Thymus and Mediastinum

The mediastinum is a potential space within the thoracic cavity that can be defined by its bordering structures or is divided into different compartments including the anterior, inferior, superior, middle, and posterior compartments. The thymus is a specialized primary lymphoid organ derived from third pharyngeal pouch and is responsible for the production and programming of T-lymphocytes. Located within the anterior mediastinum, the thymus is located within close proximity to numerous other important anatomic structures including the lungs, pleura, heart, great vessels, trachea and esophagus. The thymus has a dual epithelial and lymphoid cell population and is characterized by a lobulated architecture that is reminiscent of pancreatic architecture. The adult thymus is usually regressed and microscopically only identifiable by small islands of residual thymic tissue. As the thymus and mediastinal compartment are composed of numerous tissue types, they can give rise to a heterogenous group of disease states. These include a variety of benign, reactive, and infectious conditions, benign and malignant epithelial tumors, neurogenic tumors, and a wide variety of other mesenchymal tumor types. The thymus and mediastinum are also common locations for hematologic malignancies; particularly Hodgkin lymphoma, as well as a common site for extragonadal germ cell tumors including seminomas and non-seminomatous germ cell tumors. In addition, the thymus is also associated with numerous autoimmune deficiency conditions including DiGeorge Syndrome, Severe Combined Immune Deficiency Syndrome (SCID), Acquired Immunodeficiency Syndrome (AIDS), and Graft Versus Host Disease (GVHD).