1. Laboratory of Electron Microscopy; North Sichuan Medical College; Nanchong 637007 People's Republic of China
2. Department of Histology and Embryology; North Sichuan Medical College; Nanchong 637007 People's Republic of China
3. Department of Histology and Embryology; Chongqing Medical University; Chongqing 400016 People's Republic of China