Spermatogenesis in vitro: myth or reality?

In vitro spermatogenesis has been investigated using embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs) or spermatogonial stem cells (SSCs) by many laboratories worldwide. However, for human fertility preservation, patient-specific ESCs are not available and the safety of using these cells to generate gametes that transfer their genome to the next generation has still not been thoroughly investigated. Therefore, in this thesis, we aim to recapitulate spermatogenesis in vitro using SSCs that are available in most of the patients with impaired spermatogenesis and can already be cryopreserved and propagated in vitro.
We developed a three-step in vitro culture system to achieve complete meiosis using mouse SSCs cultured on immortalized Sertoli cells. Interestingly, in this culture system, although the in vitro-formed pachytene spermatocytes often displayed unsynapsed chromosomes and did not form sufficient meiotic crossovers, meiosis still progressed to the meiotic metaphase stages, and even sporadically formed spermatid-like cells. This indicated that the meiotic checkpoints that normally eliminate such aberrant pachytene spermatocytes were not completely functional in vitro. Considering that meiotic checkpoints prevent genomic instabilities from being transmitted to the offspring, we proposed that meiotic checkpoints should be thoroughly investigated before considering gametogenesis in vitro for clinical use.
Finally, we reviewed the current progress in the field of spermatogenesis in vitro, with specific focus on meiosis and meiotic checkpoint mechanisms and systematically screened current research articles. Also clinical prospects of in vitro-derived gametes and the criteria that are applied to assess in vitro meiosis are discussed.