Located in the anteriorsuperior mediastinum. Site of T-cell differentiation and maturation.- Encapsulated.- Derived from the third pharyngeal pouch.- Lymphocytes of mesenchymal origin.

Cortex is dense with immature T cells.Medulla is pale with mature T cells and Hassal corpuscles containing epithelial reticular cells.

Normal neonatal thymus "sail-shaped" on CXR, involutes with age.

Hypoplastic in DiGeorge syndrome and severe combined immunodeficiency (SCID).

Thymoma – neoplasm of thymus. Associated with myasthenia gravis and superior cava syndrome.