Thymus - Involution

Involution of the thymus is a gradual, nonreversible change, likely associated with sex steroid changes that occur during aging. It is a normal physiologic age-related change in rodents and is commonly seen in chronic bioassays. Changes typical of thymus involution include decreased cortical lymphocytes, shrinkage of thymic lobules, thinning and irregularity of the cortex, blurring of the corticomedullary junction, adipocyte infiltration of capsule and septae, increased prominence and/or hyperplasia of medullary epithelial cells, formation of follicular-like B-cell aggregates, cyst formation, and plasma cell hyperplasia. If possible, involution should be distinguished from primary or secondary treatment-related atrophy. Similar to involution, atrophy of the thymus is characterized by reduced thymus size and weight secondary to thymic lymphocyte depletion. However, atrophy in the thymus is unrelated to age, generally caused by toxic insult, and potentially reversible with removal of the inciting agent.