Vitrification of Mouse Blastocyst Using Cryoloop

Youm, Hyewon; Kim, Soo Kyung; Song, Sang Jin; Park, Yong Seog; Koong, Mi Kyoung; Kang, Inn Soo

Abstract

OBJECTIVE
The aim of this study is to compare the efficiency of a method for the cryopreservation of mouse blastocyst.
METHODS
Mouse embryos were obtained at 2-cell stage and cultured to blastocyst stage in T6 medium supplemented with 10% fetal bovine serum. Morphologically normal blastocysts were collected and randomly divided to one control and four experimental groups. In control group, blastocysts were cultured in vitro continuously for additional two days. In group 2, blastocysts were exposed to vitrification solution (ethylene glycol) only without cryopreservation (exposure only group). In group 3, 4 and 5, blastocysts were cryopreserved by slow-freezing procedure with glycerol (slow-freezing group) or by vitrification procedure using EM grids (EM grids group) and cryoloop (cryoloop group), respectively. Frozen blastocysts were thawed and cultured for additional two days. Twenty four hours after thawing, some blastocysts were fixed and stained with Hoechst 33342 (bisbenzimide) and the number of nuclei in each blastocysts were counted to confirm the survival of blastocysts in experimental groups.
RESULTS
Survival rate and hatching rate of the blastocysts in slow-freezing group (24 h: 72.4% and 66.0%, 48 h: 63.2% and 64.6%) and EM grids group (24 h: survival rate 77.3%, 48 h: 70.1% and 71.4%) were significantly lower (X2-test p<0.05) than those of control group (24 h: 93.4% and 86.0%, 48 h: 88.5% and 90.7%). In contrast, the survival rate and hatching rate of the blastocysts in cryoloop group (24 h: 84.1% and 84.1%, 48 h 79.3% and 87.7%) is well compared with those in the control group. The mean (+/-SD) cell number of blastocyst in the exposure only (89.2+/-11.5), EM grids (85.0+/-10.3) and cryoloop (89.0+/-11.0) groups, except slow-freezing group (79.0+/-10.0), were not significantly different from that of control group (93.1+/-13.9) 24 h after thawing (Student's t-test).
CONCLUSION
This study demonstrates that higher survival rate of vitrified-thawed mouse blastocyst can be obtained using cryoloop as the embryo container at freezing rather than slow-freezing or vitrification using EM grids. The results of this study suggest that vitrification using cryoloop (with ethylene glycol) may be a preferable procedure for mouse blastocyst cryopreservation and could be applied to the human blastocyst cryopreservation.