Activities of oxidative enzymes in thyroid follicles of Xiphophorin fishes.

: The activity of some intracellular oxidative enzymes was studied histochemically in the cells of the thyroid follicles of teleost fishes of the genus Xiphophorus. The experimental material consisted of animals of the red swordtail and Mexican swordtail breeds of Xiphophorus helleri and of melanotic Xiphophorus maculatus fishes. Observations were carried out on adult specimens of both sexes, including pregnant femals of Mexican swordtail. Moreover, immature Mexican swordtails of both sexes were examined. Thyroid follicles were found to be present in the subpharyngeal region of all fishes studied. The distribution of these follicles as well as their number and form depended on sex, age and on the analysed stage of prenancy. A smaller number and size of thyroid follicles were characteristic of immature specimens, whereas they were most numerous in the thyroids of pregnant fishes. The follicles were arranged in characteristic dense aggregations, especially in the melanotic platyfish. The follicular eipthelium in the fishes under study was usually cubical, but pregnant and non-pregnant adult females also contained a considerable number of larger follicles with flattened epithelium. Besides, thyroid follicles of multilayer epithelium were rather frequently encountered, especially in male fishes, irrespective of their age. The thyroid follicle cells of these fishes demonstrated invariably high activities of reduced NAD and NADP dehydrogenases and of beta-hydroxybutyrate dehydrogenase, and a low activity of succinat dehydrogenase. The intensities of alpha-glycerophosphate and lactate dehydrogenases and of cytochrome oxidase varied with sex, age and breed of the studied fishes. The immature and pregnant fishes showed the most clearly pronounced differences in the intensity of enzymic activity, the thyroid follicles of immature specimens revealing a high activity of lactate dehydrogenase and low activity of cytochrome oxidase, an inverse picture being seen in pregnant fishes. The adult forms of both sexes exhibited an enhanced activity of cytochrome oxidase and a decline in that of lactate dehydrogenase. The observed differences in the intensities of enzymic acitivities in the thyroids of the studied fishes are related with functions of this gland which in the period of growth are different from those in the period of sexual maturity, and certainly also with individual metabolic characteristics of the studied fishes.