Histopathology of Incidental Non-neoplastic Findings in Transgenic CByB6F1-Tg(HRAS)2Jic Mice Used in Toxicity Studies

This technical report presents a collection of illustrative images and concise descriptions of non-neoplastic microscopic findings noted in transgenic CByB6F1-Tg(HRAS)2Jic (Tg.rasH2) mice from 26-week-carcinogenicity studies. A unique finding in the Tg.rasH2 strain was the skeletal muscle myopathy observed in nearly all animals, particularly affecting the and muscles, diaphragm, and subcutaneous muscles. Pigment was noted in various organs, particularly in the spleen due to the C57BL/6J background. Mononuclear and/or mixed cell inflammatory infiltrates occurred in various tissues, with or without secondary changes, similar to other rodent and non-rodent laboratory species. Vascular anomalies were sporadically noted, mainly in the uterus. Other notable findings included extramedullary hematopoiesis in the spleen; alveolar macrophage infiltrate (often with eosinophilic crystals) in the lung; and proliferative findings in several tissues, such as the lung (bronchiolo-alveolar hyperplasia), adrenal cortex (subcapsular hyperplasia), and uterus (cystic-endometrial hyperplasia). This paper also includes illustrations of other less frequently incidental findings. The information presented in this manuscript aims to serve as a valuable reference for pathologists and researchers and expected to offer contextual insights for carcinogenicity and other toxicological studies utilizing this animal model.