The mechanism regulating primordial follicle formation remains largely unexplored because of the developmental particularity of female germ cells and their ultimate functional structure as follicles. Using an in vitro follicle reconstitution culture model, we explored, in the present study, the possibility of producing transgenetic follicles in vitro. We found that mouse fetal ovarian germ cells progressively lose the flexibility for gene manipulation with their oogonia-oocyte transformation upon entering meiosis, the borderline of which was at around embryonic age of 13.5 days post coitus (dpc). Interestingly, we further observed that fetal ovarian cells, only at this age or beyond achieve the capacity to reform the follicles in culture. Screening of well-known marker gene (Zp1-3, Figalpha, and Cx43) expression in cultured fetal ovarian cells of various developmental ages revealed that Figalpha is one of the determining factors for normal primordial follicle formation. By conducting reciprocal follicle reconstitution experiments, we provided further evidence that a synchronized germ-somatic cell interaction determines the normal duration of primordial folliculogenesis. Besides uncovering a potentially important regulatory mechanism for normal oocyte differentiation and follicle formation, this observation offers an alternative approach to produce transgenic oocytes/follicles, and thus animal models.