A review of sperm cryopreservation in the domestic dog and cat: part I, from science to clinic.

Semen cryopreservation is widely applied in the breeding of several domestic animal species. In the domestic dog, artificial insemination with cryopreserved semen is now routinely performed, whereas in the domestic cat the technique is more challenging. Achieving acceptable pregnancy rates requires high post-thaw semen quality. Unfortunately, sperm cells are susceptible to damage caused by temperature reduction. At temperatures below - 130 °C harmful reactions that can damage spermatozoa are essentially halted. Therefore, spermatozoa can be stored in liquid nitrogen at -196 °C for virtually unlimited periods, enabling the transport of genetic material across both time and space. To reach such low temperatures, however, sperm cells must undergo detrimental changes in temperature. Sperm samples are therefore diluted in buffered extenders containing cryoprotective agents that reduce cold shock and freezing induced damages. Despite these measures, freezing and thawing inevitably cause cell injuries, resulting in reduced longevity compared with freshly ejaculated spermatozoa. Therefore, intrauterine insemination and accurate prediction of ovulation are required to achieve acceptable pregnancy results. Protocols for cryopreservation of dog and cat spermatozoa are often adapted from those developed for other species, in which semen preservation is more established. However, sensitivity to cold shock and freezing varies both between species and among individuals, largely due to differences in sperm cell membrane composition. Moreover, spermatozoa from different species may exhibit varying degrees of sensitivity to potentially toxic effects of ingredients in semen extenders. Thus, protocols must be tailored to each species. Understanding mechanisms of cryo-induced cell damage requires a fundamental understanding of how cells are affected by low temperatures. Much of the research on basic cryobiology was conducted decades ago, yet improvements in cryopreservation protocols are still in progress, often driven by empirical studies, comparing alternative strategies. The aim of this review is to synthesize current knowledge on canine and feline semen freezing, placing recent findings in the context of historical research. Several breakthroughs in cell cryobiology have been successfully applied in these species and are still commonly used. Such examples are the basic Tris-buffer, and the use of egg yolk and glycerol in freezing extenders. Future developments may include alternatives to antibiotics and replacement of egg yolk with non-biological alternatives.