Reconstitution of gametogenesis in vitro: meiosis is the biggest obstacle

Germ-line cells are responsible for transmitting genetic and epigenetic information across generations, and ensuring the creation of new individuals from one generation to the next. Gametogenesis process requires several rigorous steps, including primordial germ cell (PGC) specification, proliferation, migration to the gonadal ridges and differentiation into mature gametes such as sperms and oocytes. But this process is not clearly explored because a small number of PGCs are deeply embedded in the developing embryo. In the attempt to establish an in vitro model for understanding gametogenesis process well, several groups have made considerable progress in differentiating embryonic stem cells (ESCs) and adult stem cells (ASCs) into germ-like cells over the past ten years. These stem cell-derived germ cells appear to be capable of undergoing meiosis and generating both male and female gametes. But most of gametes turn out to be not fully functional due to their abnormal meiosis process compared to endogenous germ cells. Therefore, a robust system of differentiating stem cells into germ cells would enable us to investigate the genetic, epigenetic and environmental factors associated with germ cell development. Here, we review the stem cell-derived germ cell development, and discuss the potential and challenges in the differentiation of functional germ cells from stem cells.