Factors that affect oocyte vitrification in small ruminants

Abstract

At the end of its growth, the mammalian oocyte, include in preovulatory follicle, is the largest cell of the organism, with about 120 µm in diameter. The size of oocyte, together with the specific arrangement of its organels and cytoskeleton, makes a real challenge for cryopreservation techniques. For such large cell, methods that do not require equilibrium to cryopreservation, such as vitrification, are more promising. It is important to highlight that the cryopreservation of oocytes is more difficult than zygotes or later stage embryos and this technique is a challenging task because of oocyte sensitive nature to chilling and toxic effects of cryoprotectants. However, the development of a reliable method for oocyte cryopreservation would be an important advance in the field of reproductive biology for the preservation of genetic resources. The vitrification of mammalian oocytes is influenced by many variables, such as different cryoprotectants, vitrification techniques, presence or absence of cumulus cells, the oocyte structure, metabolism (such as level of lipid storage) and meiotic stage. Thus, all these factors should be considered to optimize the techniques and adapt them to oocytes from each species. Therefore, the present review aims to describe the main factors that affect oocyte vitrification in sheep and goats, reporting the main findings in both species, as well as perspectives of future improvements.