Abstract
A jejunal nodular mass was identified in an aging rat. Histologically, the boundaries between the lesion and surrounding normal tissue as well as between the inner circular muscle and outer longitudinal muscle were indistinct. The lesion consisted of abundant eosinophilic matrix and cells with a large round to oval nucleus and indistinct cytoplasm. There was no characteristic proliferating pattern, nuclear polymorphism and a low mitotic figure count. Masson's trichrome stain revealed that the intestinal smooth muscles were replaced by the abundant collagen fiber. Immunohistochemistry revealed that the cells with a large round to oval nucleus were labeled with anti-vimentin antibody and not with anti-α smooth muscle actin antibody, suggesting that these cells were fibroblasts. The mass was diagnosed as jejunal fibroplasia.