Thymic cysts in harbor porpoises (Phocoena phocoena) from the German NorthSea, Baltic Sea, and waters of Greenland

Thymic cysts have not been previously reported in harbor porpoises (Phocoen a phocoena). Two hundred forty stranded or "by-caught" harbor porpoises ori ginating from the German North Sea and Baltic Sea and 14 animals from the w aters of Greenland were necropsied. Thymuses of 78 porpoises of the North S ea and Baltic Sea were considered suitable for histopathologic examination based on the extent of postmortem autolysis. Immunohistochemistry using an anti-cytokeratin antibody was employed to demonstrate thymic epithelial str uctures in selected cases. Thymic macrocysts were rare and were only found in animals greater than or equal to 7 years of age. Thymic microcysts, line d by a single cytokeratin-positive cell layer, were common in involuting th ymuses, and the prevalence increased with advancing age. Thymic cysts might have arisen from degenerating Hassall's corpuscles or condensed epithelial reticulum. Thymuses were easily identified macroscopically in most adult h ealthy harbor porpoises, suggesting that physiological thymic involution pr oceeds slowly in this species.