Dynamic three-dimensional epigenomic reorganization for the development of undifferentiated spermatogonia in mice

Germ cells are unique in their ability to acquire totipotency. Toward this end, they reorganize their three-dimensional (3D) epigenome during their development, including epigenetic reprogramming in primordial germ cells that differentiate mitotic prospermatogonia and ensuing unique epigenetic programming for generating undifferentiated spermatogonia/spermatogonial stem cells (SSCs). Advances in low-input epigenomic and 3D genomic techniques, along with complementary in-depth characterization of scalable in vitro reconstitution systems for germ cell development, that is, in vitro gametogenesis, have elucidated a number of fundamental events during these processes, including insulation augmentation in highly open chromatin following epigenetic reprogramming in mitotic prospermatogonia and insulation erasure and further euchromatization accompanied by chromosomal radial repositioning in undifferentiated spermatogonia/SSCs. These 3D epigenomic organizations likely serve as a foundation for generating fully functional gametes. Elucidating the mechanisms underlying 3D epigenomic reorganization during germ cell development will be instrumental not only for understanding the basis for totipotency but also for further advancing in vitro gametogenesis.