Somatic cell nuclear transfer in horses: effect of oocyte morphology, embryo reconstruction method and donor cell type.

The objective of the present work was to investigate and clarify the factors affecting the efficiency of somatic cell nuclear transfer (NT) in the horse, including embryo reconstruction, in vitro culture to the blastocyst stage, embryo transfer, pregnancy monitoring and production of offspring. Matured oocytes, with zona pellucida or after zona removal, were fused to cumulus cells, granulosa cells, and fetal and adult fibroblasts, and fused couplets were cultured in vitro. Blastocyst development to Day 8 varied significantly among donor cells (from 1.3% to 16%, P < 0.05). In total, 137 nuclear transfer-embryos were transferred nonsurgically to 58 recipient mares. Pregnancy rate after transfer of NT-embryos derived from adult fibroblasts from three donor animals was 24.3% (9/37 mares transferred corresponding to 9/101 blastocysts transferred), while only 1/18 (5.6%) of NT-blastocysts derived from one fetal cell line gave rise to a pregnancy (corresponding to 1/33 blastocysts transferred). Overall, seven pregnancies were confirmed at 35 days, and two went to term delivering two live foals. One foal died 40 h after birth of acute septicemia while the other foal was healthy and is currently 2 months old. These results indicate that (a) the zona-free method allows high fusion rate and optimal use of equine oocytes, (b) different donor cell cultures have different abilities to support blastocyst development, (c) blastocyst formation rate does not correlate with pregnancy fate and (d) healthy offspring can be obtained by somatic cell nuclear transfer in the horse.