Ultrastructural Changes of Sheep Oocytes Follow Vitrification by Different Methods and In Vitro Maturation

Our aim was determination of the sheep oocytes ultrastructural changes follow vitrification and in vitro maturation. Good quality isolated cumulus-oocyte complexes (COCs) were randomly divided into non-vitrified control, conventional straw, cryotop and solid surface vitrification groups. In the conventional and cryotop methods the vitrified COCs were plunged directly into liquid nitrogen (LN2), whereas in the solid surface group the vitrified COCs were cooled before plunging into LN2. Fresh and vitrified-warmed healthy COCs were matured in vitro and then their ultrastructural changes were evaluated. The results indicated that vitrification by cryotop and solid surface methods preserved the total arrangement of the ooplasm, whereas conventional straw vitrification disturbed the ooplasm organization. Additionally, the number of vacuoles in the ooplasm increased after vitrification, some of these vacuoles were filled partially or completely with lipids and some had filamentous scaffolding. Also, in the mature oocytes, the amount and the density of cortical granules decreased after conventional straw and solid surface vitrification. Cryotop group compared with other vitrification methods could preserve oocyte ultrastructure properly and create a condition the same as like as the control group.