359 ICSI AND ACTIVATION OF OOCYTES RECOVERED FROM TRANSITIONAL AND CYCLING MARES

Abstract

Ovarian follicular development in mares during the transitional period before the breeding season leads to an accumulation of antral follicles of various sizes. The quality of oocytes at this stage may be compromized until the first seasonal ovulation. In this study, we evaluated the developmental competence of oocytes recovered from transitional and cyclic mares, and the effect of zygote activation after intracytoplasmic sperm injection (ICSI). A 2 × 2 × 2 factorial experiment consisting of oocytes from transitional and cyclic mares, two follicle sizes (10 to 20 and 20+ mm), and two treatments (control and activated) was conducted. Follicular oocytes of 14 mares were aspirated in March and April (transitional) and May to July (cyclic) five times per each period at 10-day intervals, without use of hCG. Oocytes aspirated from mares were matured in vitro in a defined medium similar to SOF plus FSH, LH, epidermal growth factor (EGF), insulin-like growth factor (IGF), estradiol (E₂), prostaglandin (P₄) and 10% FCS, for 30 ± 1 h under 5% CO₂ in air at 38.5°C; oocytes with a first polar body were used for ICSI. Motile sperm from frozen-thawed semen were used for sperm injection with a piezo-driven pipet. For activation after ICSI, presumptive zygotes were cultured in G1.3 containing 0.02 µM phorbol 12-myristate 13-acetate (PMA) for 2 h, and then in 2 mM 6-dimethylaminopurine (6-DMAP) for 3 h under 6% CO₂ in air at 38.5°C. Zygotes were cultured in 50 µL drops of DMEM/F12 containing 10% FCS for 9 days at 38.5°C in 5% CO₂/5% O₂/90% N₂. Medium was replaced every 3 days. Cleavage and blastocyst rates were calculated based on non-degenerating injected oocytes. Data were analyzed by Fisher's exact test. A total of 115 and 78 oocytes were recovered from cyclic and transitional mares. Average maturation rates to MII in the respective groups were 76.5 and 65.4%, respectively (P < 0.07), and those of 10 to 20 and 20+ mm follicle groups were 70.6 and 80.0%, respectively (P > 0.05). The average cleavage rate in cyclic mares was higher than in transitional mares, and that of the activated group averaged over follicle sizes was higher than that of controls (P < 0.05; Table 1); those of 10 to 20 and 20+ mm follicle groups were not different (P < 0.05; Table 1). Blastocyst rates per oocyte within main effects were not different (P < 0.05; Table 1). Oocytes from transitional mares had lower cleavage rates than those of cyclic mares, but blastocyst development was similar. Activation of zygotes clearly improved cleavage rates of in vivo-derived immature equine oocytes after ICSI.

Table 1. Main effect means of responses after ICSI