The oocyte (egg) is the mother of all cells, propagating genetic and epigenetic information, integrating paternal components, and reprogramming the fertilized embryo to generate offspring. However, the oocyte is mysteriously short-lived. Female reproductive potential is limited in mammalian species including humans where cessation of fertility is reached by mid-age, and 10-15% of couples are infertile due to female factors in half of these instances. Because they are restricted in number, differentiation of oocytes from embryonic stem cells (ESCs) will facilitate analyses of the genetic, epigenetic, and environmental factors affecting oocyte development and contributing to infertility and/or birth defects. The differentiation of functional oocytes from autologous embryonic stem cells will ultimately enable the understanding, potential treatment, and/or prevention of infertility in women. Additionally, ESC-derived oocytes may be used as nuclear transfer recipients to study cellular reprogramming and early embryo development.
ESCs can differentiate into cell types of three somatic germ layers and the germline. Although putative germ cells with oocyte-like characteristics were previously reported to spontaneously differentiate from mouse ESCs in vitro, functional analyses and correlation to endogenous oocyte development in vivo have been limited. In this study, I developed germ cell-specific reporter and surface marker strategies to isolate and characterize mouse ESC-derived germ cells. Using these strategies, I established a developmental timeline and genetic program of female germ cell differentiation from ESCs in vitro that initially paralleled endogenous oocyte development in vivo. However, ESC-derived oocyte maturation eventually failed, as complete meiotic progression and ovarian follicle formation were not detected.
To overcome this in vitro maturation bottleneck, I examined endogenous mouse fetal oocyte development following transplantation to develop a synchronized ovarian niche that was competent to direct oocyte maturation. I then transplanted ESC-derived oocytes into this ovarian niche and analyzed transplants for oocyte maturation in ovarian follicles. Indeed, ESC-derived oocytes functionally integrated into the ovarian niche, recruited endogenous somatic granulosa cells, and successfully developed to the primary follicle stage. In summary, this study provides rigorous evidence of physiologically relevant ESC-derived oocyte identity and function, and presents a potential clinical strategy for treatment of infertility through ovarian niche-based transplantation.