35 IN VITRO BOVINE EMBRYO DEVELOPMENT AFTER NUCLEAR TRANSFER BY HANDMADE CLONING USING A MODIFIED WOW CULTURE SYSTEM

Abstract

The effect of the microenvironment on embryo development during in vitro culture of zona-free embryos after nuclear transfer is still unclear. The aim of this experiment was to determine the effect of the dimensions of the well (WOW; Vajta et al. 2000 Mol. Reprod. Dev. 55, 256-264) culture system on the in vitro development of handmade cloned bovine embryos to the blastocyst stage. Appropriately ground steel needles were pressed slightly by hand to the bottom of the well of a polystyrene four-well dish (176740, Nunc, Life Technologies AS, Roskilde, Denmark). Embryos were produced by the handmade cloning (HMC) technique (Vajta et al. 2003 Biol. Reprod. 68, 571-578) with modifications, using primary cultures of skin fibroblast cells from an adult cow as nuclear donors. Cumulus-oocyte complexes were in vitro-matured in M-199 supplemented with 10% estrous cow serum (ECS), FSH, hCG, and estradiol (E2) for 17 h. After maturation, cumulus cells were removed by pipetting. Following zona pellucida removal in 0.5% protease (Sigma, Brazil), zona-free oocytes were incubated for 15 min in 5 mg/mL cytochalasin B (Sigma) and subsequently hand-bisected and screened for nuclear material under UV light after incubation in 10 mg/mL bisbenzimide (Hoechst 33342). Next, two enucleated halves and one donor cell were aggregated after a quick exposure to phytohemagglutinin (PHA) and subsequently fused by two electrical DC pulses of 1 kV/cm for 20 µs, in a BTX 453 chamber coupled to an ECM 2001 Electro Cell Manipulator System (BTX, Inc., San Diego, CA, USA), with additional exposure to brief pre- and post-fusion AC pulses of 15 V. Reconstructed embryos were chemically activated in 5 mM ionomycin (Sigma) for 5 min, followed by 2 mM 6-DMAP (Sigma) for 2.5 h. Finally, activated reconstructed cloned embryos were in vitro-cultured in one of two WOW culture systems (larger vs. smaller micro-wells) in 4-well plates containing 400 mL modified SOF medium supplemented with 10% ECS, under mineral oil, at 5% CO₂, 5% O₂ and 90% N₂, and 39°C for 7 days. In Group 1 (large-size micro-well), embryos were cultured in individual cylindrical micro-wells with an inner diameter and depth of approximately 280 and 250 mm, respectively, whereas in Group 2 (small size micro-well), embryos were cultured in individual conical micro-wells with approximately 130 mm inner diameter and 150 mm depth. Data analysis was performed by the chi-square test. After four replicates, cleavage rates were significantly higher (P < 0.05) in Group 2 (51/63, 80.9%) than in Group 1 (43/67, 64.1%). Embryo development to the blastocyst stage was also greater (P < 0.05) in the small micro-wells (16/63, 25.3%) than in the large ones (8/67, 11.9%). In summary, these results show a significant increase in cleavage and blastocyst developmental rates in handmade cloned embryos cultured in a modified WOW system using individual small size micro-wells, suggesting that a small, tighter micro-well provides favorable in vitro conditions for embryo development.