Positive effect of partial zona-pellucida dissection on the in vitro fertilizing capacity of cryopreserved C57BL/6J transgenic mouse spermatozoa of low motility

Although cryopreservation of mouse spermatozoa has recently become available for use, as yet there are considerable differences in fertilization efficiency of cryopreserved spermatozoa among various mouse strains. In this study, oocytes subjected to partial dissection of the zona pellucida (PZD) were inseminated with frozen-thawed C57BL/6J mouse spermatozoa. At 30, 60, 120, and 240 min after insemination, the oocytes were washed in human tubal fluid medium and cultured for 36 h. The fertilization rates of the PZD oocytes in each group at 6-7 h after insemination were significantly higher than that of the zona-intact control (73-88% vs 12%, respectively) (p < 0.01); but the incidence of polyspermy was nevertheless quite low (1.3-2.4%). The development rates of the monospermic oocytes to the morula and early blastocyst stages were in the 87-92% range, with 31-40% of those developing into offspring after embryo transfer. When the cryopreserved spermatozoa of C57BL/6J transgenic mice were used to fertilize PZD oocytes, the fertilization rates were as high as (73-76%) those of the PZD oocytes inseminated with the cryopreserved C57BL/6J spermatozoa, with 30-31% of the morulae and early blastocysts derived from the monospermatic oocytes developing into offspring. These results indicate that PZD of oocytes may provide an alternative when the fertilizing capacity of mouse spermatozoa has been compromised by cryopreservation.