Spermatozoa Cryopreservation: State of Art and Future in Small Ruminants.

The cryotolerance of farm animal spermatozoa varies according to their specific features, such as size, shape, and lipid composition. Thus, it is impossible to develop a standardized freezing procedure for different kinds of livestock species. The establishment of an efficient semen cryopreservation procedure will facilitate long-term conservation of small ruminant genetic resources and extension of artificial insemination in daily production. Different from sheep, goat seminal plasma contains a phospholipase, which can affect spermatozoa viability through interaction with milk or egg yolk. Currently, soybean lecithin is a viable alternative that replaces the components of animal origin in freezing extenders for goat semen. In addition, vitrification or freeze-drying may act as another alternative to replace traditional cryopreservation. However, these two methods, especially freeze-drying, may require the aid of the intracytoplasmic spermatozoa injection technology. Furthermore, the cryoinjury mechanism of mammalian spermatozoa has remained unclear until now. The emergence of proteomics and transcriptomics may provide some inspiration concerning this problem. In this review, we summarize the state of art relating to small ruminant semen cryopreservation, mainly focusing on the current status of the freezing procedures. In the meantime, some highlights such as protectants, vitrification, and freeze-drying are also reviewed. Finally, the future perspectives in the field of small ruminant spermatozoa preservation are discussed.