The development of thyroid neoplasia in old age in the amazon molly, Poecilia formosa

Abstract

In a previous paper we have described the appearance of a thyrotrophin sensitive thyroid neoplasm in an few senile individuals from a laboratory population of guppies, Lebistes reticulatus (Peters). Material has now been examined from the closely related species, Poecilia formosa, the Amazon molly, in which the incidence of thyroid neoplasms in the senile fish of our clones approaches 100%. The spontaneous growth seen in the old mollies was large, occupying much of the pharyngeal region. It was formed of three zones; at the center were inactive follicles with flattened epithelial cells, enclosing a dense, uneroded colloid. Peripherally, there was an area which microfollicles predominated and, the follicular epithelial cells frequently were swollen and balloon-like. The outermost area of the growth contained many afollicular groups of epithelial cells, lying amongst a great deal of loose connective tissue. The thyroid tissue was invasive, destroying muscle and bone. We have suggested that the spontaneous thyroid neoplasia in old P. formosa and L. reticulatus arise in a similar way. They develop as a result of the inability of the aging thyroid to respond to thyrotrophic hormone. Under continued pituitary stimulation the thyroid cells greatly proliferate and hypertrophy, and form a neoplasia. The appearance is described of massive thyroid growths which were experimentally induced by the injection into young mollies of cells with damaged DNA. There were striking differences between the induced and the spontaneous thyroid growths. The neoplasia of old fish was highly variable in structure, but the induced neoplasia had a regular appearance. Neoplastic tissue closely resembled normal thyroid tissue—but the growth was exuberant and highly invasive. We suggest that the differences between the neoplasm of old age and the induced neoplasm may reflect a different hormonal status.