Seasonal Changes in Testicular Steroidogenesis and Spermatogenesis in a Northern Fur Seal, Callorhinus ursinus

抄録

Seasonal changes in testicular steroidogenesis and spermatogenesis were studied in a captive male northern fur seal, Callorhinus ursinus, of Izu-Mito Sea Paradise. Under anesthesia, monthly blood samplings and testicular biopsies at intervals of 3 months were performed to measure serum testosterone concentrations by radioimmunoassay and to observe spermatogenic activity histologically and the immunolocalization of steroidogenic enzymes. Histological observation of testes revealed full spermatogenesis, including spermatogonia to spermatozoa in the seminiferous epithelium in June and a limited generation of germ cells such as spermatogonia and primary spermatocytes in September and December. The seminiferous tubule diameter and serum testosterone concentrations showed maximum fluctuations corresponding to full spermatogenic activity in June. Three kinds of steroidogenic enzymes, P450scc, 3βHSD and P450c17, which are necessary for androgen synthesis, were immunolocalized in Leydig cells, and P450arom, the key enzyme for estrogen synthesis, was immunolocalized in spermatogonia in the testes in all 3 seasons. These results indicate a definite seasonality in high-level testicular steroidogenic and spermatogenic activity of this northern fur seal during the breeding season in spite of there being no changes in the immunostaining of intratesticular steroidogenic enzymes throughout the year.