Endoplasmic reticulum stress is involved in small white follicular atresia in chicken ovaries

During the hierarchical development of follicles in laying hens, a large number of small white follicles (SWFs) undergo atresia caused by apoptosis rather than growing into large white follicles (LWFs), which reduces the number of follicles developing into hierarchical preovulatory follicles and results in declines in egg production. Therefore, figuring out factors responsible for SWFs atresia is necessary to improve egg production of laying hens. Endoplasmic reticulum stress (ERS) is one of the possible causes of cell apoptosis. However, the association between SWFs atresia and ERS in hens is yet to be answered. In this study, cell proliferation and apoptosis, expression levels of proteins and genes related to ERS, and cell ultrastructure were compared to elucidate whether ERS occurred in atretic small white follicles (ASWFs). Subsequently, a tunicamycin-induced ERS model and a serum withdrawal-induced atresia model of SWFs in vitro were established to investigate whether ERS could lead to follicular atresia. The results showed that the cell apoptosis increased significantly while the cell proliferation decreased evidently in ASWFs compared with SWFs. Meanwhile, swollen and vacuolization deformation of ERs was observed in the GCs but not TCs of ASWFs. Results of experiments in vitro demonstrated that treatment with serum withdrawal or tunicamycin inhibited cell proliferation and promoted cell apoptosis in SWFs, and the adverse effects of tunicamycin on SWFs manifested a dose-dependent increase. In comparison with the controls, treatment with serum withdrawal or tunicamycin upregulated expression levels of proteins and genes in ERS pathway. Moreover, the dose of 5 μg/mL tunicamycin seemed to be the optimal concentration for establishing the tunicamycin-induced ERS model. Meanwhile, the results of cell ultrastructure observation showed that ERs in GCs of SWFs of serum withdrawal group and treatment with 5 μg/mL tunicamycin group became swollen, dilatation and vacuolization. In conclusion, the data of this study indicated that ERS is involved in SWF atresia in laying hens, and ERS in GCs might be more severe than that in TCs.