The effect of follicle size and cryoprotectants on nuclear maturation and early embryonic development of vitrified - thawed Awassi sheep oocytes (Ovis aries)

In this study, two experiments were conducted to study the effect of both the follicle size and the cryoprotectants dimethyl sulfoxide (DMSO) and ethylene glycol (EG) on the main phases of nuclear maturation (Experiment I), cleavage stages and embryo quality (Experiment II) of Awassi sheep oocytes. Follicles were classified into two groups: small follicles (SF) (1-2 mm) and large follicles (LF) (> 2 mm). Oocytes were vitrified in three solutions: A (30% DMSO), B (30% EG) and C (15% DMSO and 15% EG). In Experiment I, the resulting vitrified-thawed oocytes in solution C achieved the best rates after the control group (fresh), respectively as the rates of maturation, germinal vesicle (GV), metaphase II(M-II), arrest, and lyses were 85.71% (P = 0.04), 8.33% (P = 0.02), 72.92% (P = 0.04); LF group, 15.25% (P = 0.04), and 5.08% (P = 0.04); SF group, respectively. In Experiment II, the same group of oocytes achieved the best rates after the control group, as the rates of fertilization, cleavage, 2-16 cell, Type3, blastocyst, and Type1 embryos were 63.28% (P = 0.001), 57.46% (P = 0.001), 40.38% (P = 0.04), 38.46% (P = 0.04); LF group, 30.00% (P = 0.01), and SF group 36.67% (P = 0.001), respectively, while the vitrified-thawed oocytes in A solution (SF group) reached the highest rate of Type 2 embryo quality (58.06%; P = 0.01). No significant differences were noticed in the germinal vesicle breakdown (GVBD), metaphase I (M-I) and morula stage. Vitrification of oocytes obtained from follicles with a diameter of more than 2 mm in a cocktail solution of DMSO (15%) and EG (15%) led to a significant increase in the yield and quality of the resulting sheep embryos.