Comparison of open pulled straw (OPS) vs glass micropipette (GMP) vitrification in mouse blastocysts

The purpose of this study was to investigate the use of a glass micropipett e (GMP) as a vessel for vitrification of mouse blastocysts, and to compare the post-thaw survival of these blastocysts with those cooled in open pulle d straws (OPS). The GMP vessel permits higher freezing and warming rates th an OPS due to the higher heat conductivity of glass and lower mass of the s olution containing the embryos. Groups of 6 mouse blastocysts were sequenti ally placed into 2 vitrification solutions before being loaded into either the OPS or GMP vessels and immersed into LN2 within 20 to 25 sec. Post-thaw blastocysts were serially washed in 0.25 and 0.15 M sucrose in holding med ium (HM) and modified human tubal fluid medium (mHTF), each for 5 min, and then cultured in mHTF supplemented with 10% FCS for 24 h. The rate of blast ocyst re-expansion did not differ significantly for OPS (93.5%) and GMP (95 .0%) methods (P < 0.05). The hatching rate in OPS (88.7%) was similar to th at in GMP (90.0%) but was lower than for the unvitrified control embryos (9 8.3%, P < 0.05). To determine the optimal embryo population per GMP vessel, the pipettes were loaded with 2 to 10 embryos. The rate of blastocyst re-e xpansion after vitrification was significant for 2 to 4 embryos than for 6 to 10 embryos per vessel. In addition, the rate of blastocyst re-expansion was significantly lower if blastocysts were vitrified in the wide rather th an the narrow portion of the micropipette (100 vs 87.5%; p < 0.05) even whe n only 4 blastocysts were loaded per vessel. These results indicate that bo th vitrification vessels can provide high rates of embryo survival. However , the GMP vessel does not need a cap to protect the vessel from floating af ter immersion in LN2. The number and location of the embryos (narrow versus wide portion of capillary) were considered to be limiting factors to the v iability of mouse embryos. (C) 2000 by Elsevier Science Inc.