Development of early-stage embryos of the Japanese field vole, Microtus montebelli, in vivo and in vitro.

Although ovulation could be easily induced in the Japanese field vole by administering pregnant mares' serum gonadotrophin and hCG, the number of embryos obtained varied from 1 to 47 (mean, 9.6). One-cell embryos were small (57.8-63.3 microns in diameter; mean, 61.0 microns) compared with those in other mammals. Development of the preimplantation vole embryos in vivo was similar to that of mouse embryos. The first cleavage occurred between 24 and 26 h after mating. The second cleavage was between 46 and 52 h after mating, and subsequent cleavages occurred at about 12 h intervals. Blastocysts were clearly observed in the uterus 4 days after mating. Vole embryos could be cultured in vitro from the late two-cell to the blastocyst stage in M16 medium. However, development of one-cell and early two-cell embryos in vitro was limited, and few cleaved beyond the four-cell stage. Eliminating sodium pyruvate from M16 medium significantly improved the development of early two-cell embryos into blastocysts (P < 0.05). The Japanese field vole may be a useful experimental animal for reproductive biology, comparable with the mouse.