Photoperiodic Regulation of Ovine Spermatogenesis: Relationship to Serum Hormones1

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Abstract
Serum concentrations of reproductive hormones and spermatogenesis were studied in rams exposed to either decreasing (group 1) or increasing (group 2) photoperiods. Six rams were assigned to each group in late February and maintained in individual pens throughout the study. Room temperature was kept at approximately 18.degree. C for both groups of rams. Light schedules were decreased or increased stepwise from a 12 h light; 12 h dark photoperiod so that within 8 wk, rams were exposed to either short days (8 h light:16 h dark photoperiod) or long days (16 h light: 8 h dark photoperiod). Scrotal circumference (testis size) decreased (P < 0.05) .apprx. 10% throughout the experiment in long day rams; after an initial decrease, scrotal circumference in short day rams increased (P < 0.05) .apprx. 15%. When compared to long day rams, short day rams had 45% heavier (P < 0.05) testes, 30% larger (P < 0.01) seminiferous tubule diameters and produced nearly twice as many sperm. Histological examination of the germinal epithelium revealed that long days exerted deleterious effects at several points in spermatogenesis. The changes in sperm production of photoperoid-treated rams could be the result of changes in several endocrine systems. Short day rams were characterized by low concentrations of serum prolactin and elevated concentrations of serum testosterone, LH [lutropin] and FSH [follitropin]. Testicular growth in rams exposed to short day lengths is apparently a response to increased gonadotropin activity. The action of these gonadotropins may be direct on the germinal epithelium of the ram testis or may stimulate spermatogenesis indirectly by enhancing testosterone secretion in interstitial cells.