Charting the epigenetic landscape of the male germline: Towards safe assisted reproductive technology

Infertility is a pathological condition in which a couple is unable to achieve pregnancy after a year or more of unprotected intercourse. Approximately 1 in 8 couples are affected by infertility and in 10-15% of these cases infertility is caused by a complete absence of mature germ cells in the ejaculate of the male partner. The inability to produce functional mature sperm cells can also manifest as a side effect of gonadotoxic cancer treatments that do not only target the tumor but also healthy cells, including the fragile pool of spermatogonial stem cells in the testis. In adult patients, sperm cryopreservation followed by IVF/ICSI can be an option to preserve the ability to conceive, but this is not possible for prepubertal patients that do not produce sperm yet. In this thesis we comprehensively determined the epigenetic and transcriptomic states of (1) different human male germ cells currently used in assisted conception and (2) cultured testicular cell populations including spermatogonial stem cells that can potentially be used in future novel assisted reproductive techniques, with the ultimate goal of assessing their suitability for clinical practice. Collectively, our findings describe previously unknown variations in the DNA methylation pattern of in vivo testicular germ cells that could be of clinical importance. We also identified a number of aspects regarding cultured spermatogonial cell fractions that might impact the way in which spermatogonial stem cells are propagated in vitro.