Vitrification of immature and in vitro matured bovine cumulus-oocyte complexes: effects on oocyte structure and embryo development

This study aimed to verify the effects of cryoprotectant and Open Pulled Straw (OPS) vitrification on ultrastructural changes in bovine oocytes. In experiment 1, the cryoprotectant exposure was analyzed by distributing immature and matured cumulus-oocyte complexes (COCs) in groups: control immature, immature and exposed to one vitrification solution (IVS1), immatured and exposed to two vitrification solutions (IVS1-2), matured and exposed to one vitrification solution (MVS1), matured and exposed to two vitrification solutions (MVS1-2), control matured, IVS1 post in vitro maturation (IVS1 post IVM) and IVS1-2 post in vitro maturation (IVS1-2 post IVM). In experiment 2, immature and matured COCs were distributed in: control and vitrified by OPS using VS1-2. Sample COCs from both experiments were evaluated for mitochondrial distribution, nuclear lamins (immature COCs) and meiotic spindle (matured COCs). In both experiments, COCs were in vitro matured and fertilized. In experiment I, nuclear lamins in immature COCs exhibited higher proportion of structures with irregular shape when treated with VS1 and VS1-2. Immature and matured COCs treated with VS1-2, and matured COCs treated with VS1 had lower (P < 0.05) blastocyst development (29%, 20% and 8%, respectively) than control and immature COCs treated with VS1 (51% and 41%, respectively). In experiment 2, immature oocytes exhibited more (P < 0.05) structures with irregular lamin (72%) than control (9%). Vitrification of matured COCs did not induce abnormalities of meiotic spindle, compared to control matured (25% and 37%, respectively) (P > 0.05). The cleavage rate was higher (P < 0.05) in matured vitrified than immature vitrified group (19% vs. 8%). In conclusion, immature COCs were more tolerant to cryoprotectant exposure. However, matured oocytes showed better embryo development (cleavage) after vitrification.