The role of hormones in initiation of sexual maturation in male river lampreys (Lampetra fluviatilis, L.): Gonadotropin and testosterone

Abstract Previous experiments with implantation of sex hormones indicated that development of increased sensitivity toward sex hormones in the peripheral tissues responsible for secondary sex characteristics is an essential feature in initiation of normal sexual maturation. The extent to which this sensitivity change depends on hormones from the pituitary gland was investigated by implanting testosterone pellets into 15 hypophysectomized (pro- and mesoadenohypophysis) immature male lampreys. Fin changes and growth of the urogenital papilla were elicited by testosterone in all hypophysectomized males, but were delayed in some in comparison with intact males. The testosterone-stimulated rate of growth of the urogenital papilla was slow in the hypophysectomized lampreys, but eventually normal size (8 mm) was reached. In untreated hypophysectomized lampreys the liver never turns green as it does in the majority of intact males, but testosterone induced green livers in 11 out of 14 hypophysectomized males. Thus it appears that the development in peripheral tissues of sensitivity toward sex hormones can take place without the presence of the pro- and mesoadenohypophysis. Furthermore, together with previous results obtained by hypophysectomy, the results support the idea that gonadotropin from the pro- and mesoadenohypophysis of lampreys is needed for the whole process of sexual maturation to occur, stimulates the rate of development, and allows simultaneous readiness for spawning in the population by especially stimulating slowly developing lampreys. On the basis of recent results and previous data it is suggested that the metaadenohypophysis and the interrenal cells also may be involved in initiation of sexual maturation.