Efficiency of AsynchronouslyIn Vitro-Matured Oocytes as Recipients for Nuclear Transfer and of Blind Enucleation in Zona-Free Bovine Cloning

Timing factors affecting blastocyst development in equine somatic cell nuclear transfer

In nuclear transfer (NT), exposure of donor cell chromatin to the ooplast cytoplasm may aid reprogramming; however, the length of exposure feasible is limited by the developmental life span of the oocyte. We examined the effect of duration of nucleus-cytoplasmic exposure before activation and of in vitro maturation (IVM) in equine NT. In experiment 1, 24 h IVM and a delay of 2, 5, or 8 h between reconstruction and activation yielded 4%, 15%, and 11% blastocysts, respectively. In experiment 2, a 5-h activation delay yielded 17% and 22% blastocysts with two donor cell lines. In experiment 3, using a 5-h activation delay, the blastocyst rate was significantly higher using oocytes after 20 h IVM than after 24 h IVM; however, only 28% of oocytes were in metaphase II (MII) at 20 h. In experiment 4, oocytes were denuded of cumulus at 20 h, and those in metaphase I (MI) were returned to culture for 3 h (20+3H treatment); blastocyst rates were 30% and 27%, respectively (8-h and 5-h delay to activation, respectively). Four live foals resulted from the transfer of 17 blastocysts (24%) produced using MII oocytes and a 5- or 8-h activation delay. Use of equine oocytes immediately after reaching MII, combined with a longer delay from reconstruction to activation, increased developmental competence after equine NT.

Effects of the timing of cumulus cell removal from bovine oocytes on enucleation rate and subsequent development after somatic cell nuclear transfer

We examined the effects of the timing of cumulus cell removal from in vitro-matured (IVM) bovine oocytes on enucleation efficiency and subsequent in vitro development after nuclear transfer (NT). Cumulus cells were removed from IVM oocytes by pipetting, by low-speed vortexing and pipetting or by high-speed vortexing at 12, 15 or 18 h of IVM, and then denuded oocytes were further cultured for 6, 3 or 0 h, respectively (18 h of IVM in total). There was no difference in the rate of extrusion of the first polar body (PB1) among the groups. The success rate of blind enucleation of oocytes denuded at 12 h (before PB1 extrusion) by high-speed vortexing (81.7%) was significantly higher than that of oocytes denuded at 18 h by high-speed vortexing (67.8%) but similar to those of oocytes denuded by pipetting after low-speed vortexing (78.6-84.1%) or pipetting alone (84.6-86.9%). After high-speed vortexing, the percentage of oocytes in which metaphase II (MII) chromosomes were located adjacent to the PB1 tended to be lower for the oocytes denuded at 18 h than that for the oocytes denuded at 12 h. In contrast, by pipetting or low-speed vortexing and pipetting, the timing of denuding did not affect the relative location of MII chromosomes. The timing and method of denuding did not affect the fusion and blastocyst formation rates of NT embryos. These results suggest that high-speed vortexing is applicable only to cumulus cell removal from oocytes prior to PB1 extrusion, while pipetting or low-speed vortexing followed by pipetting is useful regardless of the PB1 formation status and leads to successful blind enucleation of IVM oocytes.