ENHANCING SPERM PRODUCTION IN THE BULL
Waqas, Muhammad Salman
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Spermatogenesis is a complex process highly regulated mainly by somatic cells of testes, the Sertoli cells. The number of Sertoli cells for a male is fixed between birth and puberty, and does not increase after puberty except in the horse. Prepubertal Sertoli cells highly express receptors for thyroid hormone and the expression significantly decreases after puberty. Studies in rodents showed that the size of the Sertoli cell population is set via prepubertal signaling from thyroid hormones. Spermatogenic capacity of all mammals is strongly correlated to the number of Sertoli cells in the testes (R=0.83 for the bull). Male spermatogenic capacity is a critical component of cattle production and the majority of genetic gain is made via selective utilization of gametes from desirable sires. Thus, strategies that enhance sperm production increase the availability of elite genetics for use in improving production characteristics of the populations on a worldwide scale. Here, we devised a strategy to increase Sertoli cell number in bulls via induction of a transient hypothyroidic state just prior to and extending beyond the period of Sertoli cell proliferation that we found to normally cease between 4.5 and 5 months of age. We treated Angus calves (n=3 vs n=3 age matches controls) with the drug Methimazole®, 2mg/kg body weight twice a day, from 4-6 months of age to transiently induce hypothyroidism. Analyses for serum levels of Thyroxine and Triidothyronine demonstrated effectiveness at inducing transient hypothyroidism and reversion to euthyroidism by approximately 9 months of age. In adulthood, these bulls produced a significantly greater number of sperm, up to 282%, compared to age matched controls and their testes contained nearly two times more Sertoli cells. Importantly, sperm morphology; fresh and post-thaw sperm motility, fertilizing ability, and viability were found to be no different for treated bulls compared to untreated control bulls. This strategy of transient induction of hypothyroidism during a defined period of pre-pubertal development in bulls could prove to be an efficacious approach for enhancing daily sperm production in genetically desirable sires that will, in turn, provide an avenue for improving the efficiency of commercial cattle production.