CONICET | Buscador de Institutos y Recursos Humanos

Background (incidental) lesions have been described as findings that are usually thought of as a change in tissue morphology outside of the range of normal variation for a particular species or strain. The myocardium is susceptible to a variety of morphologic changes in routine toxicity studies and background lesions need to be differentiated from real lesions related to drug treatment. The aim of this study was to do a retrospective analysis of background lesions in the heart in Wistar/Cmedc rats and Balb/c Cmedc mice. They were healthy animal from production area and control groups of preclinical and toxicological studies reported at Centro de Medicina Comparada (ICiVet-Litoral) from 2016 to 2019. Animals were necropsied and hearts were fixed in 10% buffered formalin, embedded in paraffin, sectioned at 4 µm, and stained routinely with hematoxylin and eosin. Heart lesions from 29 rats (22 male and 7 female) and 34 mice (14 male and 20 female) were reviewed. Background lesions in the heart were observed in 22 of rats (76%) and 17 of mice (50%). The following lesion were describer: cardiomyocyte vacuolation in 9 rats (31%) and 2 mice (6%), inflammatory cell infiltration in 5 rats (17%) and 6 mice (18%), valvular myxomatous degeneration in 7 rats (24%), aortic cartilaginous metaplasia in 6 rats (21%), hemorrhage in 1 rat (3%), thrombi in the atrium in 3 mice (9%) and valvular endocarditis, cardiomyocyte atrophy, mineralization and fibroplasia in 1 mouse (3%). Pathologists need to recognize background lesions in acute and chronic toxicity studies because many of the lesions can be mistaken or increased by treatment-related findings in preclinical studies. The importance of background lesions in toxicopathology studies is variable and related to the change type, severity and size.