The slow freezing of oocytes was the first oocyte conservation technique to develop. Although it offers good results in sperm conservation, it does not show good survival results in oocytes. The crystals formed as a consequence of slow freezing damage the oocyte, decreasing the success rates in assisted reproduction treatments.
Since the last decade, oocyte vitrification has been the main technique of choice for the conservation of oocytes. Cryopreservation allows us an ultra-fast freezing, which prevents the formation of ice from the water contained in the cell in question (the oocyte), thus avoiding any deterioration. During the vitrification process, we introduce the oocyte into a liquid nitrogen solution and reach a temperature of -196 °C in less than one second. The rapid speed means that the water contained in the oocyte does not freeze and vitrify, becoming a state similar to that of a consistent gelatine.