Refreezing of murine intact and biopsied embryos by rapid-freezing procedure

Double vitrification-warming cycles, coupled with blastocyst biopsy, impair live birth but do not affect neonatal outcomes

OBJECTIVE

To identify whether the transfer of blastocysts that have been vitrified, thawed, biopsied, revitrified, and subsequently rethawed affects clinical outcome and neonatal outcome.

METHODS

A retrospective study was conducted in a single assisted reproduction technology center from September 2016 to March 2021. Women undergoing single frozen euploid blastocysts transfer were stratified into two groups based on number of vitrification-thawing cycles: single vitrification coupled with single biopsy (group A, n = 177) and double vitrification coupled with single biopsy (group B, n = 30). Pregnancy and perinatal outcomes of the two groups were compared.

RESULTS

Clinical pregnancy rates were similar between the two groups. Group B was associated with an increased likelihood of live birth when compared with group A by different multivariable analysis models (model 1: odds ratio, 0.42 [95% confidence interval, 0.18-0.97], P = 0.041; model 2: odds ratio, 0.38 [95% confidence interval, 0.16-0.92], P = 0.033). No major obstetrical complication was reported in the two groups and only one malformation live birth was reported in group A.

CONCLUSION

The procedure of double vitrification-warming cycles, coupled with single biopsy, increases pregnancy loss and ultimately diminishes live birth but does not affect perinatal outcome. Future studies with a larger sample size would help to validate the results.

Offspring resulting from transfer of cryopreserved embryos in camel (Camelus dromedarius)

The dromedary embryos, collected at hatched blastocyst stage, survived freezing and thawing in the presence of a high concentration of ethylene glycol (7.0 mol/L) with sucrose (0.5 mol/L) and direct plunging in liquid nitrogen. The rate of survival, as judged by the morphological appearance of the embryos after thawing, was high (92%). The transfer of frozen-thawed embryos into the recipients during the breeding (n=20) and non-breeding season (n=25) resulted in two and one pregnancy, respectively. One of the two pregnant recipients, with embryos transferred during the breeding season, delivered a normal healthy male calf at term. To our knowledge, this offspring is the first camelid produced following transfer of a frozen-thawed embryo.

Vitrification of biopsied mouse embryos

Solid surface vitrification (SSV) was compared with in-straw vitrification for cryopreservation of biopsied mouse embryos. Eight-cell stage embryos were zona drilled and one blastomere was removed. Developed morulae or blastocysts were vitrified in microdrop (35% EG + 5% PVP + 0.4 M trehalose) or in straw (7.0 M EG + 0.5 M sucrose). Following recovery, embryos were cultivated in vitro or transferred into recipients. Cryopreservation had an effect not only on the survival of biopsied embryos but also on their subsequent development in vitro. Cryosurvival of biopsied morulae vitrified in straw was significantly inferior to SSV. The post-warm development of biopsied and non-biopsied morulae was delayed on Day 3.5 and 4.5 in both vitrification groups. A delay in development was observed on Day 5.5 among vitrified non-biopsied blastocysts. The percentage of pups born from biopsied morulae or blastocysts following cryopreservation did not differ from that of the control. No significant differences could be detected between methods within and between embryonic stages in terms of birth rate. The birth rate of biopsied embryos vitrified in straw was significantly lower compared to the non-biopsied embryos. The novel cryopreservation protocol of SSV proved to be effective for cryopreservation of morula- and blastocyst-stage biopsied embryos.

Vitrification versus programmable rate freezing of late stage murine embryos: a randomized comparison prior to application in clinical IVF

A prospective randomized trial was performed to compare post-thaw development of murine blastocysts following programmable rate freezing and two methods of vitrification. Frozen 2-cell murine embryos (n = 429) thawed and cultured for 48 h, were randomly allocated by stage of development into four groups: control (not refrozen), programmable rate freezing (PR) in 0.25 ml straws, vitrification in flexible micropipettes by immersion in super-cooled (VSC) liquid nitrogen (LN2), and vitrification in flexible micropipettes by immersion in LN2 (VLN). Survival, developmental stage progression, presence or absence of an inner cell mass (ICM), and cell counts were recorded 24 h post-thaw. All measured outcomes were different between embryos from the control group and all freezing methods. Controlled-rate freezing resulted in the lowest total cell counts and fewest embryos with a distinct ICM. A higher percentage of embryos survived 24 h post-thaw, progressed to more advanced developmental stages and had higher total cell counts after VLN compared with PR. Moreover, fewer embryos, frozen by either PR or VSC, contained a detectable ICM compared with VLN. These data demonstrate that vitrification may be a better method for freezing murine blastocysts than PR, and may prove to be a superior method for freezing human blastocysts.

Effect of freezing rate and exposure time to cryoprotectant on the development of mouse pronuclear stage embryos

BACKGROUND

The effects of exposure time (20 versus 45 s) to a high concentration of cryoprotectant (7.0 mol/l ethylene glycol with 0.5 mol/l sucrose) and freezing rates (1200-10 300 degrees C/min) during rapid freezing of mouse pronuclear stage embryos on survival and development to blastocysts were investigated. Different freezing rates were achieved by directly plunging the straws (rapid freezing) and open pulled straws (OPS) in liquid nitrogen (OPS freezing) and by plunging the straws (super rapid) and OPS (super OPS) in a super cooled liquid nitrogen chamber (at -212 degrees C) before storage in liquid nitrogen.

METHODS

Morphologically intact mouse zygotes (n = 891) pre-equilibrated in 1.5 mol/l ethylene glycol for 5 min were either loaded in 0.25 ml straws containing cryoprotectant or loaded in OPS with 2 microl cryoprotectant. After 20 or 45 s of loading the straws or mixing in cryoprotectant and loading in OPS, they were plunged either directly in to liquid nitrogen or were plunged first in to liquid nitrogen in a super cooled chamber and then stored in liquid nitrogen. Zygotes were thawed and intact embryos cultured in vitro.

RESULTS

The rate of survival was higher (91%, P < 0.01) when zygotes were frozen with rapid freezing compared with super rapid, OPS and super OPS freezing rates with an exposure time of 20 s (70, 65, and 76% respectively). When zygotes were exposed to cryoprotectant for 45 s and frozen with rapid freezing rates, the survival was higher (86%, P < 0.01) compared with those frozen with OPS (62%) but was not different from those frozen with super rapid and super OPS freezing rates (81 and 75%). A higher rate of survival was observed when zygotes were exposed to cryoprotectant for 45 s and frozen with super OPS than with OPS freezing (75 versus 62%; P < 0.05). The rate of cleavage and development of intact zygotes to blastocysts was not different among the different groups.

CONCLUSION

Exposure of zygotes to a high concentration of cryoprotectant (7.0 mol/l ethylene glycol with 0.5 mol/l sucrose) for 20 or 45 s did not influence their survival and development and increasing the freezing rate from 1200-10 300 degrees C/min was of no advantage when using a rapid freezing procedure for freezing mouse pronuclear stage embryos.