Studies on the pituitary and thyroid glands during reproduction in the teleost Poecilia reticulata.

Cyclical activity was demonstrated in both the pituitary and thyroid glands during the 3 - 4 week gestation cycle of the viviparous cyprinodont fish Poecilia reticulata. These cyclical variations were not seen in unfertilized, non-cycling fish and thus it is thought that both glands are involved in the maintenance or control of gestation. In the pituitary gland, cyclical changes occurred in the histology of the gonadotropic, STH and TSH cells and in the amounts of neurosecretion in the neurohypophysis during the gestation cycle. The histological changes in the gonadotropic cells during gestation were interpreted in terms of secretory activity. This was achieved by experimentally raising or lowering the levels of circulating gonadal steroids, which exert a negative-feedback on the gonadotropic cells, and observing histologically the results of these experiments. Further evidence was provided by observing the development of the pituitary and thyroid glands and gonads from birth to sexual maturity. The results of these experiments were used to define the characters typical of inactive and active gonadotropic cells. The activity of the thyroid gland was measured using the technique of interference microscopy. A peak of thyroid activity was demonstrated during the first eight days of the gestation cycle. The lowest thyroid activity occurred during the next eight days, and during the last eight days of the cycle there was an increase in thyroid activity. The levels of thyroxine were found to affect the gonadotropic cells; high levels of thyroxine decreased and low levels increased gonadotropic activity. Thus, in addition to the more well known effect on TSH activity, it is likely that thyroxine also exerts a feedback effect on the gonadotropic cells. The role of the pituitary and thyroid glands in the control of gestation and gonadal maturation, and the complex nature of the interaction of hormonal feedback mechanisms on these glands and on the gonads, is discussed. The hypothalamic mechanisms controlling pituitary-gonadotropic activity are also discussed. It is thought that several different hormones may be involved in the control of reproduction in the guppy.