At 3 dpp, the primordial follicular pool is established gradually. In our study, almost all of the germ cells in the cortex,in which the nuclei of oocytes expressed PAR6 highly could be classified as primordial follicles. Only a few of germ cells had not formed primordial follicles, which showed negative PAR6 staining and degenerate characteristics: small size and shrink. The primary follicles surrounded by cubical-type granulosa cells were observed in the medulla, and the expression of PAR6 became weak in the oocyte nuclei. These phenomena were also observed in the ovaries of mature mice,. However, the expression of PAR6 was not detected in all stages of prespermatogonial and spermatogonial developments. During the establishment of the primordial follicular pool, the Terbuthylazine number of germ cells decreased sharply, since a large number of germ cells that could not form primordial follicles had undergone apoptosis. We counted the numbers of total germ cells, PAR6 positive germ cells and follicles in the section from the largest cross-section through the center of the ovary at 19.5 dpc, 1 dpp and 3 dpp. The number of germ cells declined, the number of follicles increased, but the PAR6 positive germ cells remained steady. At 3 dpp, the number of follicles was the same as that of PAR6 positive germ cells since the germ cells in follicles showed positive staining, and others without follicular structures showed negative staining. Primordial follicle forming starts from the 17.5 dpc, and about one third of germ cells Phosalone contribute to the establishment of the primordial folliclular pool by 3 or 4 dpp, which endow all the available follicles for reproduction of animals. The peak of germ cell apoptosis occurs at prenatal, as well as neonatal when germ cells transit to the meiotic diplotene stage from the pachytene stage. In our study, about one third, half and almost all of germ cells expressed PAR6 at 19.5 dpc, 1 dpp and 3 dpp, respectively, but the number of PAR6 positive germ cells was constant during these stages. Several germ cells without PAR6 expression at 3 dpp had not formed primordial follicles and presented the degenerate characteristics.