The thymus is a lobulated organ encapsulated and divided into cortical and medullary portions. The thymus functions in the production of T-lymphocytes (T-cells), These specialized cells function in the adaptive immune response.
Histologically, The thymus has two main areas of interest, cortex and medulla.
The cortex is the outer (darker) region and is the sight of T-cell development. This is where the T-cells are given their receptors and are tested for positive reactions to various antigens, they are also exposed to a few negative receptors, which may be presented to the mature T-cells.
The medulla is the inner (lighter colored) region and is the site where specialization occurs. The T-cells undergo additional rounds of negative selection in an effort to remove auto-reactive T-cells from the mature population.
Other cells typically found in the tymus include; B cells (present both in the thymic medulla and the perivascular compartments), Langerhan’s cells, mast cells, eosinophils, interdigitiating reticulum cells, and stromal cells.
Thymic involution, with progressive diminution of cortical thymocyte numbers, is commonly seen as a function of age in longer term rodent studies. Thymic cortical atrophy is also seen experimentally in pre-clinical and discovery research following exposure to nonspecific stress or to various immunosuppressive agents. The thymic medulla is usually less affected than the cortex. Consequently, the use of image analysis to more accurately determine the ratio of thymic cortex to medulla can be important in assessing threshold effects of pharmaceutical agents on the thymus.
Histological whole slide image of rat thymus, showing cortex in dark purple and medulla in lighter purple
