356 IMPROVED ENUCLEATION EFFICIENCY FOR PIG SOMATIC CELL NUCLEAR TRANSFER BY DENUDING OOCYTES AT 30 HOURS OF IN VITRO MATURATION

Abstract

Oocytes for somatic cell nuclear transfer (SCNT) have to be removed from their cumulus cells before enucleation. Denuding oocytes by vortexing or repeated pipetting makes the polar body (PB) deviate from the metaphase (MII) plate, which in turn makes it difficult to remove DNA materials completely during enucleation. We hypothesized that denuding oocytes at 30 h of IVM maintains the MII plate and PB in a closer position and therefore makes it easy to enucleate. To test this hypothesis, oocytes were matured in TCM-199 supplemented follicular fluid, hormones, EGF, cysteine, and insulin for first 22 h, and in a hormone-free medium for 18 h with three modifications: (1) cumulus cells were removed from oocytes just prior to enucleation at 40 h of IVM (control), (2) oocytes were denuded at 30 h of IVM and co-cultured with their detached cumulus cells for 10 h (D+), and (3) oocytes denuded at 30 h of IVM were cultured without cumulus cells (D-). After IVM, some oocytes were stained with Hoechst 33342 and photographed by a digital camera; the distance between the MII plate and the PB were measured using an image analysis program (ImageJ 1.36; http://rsb.info.nih.gov/ij). Also, the enucleation rate after blind enucleation and the in vitro development of SCNT embryos were determined. For SCNT, oocytes were enucleated, and nuclear material from donor cells (skin fibroblasts from a miniature pig) was inserted; oocytes were then electrically fused, and activated 1 h after fusion. SCNT embryos were cultured in a modified NCSU-23 (Park et al. 2005 Zygote 13, 269-275) for 6 days. Embryos were examined for their cleavage and blastocyst formation on Days 2 and 6, respectively (the day of SCNT was designated Day 0). Data were analyzed by the GLM procedure and the least significant difference test in SAS (SAS Institute, Cary, NC, USA). The distance between the MII plate and the PB was significantly (P < 0.01) shorter in D+ and D- embryos (19.4 and 18.9 µm, respectively) than in the controls (25.5 µm). Enucleation rates after blind enucleation were significantly (P < 0.01) higher in D+ and D- groups (77% and 72%, respectively) than in the controls (60%). Oocyte maturation (89–91%), SCNT embryo cleavage (71–77%), blastocyst formation (4–5%), and embryo cell number (39-45 cells/embryo) were not altered by different denuding methods. The perivitelline space (PVS) increases with time during maturation and denudation, after PB extrusion markedly enhances PB deviation. It is likely that increased PVS in control oocytes enhanced PB deviation during denudation and then resulted in lower enucleation rate. In conclusion, the results of this study indicated that denuding at 30 h of IVM maintained the MII plate and the PB in a closer position and improved enucleation efficiency without impairing developmental capacity of SCNT embryos.

This work was supported by the Research Project on the Production of Bio-organs (No. 200506020601), Ministry of Agriculture and Forestry, Republic of Korea.