Improvement in the development of oocytes from C57BL/6 mice after sperm injection.

The C57BL/6 mouse strain is used widely for producing transgenic and knockout strains. Sperm motility is extremely low after a freeze-thaw process. Although intracytoplasmic sperm injection (ICSI) can be used to produce embryos from sperm with low or even no motility, its success rate is poor in the C57BL/6 strain. In particular, the survival of C57BL/6 oocytes after ICSI is extremely low compared with that of hybrid strains. We found that the survival percentages of C57BL/6J oocytes (63% and 64%) were lower than those of B6D2F1 oocytes (80% and 80%) when B6D2F1 and C57BL/6J sperm were injected, respectively. For C57BL/6J mice, 87%, 72%, 64%, 56%, and 59% of oocytes survived after ICSI in media containing 61.62, 71.62, 81.62, 91.62, and 101.62 mM NaCl, respectively. In addition, 64%, 81%, and 79% of oocytes survived after ICSI in media with 4.83, 14.83, and 24.83 mM KCl, respectively. Our results suggest that the survival of C57BL/6J oocytes after ICSI is improved by using Na(+)-deficient and K(+)-rich media.