Hyperplastic and inflammatory pathology of the thymus

Abstract

Thymic hyperplasia describes a nonneoplastic condition of the thymus, characterized by an increased number of the constituent cells of the organ. This concept was initially introduced by Castleman in the late 1940s to define the thymic changes seen in association with myasthenia gravis [1]. Years later, Rosai and Levine [2] in the A.F.I.P. fascicle on Tumors of the thymus identified two distinct types of thymic hyperplasia on the basis of histopathologic criteria: true thymic hyperplasia, defined by an increase in the size and weight of the organ that retains a normal microscopic morphology, and lymphoid (or follicular) hyperplasia, characterized by the presence of lymphoid follicles with active germinal centers in the thymic medulla. In the last case, the size and weight of the thymus can be increased, but in most instances they are within normal limits. The term thymitis (or autoimmune thymitis), originally coined by Goldstein in 1966 [3], has been used interchangeably by some authors to denote the lymphoid hyperplasia of the thymus, since its morphological features, both at the light and electron microscopic levels, are markedly similar to those observed in any chronic inflammatory process of other organs [4, 5], being characterized by the presence of peripheral B lymphocytes, lymphoid follicles, and diffuse plasmacytosis. © 2008 Springer Milan.