Seasonal changes in spermatogenic activity and in plasma levels of FSH, LHand testosterone, and the effect of immunization against inhibin in the male silver fox (Vulpes vulpes)

The cellular composition of the silver fox testis assessed by DNA flow cyto metry and histological analysis exhibited marked circannual alterations. Th e proportion of haploid cells increased from late October to the breeding s eason in February, while that of diploid cells decreased and that of tetrap loid cells fluctuated during the same period. Towards the end of March thes e changes were reversed. The seasonal variations in testicular histology pa ralleled the changes in distribution of cells from the different DNA popula tions. In August, 69% of the tubules contained spermatogonia as the only ty pe of germ cell, while the remaining 31% also contained a few primary sperm atocytes. In late October more than 50% of the tubules contained spermatocy tes, and during the period of further activation from early December-Februa ry the seminiferous epithelium included round and/or elongated spermatids a s well. In February, all tubules contained complete associations of germ ce lls, whereas in late March tubules with spermatogonia only and spermatogoni a together with a few spermatocytes reappeared. In May, only such tubules c ould be found indicating total regression. Plasma concentrations of FSH and LH increased from early November. both gonadotrophins reaching maximum lev els in December or early January, and then both declined during the second part of January, immediately prior to the actual breeding season. LH values showed a few smaller peaks in the beginning of June, whereas FSH levels we re generally low until the next period of testicular reactivation. Testoste rone concentrations were also low during most of the year but rose in Novem ber and December to reach a peak in January and a second peak in June. In a nimals immunized against inhibin the distribution of haploid, diploid and t etraploid cells did not deviate to any great extent from that in the contro ls, except in March when the immunized males had a markedly lower proportio n of tetraploid cells, and in May, when they had a distinctly higher propor tion of haploid cells. These findings were partly reflected by the histolog y. In the immunized animals, plasma FSH levels started to increase at appro ximately the same time but peaked higher and remained elevated almost I mon th longer than in the controls, whereas both the rise and decline in LH lev els generally coincided with the variations in these animals, but the value s were mostly higher. The testosterone profiles were similar to those in th e controls except that the maximum values were also usually higher.