Britannica
Sources used in this answer
How is the thymus organized into a cortex and medulla?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
lymphatic system
The human lymphatic system, showing the lymphatic vessels and lymphoid organs.
The thymus is a bilobed organ organized into smaller subdivisions called lobules, each of which is structurally divided into an outer zone known as the cortex and an inner zone known as the medulla.1
The organization of these zones supports the maturation and selection of T cells:
- Cortex: This outer zone is characterized by a high concentration of lymphocytes. It serves as the primary site for lymphocyte proliferation and the initial differentiation of T cells. Here, developing T cells (thymocytes) undergo "positive selection," where they learn to recognize the body’s own major histocompatibility complex (MHC) molecules.12
- Medulla: T cells that survive selection in the cortex migrate to the medulla for further differentiation. In this inner zone, cells undergo "negative selection," where those capable of attacking the body’s own tissues are destroyed to prevent autoimmune reactions. Surviving T cells then enter the bloodstream through medullary veins.12
The entire organ is supported by a connective-tissue capsule and a meshwork of reticular cells.1