BACKGROUNDGerm cell depletion caused by chemical or physical toxicity, disease or genetic predisposition can occur at any age. Although semen cryopreservation is the first reflex for preserving male fertility, this cannot help out prepubertal boys. Yet, these boys do have spermatogonial stem cells (SSCs) that able to produce sperm at the start of puberty, which allows them to safeguard their fertility through testicular tissue (TT) cryopreservation. SSC transplantation (SSCT), TT grafting and recent advances in in vitro spermatogenesis have opened new possibilities to restore fertility in humans. However, these techniques are still at a research stage and their efficiency depends on the amount of SSCs available for fertility restoration. Therefore, maintaining the number of SSCs is a critical step in human fertility preservation. Standardizing a successful cryopreservation method for TT and testicular cell suspensions (TCSs) is most important before any clinical application of fertility restoration could be successful.OBJECTIVE AND RATIONALEThis review gives an overview of existing cryopreservation protocols used in different animal models and humans. Cell recovery, cell viability, tissue integrity and functional assays are taken into account. Additionally, biosafety and current perspectives in male fertility preservation are discussed.SEARCH METHODSAn extensive PubMED and MEDline database search was conducted. Relevant studies linked to the topic were identified by the search terms: cryopreservation, male fertility preservation, (immature)testicular tissue, testicular cell suspension, spermatogonial stem cell, gonadotoxicity, radiotherapy and chemotherapy.OUTCOMESThe feasibility of fertility restoration techniques using frozen-thawed TT and TCS has been proven in animal models. Efficient protocols for cryopreserving human TT exist and are currently applied in the clinic. For TCSs, the highest post-thaw viability reported after vitrification is 55.6 ± 23.8%. Yet, functional proof of fertility restoration in the human is lacking. In addition, few to no data are available on the safety aspects inherent to offspring generation with gametes derived from frozen-thawed TT or TCSs. Moreover, clarification is needed on whether it is better to cryopreserve TT or TCS.WIDER IMPLICATIONSFertility restoration techniques are very promising and expected to be implemented in the clinic in the near future. However, inter-center variability needs to be overcome and the gametes produced for reproduction purposes need to be subjected to safety studies. With the perspective of a future clinical application, there is a dire need to optimize and standardize cryopreservation and safety testing before using frozen-thawed TT of TCSs for fertility restoration.

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Objective: To optimize the storage medium and period during short-term preservation of human testicular tissue. Design: First, human testicular tissue fragments from five patients were kept at 4 C for 3 days in different media (Dulbecco's modified Eagle's medium [DMEM]/F12, DMEM/F12 þ 20% human serum albumin [HSA], DMEM/F12 þ 50% HSA, and HSA). Secondly, fragments from four patients were kept in DMEM/F12 for 3, 5, or 8 days at 4 C. Setting: Laboratory research environment. Patient(s): Adult human testicular tissue. Intervention(s): Biopsy and short-term storage of human testicular tissue at different conditions. Main Outcome Measure(s): Viability, general tissue morphology, Sertoli cell morphology, number of spermatogonia, and apoptosis. The experimental conditions were compared with fresh control samples. Result(s): Storing human testicular tissue in DMEM/F12 did not alter any of the investigated parameters. In most conditions containing HSA, tissue morphology was altered, and in all of them the Sertoli cell morphology was affected. The number of spermatogonia was only affected when tissue was stored in 100% HSA. In the second part of the study, tissue morphology deteriorated significantly as of 5 days of hypothermic storage, and Sertoli cell morphology after 8 days. Conclusion(s):Human testicular tissue can be preserved for 3 days at 4 C in DMEM/F12 without altering tissue morphology, Sertoli cell morphology, number of spermatogonia, or number of apoptotic cells. (Fertil Steril Ò 2016;105:1162-9. Ó2016 by American Society for Reproductive Medicine.

Upon freeze-thawing or vitrification-warming, and assessment of donor-derived spermatogenesis after transplantation, Dulbecco's modified Eagle's medium supplemented with 1.5M dimethyl-sulphoxide, 10% fetal calf serum and 60 µM of Z-VAD-(OMe)-FMK in vials at a freezing rate of -1°C/min was optimal.

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