Thymus - Necrosis, Lymphocyte

Necrosis of thymic lymphocytes can occur in rats and/or mice as a direct effect of toxicants such as corticosteroids, estrogen, and cyclophosphamide, and usually in conjunction with severe lymphocyte apoptosis. The cortical and/or medullary lymphocytes may be involved (Figure 1). Necrosis may also involve all cell components of the thymus. Necrosis is characterized by cell swelling, condensation and dissolution of the nucleus, and cell lysis with accumulation of abundant eosinophilic cytoplasmic and basophilic nuclear debris (Figure 2, arrows). Necrosis is usually accompanied by inflammation and may also be accompanied by hemorrhage and/or mineralization (Figure 2, arrowhead). Significant lymphocyte necrosis may lead to atrophy of the thymus, with or without thymic regeneration. Necrosis of lymphocytes must be differentiated from lymphocyte apoptosis, although they can occur together if apoptosis is severe and apoptotic factors such as caspases and ATP are depleted. Apoptosis typically involves shrinkage of individual lymphocytes, condensation of nuclear chromatin, and fragmentation of apoptotic cells into apoptotic bodies. Apoptotic bodies are phagocytized by macrophages (tingible-body macrophages). Apoptosis is not typically associated with inflammation, hemorrhage, or mineralization.