A NOTE ON THE DEVELOPMENT OF THYROID TUMOURS IN THE SENILE GUPPY, LEBISTES RETICULATUS (PETERS)

In a previous paper we have described the appearance of a thyrotrophin-sensitive thyroid tumour in senile guppies, Lebistes reticulatus (Peters). Further material has now been examined, amongst which were individuals showing progressive stages in the development of these structures. Generally, senile guppies have a completely inactive thyroid gland, the follicular cells being flattened and low and surrounding a dense homogeneous colloid. In one individual the gland was principally composed of inactive follicles, but towards the centre of the gland a tumour was developing, and the follicular cells had become enlarged and distended, and the enclosed colloid was coarsely granular. In a second fish these two zones were clearly represented, but a further development had taken place and the tumour cells at the centre of the gland had become very swollen and balloon-like. The appearance of these cells corresponded closely to those produced when thyroid tumours of fish were experimentally treated over a prolonged period with thyrotrophic hormone. We have previously suggested that thyroid tumours in the guppy initially develop as a result of the inability of the ageing gland to respond to thyrotrophic hormone. Under continued pituitary stimulation the tumour cells become extremely swollen and hypertrophied. The present findings would appear to strengthen this suggestion.