Abstract： Objective To study the ultrastructural features of myocytes in uterine junctional zone (JZ).Methods From August 2010 to August 2013,there were 16 pre-menopause patients who suffered from cervical neoplasm to be performed hysterectomy.Samples of JZ and outer myometrium (OM) of hysterectomy specimens were collected.There were 8 specimens from the proliferative-phase and 8 specimens from the secretory-phase of endometrium.Ultrastructural features of JZ and OM were examined by using transmission electron microscopy and the related indices of myocytes were compared by using Student's t test.Results At JZ,there were more cytoplasmic process in the myocytes.The myocytes of JZ exhibited significant difference compared with those of OM.Firstly,the contractile structural components,such as the dense patches,dense bodies and the myofilaments were less abundant.In contrast,the perinuclear cell organelles were more distinct.The mitochondria,endoplasmic reticulum and Golgi apparatus were more prominent,denoting active protein synthesis.Secondly,the mean diameter of cell and nuclei demonstrated cyclic change.In proliferative phase of endometrium,the cell diameters of JZ and OM were (4.70_±0.52) and (4.69± 1.20) μm,respectively,which there were no significant difference(P=0.987).While in secretory phase,the cell diameters of JZ and OM were (3.75±0.36) and (4.92±0.51) μm,which there were significant difference (P=0.006).In proliferative phase,the nuclei diameters of JZ and OM were (3.24±0.41) and (2.90±0.62) μm,and in secretory phase,the nuclei diameters of JZ and OM were (2.44±0.27) and (2.92±0.44) μm.There were no significantly different in both phases (P=0.374,P=0.097).The diameters of cell and nuclei had cyclical changes (P ＜ 0.05).However,the cyclical changes were absent in OM (P＞ 0.05).Thirdly,the myofilaments/cytoplasm ratio of JZ in proliferative and secretory phases were 0.27±0.04 and 0.34±0.03,which were significantly less than those of OM in respective phases (0.49±0.03and 0.54±0.03;P=0.000,P=0.000).The myofilaments/cytoplasm ratio exhibited cyclical changes in JZ (P=0.029),but in OM,the cyclical changes were absent (P=0.083).Conclusions Compared with OM,ultrastructures associated with synthetic organelles are prominent,whereas the contractile organelles are reduced.And there are the cyclical changes in ultrastructural characteristics.The ultrastructural features of JZ are the basis of its physiology.