Ultrastructural differences in the mitochondria of normal versus oocytes with a dark zona pellucida, and the morphological characteristics and density of mitochondria in normal versus oocytes with a dark zona pellucida.

The arched cristae were determined using electron microscopy (A). Mitochondrial cristae had a spherical or oval shape and presented arch-like cristae, which were located peripherally or transversely (arrows, A). In oocytes with a DZP, an intimate relationship between the SER and abnormal mitochondria was observed (arrows, B); numerous abnormal mitochondria with vesicle complexes (C); distribution and density of mitochondria were similar to the control. The whole part of the oolemma was filled with small and large vacuoles and abnormal mitochondria (arrows, D). M: Mitochondria; AM: abnormal mitochondria; V: vacuole; V1: vesicles encircled by flattened mitochondria; SER: smooth endoplasmic reticulum associated with mitochondria; Scale bar (A, B, C, and D): 500 nm. The rate of abnormal mitochondria was significantly lower in the control group (16.4 vs. 38.7%) (E). There were no significant differences between groups with long diameter/short diameter (1.73 vs. 1.62) and the density of mitochondria in each cytoplasm (1.16 vs. 1.12/µm2) (F and G). Bars indicated the standard deviation (SD) of the mean. *: compared with the control group, the abnormal mitochondria was significantly increased (P<0.05) in oocytes with a DZP. Note: Abnormal mitochondria rate: the number of abnormal mitochondria/total number of mitochondria; density of mitochondria: number of mitochondria/µm².