Thymus - Atypical Hyperplasia, Lymphocyte

Atypical hyperplasia of the thymic lymphocytes has been described in chemically treated B6C3F1 and p53-deficient mice. This lesion also occurs in other mouse strains that develop thymic lymphomas and possibly in rats with chemically induced thymic lymphomas. It may be unilateral or bilateral; when unilateral, the unaffected lobe is typically larger than the abnormal lobe (Figure 1, arrow), and atrophy of the other lobe may occur. This lesion is characterized by depletion of small lymphocytes and a diffuse change with loss of the normal thymic corticomedullary junction. Normal thymic architecture is replaced by a sheet of large, atypical lymphocytes, including lymphoblasts and fewer, admixed small lymphocytes (Figure 2); these cells do not extend beyond the capsule of the thymus. Atypical hyperplasia of the thymic lymphocytes is regarded as a proliferative change that may progress to lymphoma, although it may occur without progression. This putative preneoplastic lesion can be differentiated from lymphoma by the heterogeneous population of lymphocytes, mitotic rate, and lack of capsular invasion.