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, all 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.