Lymphoid Hyperplasia of the Spleen of an 8-Week-Old Rat

In a young adult (8 weeks old) Sprague-Dawley (SD) rat, a 7 mm-diameter hemispherical nodule with white spots was macroscopically identified on the visceral surface of the spleen. Microscopically, the nodule consisted of areas of enlarged white pulp. The periarteriolar lymphoid sheaths (PALS) area was expanded due to an increase in lymphocytes, but the marginal zone was relatively atrophied. Although minimal compressions and mitotic figures were sporadically observed, there were no structural alterations or atypical cells. Immunohistochemically, there were increased numbers of proliferating cells (Ki-67 positive cells) in the PALS and red pulp areas. In the PALS area, there were increased numbers of T-cells and macrophage/dendritic cells, while in the red pulp area, there were increased numbers of T-cells and B-cell/plasma cells. This lesion, which is believed to be hyperplastic and not neoplastic, was diagnosed as lymphoid hyperplasia, a condition considered to be uncommon in young adult SD rats. (J Toxicol Pathol 2008; 21: 243-247)