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 <i>femoralis</i> and <i>pectoralis</i> 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.