Development Testicular and Spermatogenesis in Nellore Bulls.

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Nellore and their crosses represent the largest group of beef herd in Brazil. Their reproductive performance still needs to be improved once conception rate is not consistently high at the end of breeding seasons. Part of this problem is related to the sires which were not selected considering their reproductive capacity. In this regard, the knowledge of testicular physiology and the main factors that affect sperm producing capacity is crucial for selection and management of sires. Thus, the present study was conducted to evaluate the relationships among quantitative aspects of the testis size, testosterone secretion and spermatogenesis in Nellore bulls raised in the Northeast of Brazil. For this purpose, bull scrotal circumference (SC), testis diameter and length were measured from 10 to 29 months (mo.) of age in 28 bulls. At 12, 13, 15, 17, 18, 19, 20 and 21 mo., plasma testosterone concentrations were measured before and after intramuscular injection of GnRH (0.05 mg/kg body mass). At 30 mo., all bulls were slaughtered and the testis samples taken for histological analysis. Correlations among all variables were determined by Pearson's method. We observed that several aspects of the seminiferous tubules, including the average number of AI spermatogonium and round spermatids in the tubule cross sections, number of round spermatids per Sertoli cells, Al spermatogonium and spermatocyte were correlated to concentrations of GnRH-stimulated testosterone. Animals with higher testicular mass had more tubules with mature spermatids as the most developed germ cell type (r=0.52; p<0.05). All correlations between quantitative aspects of spermatogenesis and testis diameter and length were similar to those found with testis weight. Testis and epididymal weight were closely related at the age of 30 months (r=0. 72; p<0.05). Scrotal circumference at 23, 25 and 26 months was related to the number of round spermatids per tubule per section (r=0.49 to 0.59; p<0.05) and SC from 18 to 29 months was associated with the population of Sertoli cells per testis (r=0.50 to 0.59; p<0.05). In conclusion, these results suggest that testes measure evaluated in pre and pubertal phase can be an important indicator of gonadal development in adult Nellore bulls. Confirming that animals with larger scrotal circumference have more Sertoli cells, which provide a specialized, protected environment within the seminiferous tubules of the testis for germ cell development. These results extend our knowledge in physiology of male reproductive tract mainly in spermatogenesis. This study was supported by grants from Brazilian Research Councils, CNPq and CAPES (to AAAM). (poster)