Thymomas and Associated Hyperplastic Lesions in Wistar Hannover Rats

Thymomasoccur prevalently in aged Wistar Hannover (WH) rats, along with hyperplastic lesionsthat cannot be categorized as thymomas. We compared the histological features of hyperplastic lesionsand thymomasin WH rats, the incidences of these lesions, and the relationship of these lesionsto the degree of thymic involutionand also compared these lesionswith those of Sprague Dawley (SD) rats in 4-, 13-, 26-, and 104-week studies. There were no morphological differences between hyperplastic cells and benign tumorcells in thymomas. The histological difference between hyperplastic lesionsand thymomaswas the size of the proliferative areas and the number of medullary differentiation areas. The hyperplastic lesionsof the thymus in WH rats might have a potential for progression to thymomasdue to the observed multiple hyperplastic lesionsor mixed lesionswith thymomas. The incidence of these proliferative lesionsin the thymus was higher in females than in males. Further, the incidence of these prolifer...