Recent Advances in Research on the Hormone INSL3 in Male Goats

Insulin‐like factor 3 (INSL3), previously called relaxin‐like factor (RLF), is essential for testis descent during fetal development and has been implicated in the testicular and sperm functions in adult males. However, similar functions in ruminants remain largely unknown. This chapter will cover recent advancement in our understanding of INSL3 in goats. First, testicular Leydig cells were the sole source of INSL3, with INSL3 expression increasing during development. Second, INSL3 was constitutively secreted as a B–C–A single‐chain structure with full biological activity. Third, secreted INSL3 was transported into the seminiferous compartments, where its receptor RXFP2 was expressed on germ cells, thus suggesting that the intratesticular INSL3 hormone‐receptor system operates in germ cells. Fourth, functional RXFP2 enabling INSL3 to bind was also identified in the spermatozoa and suggested the existence of the extratesticular INSL3 hormone‐receptor system in the spermatozoa. Interestingly, percentages of INSL3‐binding spermatozoa were significantly reduced in the semen of subfertile bulls compared to that of fertile bulls, sug ‐ gesting the potential of this system to diagnose fertility in breeding sires. These fascinating findings will give a new perspective in physiological and/or therapeutic actions of INSL3 on male reproductive processes in domestic ruminants, including goats.