Abstract
Thymic cysts have not been previously reported in harbor porpoises (Phocoena phocoena). Two hundred forty stranded or "by-caught" harbor porpoises originating from the German North Sea and Baltic Sea and 14 animals from the waters of Greenland were necropsied. Thymuses of 78 porpoises of the North Sea 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 structures in selected cases. Thymic macrocysts were rare and were only found in animals ≥7 years of age. Thymic microcysts, lined by a single cytokeratin-positive cell layer, were common in involuting thymuses, 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 healthy harbor porpoises, suggesting that physiological thymic involution proceeds slowly in this species.