Improving sperm production of boars
J. Ford, D. Lunstra, T. Wise, G. Rohrer and D. Nonneman
During the past 15 years, the percentage of sows in the U.S. that are artificially inseminated increased from less than 5% to 80%. This change in breeding technology compelled development of boar studs to supply the needed semen. As such, sperm production of boars acquired greater significance due to the overhead costs associated with maintenance of boars and the relatively few doses of semen produced per collection.
Meishan pigs are a Chinese breed that produces large litters. They were imported into the U.S. in 1989 for the purpose of conducting research on components of litter size. Little consideration was allotted to direct research with the boars. However, Meishan boars became an invaluable resource that stimulated a series of studies at the U.S. Meat Animal Research Center to define testicular development and sperm production in swine. The following conclusions arose from these efforts.
1. Meishan boars reach puberty at an earlier age and have smaller testes compared with commercial crossbred boars available to U.S. swine producers. This provided conclusive evidence that, within swine, ovulation rate of females is not related to testicular size of boars as Meishan females have a high ovulation rate. (Borg et al., 1993; Christenson, 1993; & Lunstra et al., 1997)
2. A technique was developed that greatly improves the ease of counting Sertoli cells within the seminiferous tubules of testes. This technique is based on immunological detection of GATA4, a transcription factor that is localized in nuclei of Sertoli cells. Sertoli cells nurture the development of germ cells as they mature to sperm cells, and the number of Sertoli cells correlates positively with testicular size and sperm production. (McCoard et al., 2001; Okwun et al., 1996; Lunstra et al., 2003)
3. The physiological mechanisms that establish the future number of Sertoli cells within the testis of boars are established by one month of age, although proliferation of these cells continues through early pubertal development. (Lunstra et al., 2003; Ford et al., 2006)
4. Mature Meishan boars have greater plasma concentrations of follicle-stimulating hormone (FSH), luteinizing hormone and testosterone. These findings were unexpected because FSH stimulates Sertoli cells, and thus, one would predict a positive association with larger testicular size rather than with smaller size. This inverse relationship of testicular size with plasma FSH concentrations is not unique to Meishan boars as it has been observed in commercial crossbred boars. These observed endocrine differences are surprising in light of the very subtle differences in reproductive hormones that were observed when concentrations of reproductive hormones in the blood of Meishan sows were compared with those in commercial crossbred sows. (Ford et al., 1997; Lunstra et al., 1997; Wise et al. 2001)
5. The above findings stimulated examination of embryonic and fetal testicular development in Meishan and commercial crossbred boars. The pattern of testicular development was strikingly similar during the first one-half of gestation in these two diverse breeds. During the last one-half of gestation, commercial crossbred fetuses had more Sertoli cells than Meishan fetal boars, but during the first 25 days of life, Meishan boars accumulated Sertoli cells more rapidly than commercial crossbred boars in preparation for their earlier onset of puberty. Proliferation of Sertoli cells continued for a longer period in commercial crossbred boars, leading to a greater number Sertoli cells, larger testes and greater sperm production as adults. (McCoard et al., 2002, McCoard et al., 2003)
6. Genetic analyses of Meishan crossbred boars revealed that a specific region of the X chromosome is associated with testicular size and sperm production. The gene for thyroxine-binding globulin (TBG) is located within this region of the X chromosome, and a mutation within the binding domain of TBG reduces binding to thyroxine that is associated with smaller testicular size. (Rohrer et al., 2001, Ford et al., 2001 & Ford et al., 2006; Nonneman et al., 2005)