Expansion
Cortex of dense thymocytes, medulla with Hassall corpuscles
Expansion
Structure
- Cortex: densely packed immature thymocytes, appearing dark; epithelial reticular cells form the blood-thymus barrier
- Medulla: fewer, more mature lymphocytes, appearing pale, containing Hassall corpuscles (concentric whorls of keratinised epithelial cells), which are unique to the thymus and diagnostic
T cell selection proceeds outward:
- Positive selection in the cortex: cells that can recognise self MHC survive
- Negative selection at the corticomedullary junction and medulla: cells reacting strongly to self antigen are deleted. Medullary epithelial cells express AIRE, which displays peripheral tissue antigens for this purpose
- Only about 2 per cent of thymocytes survive both
Failure of negative selection causes autoimmunity: AIRE mutation gives autoimmune polyendocrine syndrome type 1.
Involution begins at puberty, with lymphoid tissue progressively replaced by fat, although some function persists throughout life.
Clinical associations: DiGeorge syndrome (thymic aplasia from third pharyngeal pouch failure, with hypocalcaemia and cardiac defects); thymoma and its association with myasthenia gravis; and the normal infant thymus producing the sail sign on chest radiography.