thymus

The thymus is a primary lymphoid organ of the immune system located in the anterior mediastinum, behind the sternum and in front of the heart. It is most prominent in childhood and gradually involutes with age, often becoming smaller and more fatty. Embryologically, the thymus develops from the third pharyngeal pouch.

Anatomy and histology: The thymus consists of two lobes partitioned into lobules with a cortex and a medulla. The cortex is densely populated with immature thymocytes and is supported by thymic epithelial cells. The medulla contains fewer thymocytes, a different population of epithelial cells, dendritic cells, and Hassall’s corpuscles. The organ is surrounded by a capsule and is supplied by arteries from the inferior thyroid and internal thoracic vessels; venous drainage feeds into the brachiocephalic veins, and lymphatics drain to mediastinal nodes.

Function: The thymus provides the environment for T lymphocyte maturation. In the cortex, thymocytes undergo positive selection for recognition of self-MHC, while in the medulla negative selection eliminates cells with strong self-reactivity. The result is a repertoire of naive T cells that exit the thymus and populate peripheral lymphoid tissues, contributing to adaptive immunity.

Clinical relevance: Thymic size and activity decline with age (involution). Abnormal thymic activity can be associated with autoimmune diseases; thymic hyperplasia occurs in conditions such as myasthenia gravis and Graves’ disease. Thymic epithelial tumors include thymomas, which may be associated with myasthenia gravis and other paraneoplastic syndromes, and thymic carcinoma, a rarer and more aggressive tumor. DiGeorge syndrome involves thymic aplasia or hypoplasia, resulting in impaired T cell development.