So-called mesothelioma of the atrioventricular node.

A so-called mesothelioma of the atrioventricular node causing sudden, unexpected death is reported. The tumor is composed of tubular-glandular spaces and surrounded by connective tissue containing reticulin and elastic fibres. The tubular-glandular spaces are lined by a single, cuboidal or multilayered squamoid epithelium. The tumor epithelium contained epithelial mucin and carcinoembryonic antigen (CEA). At the ultrastructural level the epithelium showed tight junctions, desmosomes and a few epithelial cells had preserved blunt microvilli at the luminal cell surface. Many tumor cells contained a large number of cytoplasmic tonofilaments. Our results support the opinion, that the conducting tissue tumor is a lesion arisen from sequestered endodermal elements from the primitive forgut during cardiac organogenesis.