Sperm surface changes and their consequences for sperm transit through the female reproductive tract

Spermatozoa are faced with considerable challenges during their passage through the female reproductive tract. Following deposition, they must deal with several physical and biochemical barriers as well as an aggressive immune defence system before they reach the site of fertilisation. While many factors are at play, the surface characteristics of spermatozoa are central to communication with the female and successful transit. The surface proteome of spermatozoa has been extensively studied and shown to vary considerably between species that deposit semen in the vagina (ram and bull) and uterus (boar and stallion), likely due to major differences in accessory sex gland anatomy. Comparing the surface characteristics of spermatozoa from these domestic species and how individual components may equip spermatozoa to interact with different features of the female tract could help understand how spermatozoa navigate from vagina or uterus to oviduct ampulla. Furthermore, we can begin to explain why use of high quality preserved spermatozoa in artificial insemination programs may still result in reduced fertility due to altered interaction with the female. In this review, we describe the sperm surface characteristics of the ram, bull, boar and stallion and compare changes as a result of mixture with seminal plasma and/or in vitro processing. The role of these seminal components in facilitating sperm survival and transit within the female reproductive tract is summarised, drawing attention to potential implications for applied reproductive technologies.