**Figure 1.** **eNOS was involved in PF activation.** (**A**) Immunohistochemical staining of eNOS, iNOS and nNOS in ovaries at 3 dpp and 7 dpp. Scale bar, 40 μm. (**B**) The relative mRNA levels of eNOS, iNOS and nNOS were assessed in ovaries from 1-7 dpp using qRT-PCR. (**C**) Immunofluorescence staining of eNOS in ovaries at 3 dpp and 7 dpp. Sections were labeled for eNOS (red), the oocyte marker Y box protein 2 (MSY2, green) and the nuclear marker Hoechst (blue). Arrows indicate PFs and triangles indicate GFs. Scale bar, 40 μm. (**D**) Ovaries at 1 dpp were treated with the vehicle, the NO donor SN (100 μM) or the NO inhibitor NM (1 mM) for six days (n=6). The ovarian morphology was analyzed after hematoxylin staining. Triangles indicate GFs. Scale bar, 40 μm. (**E**) The numbers of PFs and GFs/the total number of follicles were analyzed. (**F**) The numbers of oocytes with diameters larger than 20 μm were counted. **, *** and **** denote statistical significance at *p* < 0.01, *p* < 0.001 and *p* < 0.0001 respectively. Different letters with the same color denote statistical significance at *p* < 0.001 (Red letters represent the proportions of PFs, while black letters represent the proportions of GFs).