Generation of live offspring from vitrified mouse oocytes of C57BL/6J strain

In mammals, unfertilized oocytes are one of the most available stages for cryopreservation because the cryopreserved oocytes can be used for assisted reproductive technologies, including in vitro fertilization (IVF) and intracytoplasmic sperm injection. However, it has generally been reported that the fertility and developmental ability of the oocytes are reduced by cryopreservation. C57BL/6J mice, an inbred strain, are used extensively for the production of transgenic and knockout mice. If the oocytes from C57BL/6J mice can be successfully cryopreserved, the cryopreservation protocol used will contribute to the high-speed production of not only gene-modified mice but also hybrid mice. Very recently, we succeeded in the vitrification of mouse oocytes derived from ICR (outbred) mice. However, our protocol can be applied to the vitrification of oocytes from an inbred strain. The aim of the present study was to establish the vitrification of oocytes from C57BL/6J mice. First, the effect of cumulus cells on the ability of C57BL/6J mouse oocytes to fertilize and develop in vitro was examined. The fertility and developmental ability of oocyte-removed cumulus cells (i.e., denuded oocytes, or DOs) after IVF were reduced compared to cumulus oocyte complexes (COCs) in both fresh and cryopreserved groups. Vitrified COCs showed significantly (P