Retained fertilizing capability in cryopreserved feline spermatozoa
Sperm cryopreservation offers a long-term preservation of male genetic materials for future assisted reproductive technologies. However, dramatic changes in temperature during freezing and thawing injure sperm cells. While motility is essential for AI and membrane integrity is crucial for in vitro fertilization (IVF), sperm DNA integrity is a common index of fertilizing capability required for AI, IVF and intracytoplasmic sperm injection (ICSI). In endangered felids died unexpectedly, attempts have been made to recover as many DNA intact spermatozoa as possible from epididymis and testis to increase the opportunity to produce offspring in future. Although sperm from caudal epididymis has shown retained fertilizing capability after freezing and thawing (27.3% conception rate after unilateral intrauterine insemination), sperm recovery from the corpus epididymis has been suggested as an alternative to increase the amount of preserved genetic materials. To improve epididymal sperm quality, pre-treatment with single-layer centrifugation resulted in selection of sperm cells with intact DNA while post-thaw treatment with extracellular ATP incubation promoted the blastocyst rate. Cold storage of domestic cat testis for 7 days at 4°C demonstrated <1% of sperm cells with fragmented DNA. Moreover, isolated testicular sperm cells, stored for 7 days at 4°C, produced after ICSI poorer percentages of cleavage, morula and blastocyst than the fresh control. In wild felids, a death-to-necropsy time of 2 hr after a jungle cat (Felis chaus) aged 10 years died during anaesthesia plus another necropsy-to-sperm recovery time of 25 hr has been reported to yield the post-thawed testicular sperm with 22.2% intact DNA. In summary, the chromatin structure of feline ejaculated and epididymal sperm seems to be tolerated to cold storage and cryopreservation; thus, fertilizing capability is well protected. In contrast, the cat testicular sperm DNA is generally damaged through the cryopreservation.