Comparative histopathology of gonadal neoplasms in marine bivalve mollusks

Abstract

Comparative histology of gonadal neoplasms in 14 marine bivalve species or hybrids from 5 countries described in the literature and/or archived in the Registry of Tumors in Lower Animals (RTLA), Washington. DC. USA, revealed 3 basic histotypes. Hundreds of cases were of germ cell origin with hfferent stages of development. They consisted of undifferentiated germ cells that filled individual follicles (stage l ) , were present throughout the gonadal area (stage 2), or had spread to outlying tissues (stage 3). Five cases were of stromal origin. The connective tissue comprising these tumors ranged from vesicular to myxoid to spindle-cell. As these tumors grew, they invaded and destroyed normal follicles. Three cases representing a third histotype appeared to be of both germ cell and stromal origin. Two of these 3 were among 15 Crassostrea virginica recently collected from the Pawcatuck River, Rhode Island, USA. In the most advanced case, basophilic hypertrophied neoplastic germ cells were rapidly proliferating along the walls of gonadal follicles and the ducts that extended into the mantle, while the central region of the tumor mass was densely flbrous. Some neoplastic cells in follicles adjacent to normal ova-bearing follicles were differentiating into spermatocytes. Tumor cells aggressively crossed the follicular basement membrane, invaded the vesicular connective tissue supporting the gill axis, and formed a cystic mass along the lumina1 wall of the branchial vein. The less advanced C. virginica case had a smaller, less aggressive tumor but its basic features were similar. The third case similar in composition, pattern, and behavior was in a C. gigas that had been collected during the 1960s from the Willapa Bay. Washington, USA, and had originally been interpreted as a fibroma. All 3 of these mixed gonadal-stromal neoplasms are presently diagnosed as gonadoblastomas.