Ectopic intestinal cyst in the liver of a Sprague–Dawley rat

Ectopic intestinal cysts are extremely rare in the rat liver. Here, we report a case of a spontaneous ectopic intestinal cyst in the liver of an 8-week-old male Crl:CD (SD) rat. Necropsy revealed a solitary white, firm nodule, approximately 3 mm in diameter on the diaphragmatic surface near the porta hepatis of the medial lobe of the liver. Histologically, the lesion exhibited a cystic structure lined with tissue resembling intestinal mucosa, located on the liver capsule. Periodic acid-Schiff and Alcian blue (pH 1.0) staining-positive mucous cells, similar to goblet cells, and Paneth cell-like cells containing eosinophilic granules were observed in the mucosal epithelium. Immunohistochemically, the mucosal epithelium demonstrated low proliferative activity, as confirmed by Ki-67 staining. The thin outer layer of the mucosa was positive for alpha-smooth muscle actin, suggesting the presence of the lamina muscularis or a poorly developed muscular layer. Based on the lesion's location and histological features, this case was diagnosed as an ectopic intestinal cyst, likely resulting from persistence of the vitelline duct. To the best of our knowledge, there are no previous reports of ectopic intestinal cysts in the rat liver that include such detailed histochemical and immunohistochemical findings. This report provides valuable insights into congenital lesions of the rat hepatobiliary system.