- The thymus is the primary organ of T-cell Development and is divided into an outer cortex and inner medulla. The entire organ is surrounded by a collagenous capsule which lobulates the organ's mass with occasional septae. As T-cells progress through their developmental stages, they migrate from the cortex to the medulla.
- The thymus is composed of a mixture of developing T-cells enmeshed in a network of Thymic Epithelial Cells. Thymic epithelial cells are thought to present self-peptides in complex with MHC on their surface which is used for both positive and negative selection during T-cell Development.
- Thymic Cortex
- The Thymic Cortex is highly cellular, consistent with the large numbers of T-cells which exist prior to the stringent selection that occurs during T-cell development.
- Thymic Medulla
- The Thymic Medulla is significantly less cellular and represents the exit point of mature T-cells into the peripheral circulation.
- The human thymus is most cellular and active during childhood and begins a long, slow process of involution following pubescence. By the time of the later decades the majority of the thymus has been replaced with adipose tissue.