Endocardial Thickening in the Sprague-Dawley Rat

Thickening of the endocardium is sometimes observed in rats and other animals as a spontaneous or experimentally induced lesion. However, its nature, pathogenesis, and significance are not well understood. In this study, 480 Sprague-Dawley CD outbred albino rats between the ages of 6 and 25 months were examined grossly and microscopically. Endocardial thickening was observed in six cases. It was diffuse and was caused by proliforation of fibroblast-like cells, most frequently involving the left ventricle. Endocardial thickening in Sprague-Dawley rats appears to be primarily due to endomyocardial fibrosis. It may represent repair of damage to subendocardial myocardium occurring spontaneously or as a consequence of exposure to chemicals or other agents.