Positive effect of partial zona pellucida digestion on in vitro fertilization of mouse oocytes with cryopreserved spermatozoa.

Cryopreservation of mouse spermatozoa has been widely used; however, fertility of frozen spermatozoa in some strains, especially when inseminating cryopreserved oocytes, is low and may be improved by assisted fertilization techniques. The present study was performed to investigate the effect of partial zona pellucida (ZP) digestion on the in vitro fertilization (IVF) capacity of frozen mouse spermatozoa. Mouse oocytes were subjected to partial ZP digestion using acidic Tyrode's solution (pH 3.1). Fertilization rates in digestion groups (30 or 45 s) were higher (P < 0.05) than that of zona-intact control (78.3% or 86.3% vs. 52.5%). The recovery rate at 45 s was lower (P < 0.05) than that at 30 s (84.2% vs. 97.3%). Among vitrified oocytes, the fertilization rate in treatment group (digested for 30 s) was higher (P < 0.05) than that of zona-intact group (50.8% vs. 22.1%). After embryo transfer at the two-cell stage, 17.7% and 11.8% of transferred embryos derived from fresh and vitrified digested oocytes developed to term and showed no significant difference as compared with that from zona-intact oocytes (24.1%, P > 0.05). These results indicate that partial ZP digestion improves IVF efficiency of fresh and vitrified oocytes with frozen mouse spermatozoa, which can provide valuable information for in vitro assisted fertilization using cryopreserved gametes in the re-establishment of mouse colonies.