52 HYALURONIC ACID IMPROVES CRYOTOLERANCE OF BUFFALO (BUBALUS BUBALIS) IN VITRO-DERIVED EMBRYOS

Abstract

Although in vitro embryo production efficiency in buffalos has greatly improved over the years, the in vitro-produced embryos show lower viability and resistance to cryopreservation. Therefore, it is necessary to optimize the in vitro culture conditions to improve embryo quality. Hyaluronic acid, a glycosaminoglican present in oviducal and uterine fluids, has been shown to successfully support in vitro development of bovine embryos (Stojkovic et al. 2002 Reproduction 124, 141–153). The aim of this study was to evaluate the influence of high concentrations of hyaluronic acid (HA) during late in vitro culture on blastocyst development, as well as on their cryotolerance after cryotop vitrification in buffalos. In vitro matured and fertilized buffalo oocytes (n = 1007) from slaughterhouse ovaries were cultured for 4 days in SOFaa supplemented by 8 mg mL^–1 of BSA in a controlled gas atmosphere consisting of 5% CO₂, 7% O₂ and 88% N₂, in humidified air, at 38.5°C. On Day 4, cleavage rate was assessed (75.2%) and all of the cleaved elements were divided into 3 different late culture groups: 8 mg mL^–1 of BSA (n = 244; group A), 8 mg mL^–1 of BSA supplemented by 6 mg mL^–1 of HA (n = 251; group B) and 1 mg mL^–1 of BSA supplemented by 6 mg mL^–1 of HA (n = 262; group C). On Day 7 after IVF, embryo outcome was assessed and all of the embryos were vitrified by cryotop [De Rosa et al. 2007 Ital. J. Anim. Sci. 6 (Suppl 2), 747–750] and cultured for 24 h. The resistance to cryopreservation was evaluated by assessing the survival rate on the basis of morphological criteria and the percentage of embryos reaching a more advanced developmental stage after 24 h culture. Data were analysed by the chi-square test. No differences in blastocyst rate were recorded among groups (43.9, 44.3 and 40.0%, respectively in A, B and C groups). However, out of the total embryos, a higher percentage of Grade 1 hatched blastocysts (Robertson and Nelson 1998 Manual of the International Embryo Transfer Society 9, 103–16) was observed in group C (P < 0.05) than in groups A and B (14.3, 18.8 and 25.5% in A, B and C groups, respectively). Although the supplementation with HA did not improve the survival rates following vitrification-warming (51.1, 59.4 and 58.4% in A, B and C groups, respectively), the percentage of vitrified-warmed embryos that resumed development and reached a more advanced developmental stage after culture increased (P < 0.01) in group C (20.7, 27.7 and 37.6% in A, B and C groups, respectively). In conclusion, the addition of 6 mg mL^–1 of HA, together with a limited protein source (i.e. 1 mg mL^–1 of BSA), during late culture improved buffalo embryo quality, indicated by both the greater percentage of advanced-stage embryos and by the resumption of development after post-warming culture.