Rat oocytes fertilized in modified rat 1-cell embryo culture medium containing a high sodium chloride concentration and bovine serum albumin maintain developmental ability to the blastocyst stage

A suitable chemically defined culture medium far 1-cell rat embryos (mR1ECM) was modified to obtain sperm penetration, and the developmental competence of oocytes fertilized in the medium was compared to that of oocytes fertilized in a traditional medium, modified Krebs-Ringer bicarbonate medium (mKRB), Sperm penetration was not observed when polyvinyl alcohol was replaced with ESA in mR1ECM (mR1ECM-BSA); the incidence was improved only when the osmolarity in mR1ECM-BSA was increased to that in mKRB (310 mOsm) by addition of NaCl, The proportion of oocytes penetrated in mR1ECM-BSA with NaCl increased (71.6 +/- 6.9%), which was not different compared to that in mKRB (76.7 +/- 13.7%). High incidences of sperm penetration (88.8 +/- 4.1% to 93.1 +/- 5.1%) were also observed when NaCl concentration in mR1ECM-BSA was increased from 76.7 mM to 100-130 mM. The incidence of embryos developing to the morula and blastocyst stages was higher when fertilized in mR1ECM-BSA containing 110-130 mM NaCl (91-94%) than in mKRB (70%), A total of 5 offspring were obtained after transfer of the morulae and blastocysts (69 embryos/ 7 females), These results demonstrate that a high developmental ability of rat embryos to the blastocyst stage is attained when the embryos have been fertilized in mR1ECM-BSA containing 110-130 mM NaCl and then cultured in mR1ECM.