Abstract: Mouse Primordial Germ Cells (PGCs) proliferate and migrate into the Genital Ridges (GRs) and the urogenital complex (UGC) is then constructed from the GR and the mesonephros. However, the mesonephros disappear at an early stage of subsequent embryonic development and the interaction between mesonephros and PGCs are not well understood. In the present study, it has been investigated the need of mesonephros in PGC differentiation by the ectopic transplant. The GR or UGC of female mouse 12.5 days post coitum (dpc) were transplanted into the kidney capsules of female immunodeficient mice. At 4 and 8 weeks after transplantation, the kidney capsules were recovered and the cell types in the grafts were identified by histological analysis. In UGCs graft of 4 weeks after transplantation, the follicles containing with growing oocyte were observed in all recipient kidney. The structures had the characteristics of secondary follicles with zona pellucida and multi-layered granulosa cells. Antibody staining indicated that these follicles differentiated from PGCs and were not apoptotic. But, no follicles were observed at 4 weeks in transplantation of GR without mesonephros. Similarly, follicles were not present at 8 weeks after transplantation of GR or UGC. The mesonephros is necessary for forming the follicles into kidney capsules of recipient immunodeficient mouse. However, the transplantation of UGCs in ectopic kidney capsules should be performed within 4 weeks in order to study the processes of follicular formation/development.