Aberrant Expression Levels of Androgen Receptor and SRD5A2 in Epididymal Epithelial Cells of Crossbred Infertile Cattle-Yak.

Although yaks and cattle belong to the same Bovinae subfamily and have the same number of chromosomes, hybrid males are sterile because of the inactivation or abnormality of gene expression related to the production of healthy normal sperm. Recently, the analysis of gene expression not only in the testis but also in the epididymis has offered hints about the mechanism of infertility, because the epididymis supports the maturation of sperm in acquiring the capacity of fertilisation. Sperm maturation processes have been thought to be androgen-dependent, and the androgen receptor (AR) can be activated by dihydrotestosterone converted from plasma testosterone by the 5α-reductase isoform 2 (SRD5A2) in epididymal cells. In the present study, we investigated the immuno-expression levels of the AR and SRD5A2 in the epithelial cells of the hybrid cattle-yak epididymal caput in comparison with yak samples using image analysis. Epididymal tissues from yaks (1-3 years of age) and hybrid cattle-yaks (2 years of age) were used in this study. In yaks, AR signal intensity did not show any changes in epididymal epithelial cells during maturation. However, in 2-year-old hybrid cattle-yaks, AR signal intensity was significantly higher in the principal cells of the epididymis compared to that of yaks of the same age, indicating that hybrid sterility is not likely related to AR deficiency in the epididymal epithelium. On the other hand, SRD5A2 signal intensity was stable during maturation in the epithelial cells of the yak epididymis. However, the epididymal SRD5A2 signal intensity in the epithelial cells of the hybrid cattle-yak was lower than that of the yak. This suggests that a deficiency in SRD5A2 production in the epididymis may result in hybrid infertility, as it can subsequently cause incomplete AR signal transduction and altered spermatozoa physiology.