Ultrastructure of human mature oocytes after slow cooling cryopreservation with ethylene glycol

The morphological characteristics of frozen–thawed human mature oocytes (n = 12) were studied by light and transmission electron microscopy following cryopreservation using a slow cooling protocol including increasing concentrations of ethylene glycol (0.5–1.5 mol/l) and sucrose 0.2 mol/l in the freezing solution. Fresh human mature oocytes (n = 12) were used as controls. Fresh and frozen–thawed oocytes appeared rounded in section, with a homogeneous cytoplasm, an intact oolemma and a continuous zona pellucida. Disorganization of mitochondria-smooth endoplasmic reticulum aggregates and a decreased complement of microvilli and cortical granules were frequently observable in frozen–thawed oocytes. Increased density of the inner zona pellucida, possibly related to the occurrence of zona ‘hardening’, was sometimes found associated with a reduced amount of cortical granules. In addition, delamination of the zona pellucida was evident in some frozen–thawed samples. Finally, numerous vacuoles and secondary lysosomes were detected in the ooplasm of most frozen–thawed oocytes. In conclusion, frozen–thawed oocytes treated with ethylene glycol may show a variety of ultrastructural alterations, possibly related, at least in part, to the use of this cryoprotectant. Thus, the ethylene glycol-based protocol of slow cooling herein described does not seem to offer significant advantages in terms of oocyte structural preservation.