Effect of partial incision of the zona pellucida by piezo-micromanipulator for in vitro fertilization using frozen-thawed mouse spermatozoa on the developmental rate of embryos transferred at the 2-cell stage

Cryopreservation of mouse spermatozoa is widely used, although considerable strain differences in fertilization rates using frozen-thawed mouse spermatozoa have been described. The C57BL/6 mouse strain is a very widely used for establishment of transgenic mice, but the fertilization rate associated with the use of cryopreserved C57BL/6 spermatozoa is very low compared with rates for other inbred strains. We have recently solved this difficulty by in vitro fertilization (IVF) in combination with partial zona pellucida dissection (PZD). However, this technique requires culture of fertilized eggs with PZD in vitro up to morula or blastocyst stage before transfer into the uterus because blastomeres are lost after transfer into the oviduct because of the relatively large artificial slit in the zona pellucida. To overcome this problem, we performed a partial zona pellucida incision by using a piezo-micromanipulator (ZIP) for IVF with frozen-thawed mouse spermatozoa. The blunt end of the micropipette touched the surface of the zona pellucida of the oocytes, and piezo pulses were used to incise the zona pellucida while the pipette was moved along by the surface of zona pellucida. The length of the incision was pir/6 mum. When cumulus-free ZIP and PZD oocytes were inseminated with frozen-thawed genetically modified C57BL/6J spermatozoa, the fertilization rates of ZIP and PZD oocytes were 52% and 48%, respectively. After embryo transfer at the 2-cell stage, 18% and 2% of the transferred embryos with ZIP and PZD developed to term, respectively. This difference was significant (P < 0.05). When ZIP and PZD zygotes were cultured to blastocyst stage and subsequently transferred to uterine horns of recipient animals, the difference between ZIP and PZD zygotes for development rate to full term was not significant. Our results indicate that ZIP is an effective alternative technique for IVF using cryopreserved mouse spermatozoa and subsequent embryo transfer.