells and dendritic cells, which provide signals to ensure that T cells can recognize foreign antigen but not self-antigen. They also undergo rapid proliferation. Only about 3% of the cells generated in the thymus exit the medulla as mature T cells. The rest die by apoptosis and are removed by thymic macrophages. The thymus is responsible for supplying the T-dependent areas of lymph nodes, spleen, and other peripheral lymphoid tissue with immunocompetent T lymphocytes.

     The thymus is a well-developed organ at birth and continues to increase in size until puberty. After puberty, however, it begins to atrophy until in old age it becomes barely recognizable. This atrophy could be driven by increased steroid levels beginning in puberty and decreased growth factor levels in adults. The atrophied thymus is still capable of producing new T cells if the peripheral pool becomes depleted as occurs after the lymphoid irradiation that accompanies bone marrow transplantation.

FIGURE: A schematic drawing of the thymus. Hassall’s corpuscles are collections of epithelial cells that may be involved in the development of certain (regulatory) T cell subsets in the thymus.