Annual changes of the gonad and reproductive endocrinology in hynobiid salamanders

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Annual changes of reproductive organs were histologically studied in two species of hynobiid salamanders, Hynobius nigrescens and H.retardatus. The most conspicuous difference found between the two species was the duration of inactive period of spermatogenesis after the breeding season. Inactive duration was 4 months in the former, and 0 month in the latter. Furthermore, the fat body developed very rapidly after the breeding period in H.retardatus. This diffrence is closely related to the length of hibernating period. The hibernating period is 4 months in H.nigrescens, and 7 months in H.retardatus. Concerning the plasma concentrations of sex steroids in H.nigrescens, the concentration of progesterone was strikingly high at the time of ovulation, and the concentration of estradiol was highest in autumn in females. In males the concentration of testosterone was remarkably high at the time of spermiation. Annual changes in the functional activity of gonadotropin-producing cells were immunocytochemically studied in H.nigrescens. The results obtained well coincided with the changes in the gonadal histology and the plasma concentration of sex steroids. In sexually active males found in the breeding pond, the anterior lobe of the pituitary was remarkably large in size. The youngest age of sexual maturation in H.nigrescens was various according to habitat, and 3 years old in a hillside and 5 in a mountain district. The longest life span was 14 years old as far as we observed.