Application of the Zona-Free Manipulation Technique to Porcine Somatic Nuclear Transfer

Abstract

The recent demonstration of a successful zona-free manipulation technique for bovine somatic nuclear transfer (NT) that is both simpler and less labor intensive is of considerable benefit to advance the applications of this technology. Here, we describe that this method is also applicable to porcine somatic NT. Porcine cumulus oocyte complexes were matured in TCM-199 medium before sequential removal of the cumulus and zonae. Zona-free oocytes were bisected using a microknife, and the halves containing the metaphase plate (as determined by Hoechst 33342 staining) were discarded. Each half cytoplast was agglutinated to a single granulosa cell (primary cultures grown in 0.5% serum for 2–5 days prior to use) in phytohaemagglutinin-P. Subsequently, each half cytoplast-granulosa cell couplet was simultaneously electrofused together and to another half cytoplast. Reconstructed embryos were activated in calcium ionophore A23187 followed by DMAP and were then individually cultured in microwells in NCSU-23 medium. On day 7 after activation, blastocyst yield and total cell numbers were counted. Of 279 attempted reconstructed NT embryos, 85.0 ± 2.8% (mean ± SEM; n = 5 replicates) successfully fused and survived activation. The blastocyst rate (per successfully fused and surviving embryo) was 4.8 ± 2.3% (11/236; range, 0–12.8%). Total blastocyst cell count was 36.0 ± 4.5 (range, 18–58 cells). The blastocyst rate and total cell numbers of parthenogenetically activated and zona-free control oocytes propagated under the same conditions was 11.6 ± 3.9% (35/335 embryos; n = 3 replicates) and 36.8 ± 5.2, respectively. Developmentally halted embryos that could still be evaluated on day 7 possessed 54.4 ± 2.3% (53/96 embryos; n = 3 replicates) anucleate blastomeres, the latter representing 53.5 ± 6.6% of the blastomeres in such embryos. In conclusion, blastocyst yield was independent of activation efficiency and was likely reduced by insufficient nuclear remodeling, reprogramming, imprinting, or other effects. The data also suggest that fragmentation was a considerable problem that could conceivably contribute to halted development in a high proportion of embryos. The results indicate that the zona-free manipulation technique can be successfully applied to pig somatic NT. Although such zona-free early cleavage stage embryos cannot be transferred to recipients at present, this technique permits simplification of the NT technique for application in basic research, until pig nonsurgical blastocyst transfer becomes a realistic option.