Dihydrotestosterone (DHT) and 17β-estradiol (E2) are sex hormones that regulate 
human hair follicle (HF) growth and are produced by peripheral reduction and 
aromatization of testosterone. However, the expression patterns of DHT and E2 
synthesis-related proteins and their receptors in male yak skin during different 
HF stages (telogen, anagen, and catagen) are unknown. In this study, we found 
that both 5α-red and androgen receptor (AR) were expressed in epithelial cells 
and AR was expressed in the dermal papilla. Additionally, the transcription 
level of 5α-red1 at different HF stages was significantly higher than that of 
5α-red2 mRNA at the same stage; 5α-red1 and 5α-red2 proteins peaked during the 
anagen and telogen periods of HF, respectively. However, AR protein was only 
expressed in the skin during the anagen phase of HF. Aromatase and estrogen 
receptors (ERα and ERβ) were expressed in cutaneous epithelial cells, whereas 
ERα and ERβ were expressed in the dermal papilla; the transcription level of ERα 
in HFs at each stage was much higher than that of ERβ. From the catagen to 
telogen phase, aromatase protein expression was down-regulated, while ERα 
protein expression was up-regulated. Based on our results, we speculate that 
5α-red1 is essential for the synthesis of DHT in male yak skin epithelial cells 
and promotes the growth of HFs through AR. E2 synthesized by male yak skin 
epithelial cells may inhibit the growth of male yak skin HFs by ERα. These 
results provide a foundation for further study on the mechanism of 
hormone-regulated male yak skin HFs.