Endocrine Abstracts

JOINT836

Background: In humans, it is well-described that spermatogenesis is tightly regulated by reproductive hormones. Spermatogenesis is also an evolutionarily conserved process, which ultimately results in the production of spermatozoa. Common across all mammals are processes that maintain a stem cell pool, a proliferation phase, meiosis and a final differentiation process. Despite being a conserved process, spermatogenesis also undergoes rapid evolutionary changes driven by selective pressures favouring male reproductive success. Primates represent an excellent population to investigate spermatogenesis since many species are evolutionary close but also differ considerably regarding testis size and mating strategies. Comparison of the spermatogenic efficiency across primates can provide valuable insights into how evolution has influenced the complex interplay between reproductive hormones and testicular function and potentially provide insight into why human male fertility has decreased.

Methods: Hormones were measured in serum samples from nine primate individuals representing six different species using ELISA or LC-MS/MS. Histological evaluations of testicular tissue were performed by immunohistochemistry using antibodies against melanoma-associated antigen 4 (MAGE-A4) to stain spermatogonia and nuclear transition protein 1 (TNP1) to stain elongated spermatids. A total of thirteen individuals representing eight species. Germ cells positive for the two antibody markers were quantified using the Qupath software, and the ratio of TNP1-positive spermatids and MAGE-A4-positive spermatogonia was calculated.

Results: Whereas the percentage of MAGE-A4-positive spermatogonia was similar between all individuals, there were remarkable differences in the percentage of TNP1-positive spermatids. Species that have a monogamous mating strategy had lower TNP1/MAGE ratios, indicating a lower spermatogenic efficiency and species with a harem mating strategy showed intermediate ratios, whereas species with polygynandrous mating systems showed the highest TNP1/MAGE ratios, suggesting a more efficient spermatogenesis when multiple males are present per female. Interestingly, all primates showed high levels of the spermatogenesis hormone marker, inhibin B compared to human reference ranges. The inhibin b levels were particularly high in the chimpanzees, likely caused by their large testis size. The harem-living species had the highest testosterone levels and their dihydrotestosterone (DHT) levels were above human reference ranges suggesting that these hormones could reflect male dominance.

Conclusions: Our preliminary data indicates that the mating strategy among primates affects both testis size and the efficiency of spermatogenesis, which again is reflected by inhibin B levels. Humans had the lowest inhibin B levels of all investigated primates, which corroborates the poor testicular function observed among humans.