Co-culture with bovine intact COCs during IVF of buffalo denuded oocytes completely restores their fertilizing and developmental competence

Removal of cumulus cells is necessary for several technologies such as vitrification, intracytoplasmic sperm injection, and nuclear transfer. However, it is known that the presence of cumulus cells during IVF of buffalo oocytes is fundamental for fertilization and embryo development (Gasparrini et al. 2007 Anim. Reprod. Sci. 98, 335–342; Nandi et al. 1998 Theriogenology 50, 1251–1262). The aim of this work was to evaluate whether co-culture with intact bovine cumulus–oocyte complexes (COC) during IVF would restore the developmental competence of denuded buffalo oocytes. Due to the scarce availability of buffalo ovaries, the somatic support was provided by bovine cumulus cells. Abattoir-derived COC were matured in vitro according to our standard procedures (Gasparrini et al. 2006, Theriogenology, 65, 275–287) and randomly distributed in 3 fertilization groups: 1) a control group of COC (n = 122), 2) a negative control of denuded oocytes (DO; n = 119), and 3) DO co-cultured with in vitro matured bovine COC (DO+COC; n = 103) in a 1:1 ratio (3 bovine COC + 3 denuded buffalo oocytes/50 μL drop). Fertilization was carried out with frozen–thawed spermatozoa from a tested bull in TALP medium supplemented by 0.2 mM penicillamine, 0.1 mM hypotaurine, and 0.01 mM heparin at 38.5°C under a controlled gas atmosphere of 5% CO2 in humidified air. After fertilization the zygotes were cultured in SOF medium including essential and nonessential amino acids and 8 mg mL–1 BSA, at 38.5°C under humidified 5% CO2, 7% O2, and 88% N2, up to the blastocyst stage. On Day 5 and on Day 7 (Day 0 = IVF) cleavage and blastocyst rates were respectively recorded. Data were analysed by chi-square test. As expected, cleavage and blastocyst rates were lower (P < 0.01) in DO (36.1 and 9.2%, respectively) compared with the control (67.2 and 27.1%, respectively). However, co-culture during IVF (DO+COC) significantly increased (P < 0.01) both parameters compared with DO, giving cleavage (70.9%) and blastocyst (27.2%) rates similar to the control. The results of this study demonstrated that co-culture with bovine intact COC during IVF of buffalo denuded oocytes completely restores their fertilizing capability and blastocyst developmental competence. We conclude that this may be a suitable strategy for preserving the developmental competence of oocytes devolved to technologies, such as oocyte vitrification, that require cumulus removal.