Disruption of cell proliferation and apoptosis balance in the testes of crossbred cattle‐yaks affects spermatogenic cell fate and sterility

The balance between proliferation, differentiation and apoptosis is well‐coordinated in spermatogenesis for the timely production of appropriate numbers of sperm in animals. Disruption or decrease in sperm production is due to many conditions, including changes in testicular cell fate balance. Interspecies hybridization of domestic yaks and cattle results in sterility in males because of spermatogenic arrest; however, the underlying mechanisms involved in sterility are still unclear. In the present study, we investigated the proliferation and apoptosis status during the development of yaks and crossbred cattle‐yaks using immunohistochemistry of proliferating cell nuclear antigen and terminal deoxynucleotidyl transferase‐mediated dUTP nick end labelling assays. Testicular tissues from yaks (immature: 1 year old, mature: 2–3 years old) and backcrossed hybrids (2 year old) were collected and used to investigate the expression of each parameter in testicular cells. During the maturation of yak testes, proliferation and apoptosis became active only in spermatogenic cells, and not in other somatic cells, such as Sertoli cells, myoid cells and Leydig cells. Furthermore, hybrid cattle‐yak testes maintained proliferation ability but less apoptotic ability in spermatogenic cells when compared to yaks of the same age, suggesting that normal spermatogenic cell fate control is disrupted by changes in the balance between proliferation and apoptosis. In addition, Leydig cell proliferation rate was higher than apoptosis rate in the cattle‐yak testes, indicating an increased number of Leydig cells, which may affect spermatogenesis through changes in steroidogenesis. Although epigenetic changes may be involved in cattle‐yak testes, further studies are needed to clarify the modulation of proliferation and apoptosis to elucidate the mechanisms of infertility in hybrid cattle‐yak males.