Endocrine aspects of ageing in the guppy, Lebistes reticulatus peters—I. The thyroid gland

Abstract

The growth and histological appearance of the thyroid gland of laboratory bred population of guppies, Lebistes reticulatus Peters, have been described throughout life. Measurements were made of body length, of thyroid size and of the height of the epitholial cells of the follicles. An estimate was obtained of the total number of follicles in the gland. The rate of increase in thyroid size and total number of follicles exceeded that of body length. In young Lebistes the thyroid gland was composed of uniform, rounded follicles, lined with high columnar epithelial cells surrounding a granular colloid. The activity of the gland decreased with advancing age; in the oldest fish, the gland had become completely inactive, the follicular cells were extremely flattened and enclosed a dense, homogenous colloid. The appearance of the thyroid in old age is closely similar to that of hypophysectomized fish, and it was suggested that the decrease in glandular activity with age is caused by a progressive fall in the release of thyrotrophic hormone from the pituitary gland. A few very old guppies had atypical thyroid glands in which numbers of microfollicles had been produced, and the typical follicular organization of the gland had almost been lost. The appearance of the thyroid in the old Lebistes most closely resembled the thyroid tumours of fish treated with thyrotrophic hormone. It was suggested that the appearance of these thyroid tumours was the result of a combination of abnormal function of the pituitary and the inability of the thyroid gland to respond to pituitary thyrotrophic hormone.