Influence of the developmental stage and the equilibration time on the outcome of ultrarapid cryopreservation of mouse embryos

Effect of vitrification on mitochondrial distribution and membrane potential in mouse two pronuclear (2-PN) embryos

The present study was designed to investigate the effect of vitrification on mitochondrial distribution, membrane potential (Deltapsi) and microtubule distribution in mouse 2-PN embryos, as well as to document the relationship between mitochondrial distribution and developmental ability of those embryos. Mitochondrial distribution was examined by fluorescence microscopy technology. Results indicated that: (1) The rate of mitochondrial ring formation around pronuclei in vitrified 2-PN embryos was significantly lower than in fresh ones (67.3 +/- 3.0% vs. 84.9 +/- 3.1%) (P < 0.05). (2) Blastocyst development rate of vitrified 2-PN embryos without mitochondrial rings (61.7 +/- 4.5%) was significantly lower than that of vitrified embryos with mitochondrial rings (82.1 +/- 2.8%). (3) Following staining by 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethyl-imidacarbo-cyanine iodide (JC-1), most red-colored mitochondria (high Deltapsi) were distributed peripherally around pronuclei and along cell membranes of fresh 2-PN embryos. Conversely, red-colored mitochondria were greatly diminished in vitrified embryos, with green mitochondria (low Deltapsi) evenly distributed throughout the cytoplasm. The proportion of fresh 2-PN embryos with obvious aggregation of high Deltapsi mitochondria (84.2 +/- 2.2%) was significantly higher than that of vitrified embryos (26.7 +/- 3.0%) (P < 0.05). (4) The proportion of fresh embryos with microtubules distributed around pronuclei (83.5 +/- 3.4%) was similar to that of vitrified embryos (74.7 +/- 2.5%). In conclusion, vitrification affected mitochondrial distribution and decreased the mitochondrial membrane potential in mouse 2-PN embryos, events which may affect subsequent developmental viability of such embryos.

Conventional freezing, straw, and open-pulled straw vitrification of mouse two pronuclear (2-PN) stage embryos

Little is known on the cryopreservation of mouse pronuclear (PN) stage embryos. In the present experiment the mouse 2-PN stage embryos were cryopreserved by conventional freezing, straw, or open-pulled straw (OPS) vitrificaiton methods. The conventional freezing solution was 1.5 mol/L ethylene glycol (EG), and vitrification solutions were EFS30 (30% EG, Ficoll, and sucrose), EFS40 (40% EG, Ficoll, and sucrose), EDFS30 (15% EG, 15%dimethyl sulfoxide [DMSO], Ficoll, and sucrose), or EDFS40 (20% EG, 20%DMSO, Ficoll, and sucrose). The blastocyst rate of 2-PN stage embryos cryopreserved by conventional method (30.4%) was lower than those vitrified by straw method with EDFS (56.9% to 69.1%), by OPS method (66.0% to 85.7%), and that of control (80.8%) (P < 0.05). With a given vitrificaiton solution EFS30, EFS40, EDFS30, or EDFS40, the blastocyst rate of embryos vitrified by the OPS method (66.7%, 66.0%, 85.7%, or 76.9%) was higher than that of those vitrified by the straw method (46.8%, 43.8%, 69.1%, or 56.9%) (P < 0.05). When mouse 2-PN-stage embryos were vitrified with EDFS30 by straw or OPS method, the highest blastocyst rate was achieved (69.1% or 85.7%) and was similar to that of the control, respectively. The embryos transfer results revealed that the full-term development of blastocysts derived from 2-PN stage embryos vitrified by OPS method with EDFS30 (19.9%) was similar to that of the control (23.5%), and higher than that of those cryopreserved by conventional freezing (9.3%) (P < 0.05). The present research demonstrates that the OPS method, especially with EDFS30, is more effective in cryopreserving mouse 2-PN embryos.

Gene expression profiles and in vitro development following vitrification of pronuclear and 8-cell stage mouse embryos

The analysis of differences in gene expression, responding to cryopreservation may explain some of the observed differences in further development of the preimplantation stage embryos. The aim of this study was to create a link, for the first time, between morphological/developmental observations and gene activity changes following cryopreservation of embryos. Efficiency of two vitrification methods, solid surface and in-straw vitrifications for pronuclear-stage mouse zygotes and 8-cell stage mouse embryos was compared based on morphological survival, blastocyst formation, and changes in embryonic gene expression. Both stages of embryos were vitrified by SSV using 35% ethylene glycol (EG) for vitrification solution (VS) and in-straw vitrification using 40% EG for VS. No significant differences were found between immediate survival rates of embryos vitrified by SSV and in-straw vitrification in both stages. Blastocyst rates were significantly higher with SSV and not significantly different from that of control. These results showed that SSV was more efficient than in-straw vitrification. Treatment with cytochalasin-b did not improve cryosurvival during SSV. The quantification of selected gene transcripts from single embryo (6 embryos/treatment group) were carried out by quantitative real-time RT-PCR. It was performed by adding 1/8 of each embryo cDNA to the PCR mix containing the specific primers to amplify housekeeping gene (beta-actin), heat shock protein gene (Hsp70), genes related to oxidative stress (MnSOD and CuSOD), cold stress (CirpB, Rbm3), and cell-cycle arrest (Trp53). We found upregulation of all six stress-related genes at 3 hr post-warming in pronuclear stage embryos. Expression of these genes showed much higher level (2-33-fold) in in-straw vitrification than in in vitro control embryos. In SSV-treated embryos we could detect only slight changes (0.3-2-fold). At 10 hr post-warming, all genes were downregulated in embryos vitrified by in-straw method. In SSV-treated group expression of Hsp70 showed slight increase and Trp53 showed decrease. In contrast to pronuclear stage, there was no clear pattern of gene expression changes after vitrification in 8-cell stage embryos. Several genes were upregulated both at 3 and 10 hr post-warming. Moreover, we found upregulation of beta-actin gene which we expected to use as a reference gene in in-straw treated embryos in both 3 and 10 hr post-warming, while in pronuclear stage embryos and in SSV treatment there was no effect on beta-actin expression level. There was no difference in gene expression between blastocysts developed from fresh or vitrified embryos. In conclusion, the real-time RT-PCR method from single embryo opened new opportunities for the understranding of molecular events following cryopreservation. The upregulation of stress-related genes at 3 hr post-warming in pronuclear stage embryos might have been an early indicator of reduced viability following in-straw vitrification in good correlation with the developmental data to blastocyst stage.

Vitrification of pronuclear-stage mouse embryos on solid surface (SSV) versus in cryotube: Comparison of the effect of equilibration time and different sugars in the vitrification solution

The cryopreservation of pronuclear-stage embryos has particular importance in transgenic technology and human assisted reproductive technology (ART). The objective of this study was to improve the efficiency of cryopreservation of pronuclear-stage mouse embryos. Two vitrification methods (solid surface vitrification (SSV) vs. vitrification in cryotube) have been compared with special emphasis on the effect of the exposure of the embryos to the solutions for various times and the sugar content (trehalose, sucrose, or raffinose) of the vitrification solutions. Pronuclear-stage embryos were either exposed to 1 M dimethyl sulfoxide (DMSO) + 1 M propylene-glycol (PG) solution for 2, 5, 10, or 15 min or not exposed to this "equilibration" solution. The vitrification solutions consisted of 2.75 M DMSO and 2.75 M PG in M2 medium supplemented with 1 M trehalose (DPT), 1 M sucrose (DPS), or 1 M raffinose (DPR). In the cryotube method, groups of 15-25 embryos were transferred into a 1.8 ml cryotube containing 30 microl of DPT, DPS, or DPR. After 30 sec, the cryotubes were directly plunged into liquid nitrogen (LN(2)) and stored for 1 day to 1 month. Vitrified samples were warmed by immersing the cryotubes in a 40 degrees C water bath and then immediately diluted with 300 microl of 0.3 M trehalose, sucrose, or raffinose in M2. In the SSV method, after equilibration 15-20 embryos were exposed to DPT, DPS, or DPR solutions for around 20 sec before being dropped in 2-microl drops onto a pre-cooled (-150 to -180 degrees C) metal surface. Vitrified droplets were stored in cryovials in LN(2). Warming was performed by transferring the vitrified droplets into 0.3 M solutions of trehalose, sucrose, or raffinose at 37 degrees C, respectively. Results showed that both SSV and cryotube vitrification methods can result in high rates of in vitro blastocyst development (up to 58.3 and 68.5% with DPR, respectively), not statistically different from that of the controls (58.3 and 64.4%). Even without the equilibration step prior to vitrification, relatively high-survival rates have been achieved, except for the DPS solution. In conclusion, vitrification of pronuclear-stage mouse embryos can result in high rates of in vitro development to blastocyst, and the use of raffinose in the vitrification solution is advantageous to improve cryosurvival.

Production of transgenic mice from vitrified pronuclear-stage embryos

Cryopreservation of pronuclear-stage embryos would be useful for transgenic technology and genome preservation purposes. We compared a novel vitrification technique (solid surface vitrification, SSV) with another vitrification method in straws for cryosurvival and to generate transgenic progeny from cryopreserved mouse zygotes following microinjection. The SSV solution consisted of 35% ethylene glycol (EG), 5% polyvinyl-pyrrolidone (PVP), and 0.4 M trehalose in M2 supplemented with 4 mg/ml BSA; the in straw vitrification solution was 7 M EG in M2 plus BSA. In experiment I, we compared the effect of the vitrification solutions alone, without cooling. No reduction was detected in survival and cleavage rates. In experiment II, SSV yielded a significantly higher percentage of morphologically normal zygotes (96%) that also cleaved at significantly higher rates (80%) when compared to that following "in straw" vitrification (68 and 66%, respectively). Cleavage rate in the non-vitrified control group (93%) was significantly higher than that of both vitrified groups. Following embryo transfer, there was no difference in the rate of pups obtained from the SSV, "in straw" vitrified, and control groups (97/457, 21%; 15/75, 20% and 56/209, 27%, respectively). In experiment III, SSV vitrified and fresh embryos were used for pronuclear DNA injection. Survival rate of vitrified embryos after microinjection was reduced compared to nonvitrified ones (64 vs. 72%, respectively; P < 0.05); however, development to two-cell stage was not different (76 vs. 72%, respectively). Following embryo transfer of vitrified vs. fresh microinjected embryos, in both cases 10% live pups were generated, including transgenic pups. The results demonstrated that the efficiency of generating transgenic pups from SSV vitrified pronuclear zygotes is comparable to that from fresh embryos.

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.

Production of transgenic rats using cryopreserved pronuclear-stage zygotes

We investigated the application of cryopreserved pronuclear-stage zygotes for the production of transgenic rats. Most of the pronuclear-stage zygotes cryopreserved by conventional two-step freezing or vitrification appeared morphologically normal, but the proportion of frozen zygotes that developed into fetuses following transfer (59.7-60.2%) was higher than that of vitrified zygotes (5.5-22.1%). When the frozen-thawed zygotes were used for DNA microinjection, 97.5% survived after DNA microinjection and 25.1% of the transferred zygotes developed into fetuses. These proportions were comparable to those of the fresh control zygotes (97.0 and 30.0%, respectively). The integration efficiency of the exogenous DNA into fetuses was similar between the frozen group (3.3% per injected zygote) and the control group (3.5%). These results indicate that pronuclear-stage rat zygotes can be successfully cryopreserved by conventional two-step freezing for production of transgenic rats.

Effect of cryoprotectants and their concentration on post-thaw survival and development of expanded mouse blastocysts frozen by a simple rapid-freezing procedure

Experiments were conducted to develop a simple rapid-freezing protocol for expanded mouse blastocyst-stage embryos. The effect of type of cryoprotectant (ethylene glycol and propylene glycol) and its concentrations (4.5, 6.0 and 7.0 mol/L each with 0.5 mol/L sucrose) on morphological survival and development in vitro were studied. The survival and development of embryos frozen with best concentration of each cryoprotectant pre-exposed to either a low concentration (1.5 mol/L with 0.25 mol/L sucrose) of the respective cryoprotectant or ascending concentrations of sucrose were also compared. The in vivo development of embryos frozen with best protocol (pre-exposure to 1.5 mol followed by 7.0 mol ethylene glycol) was compared with nonfrozen embryos. The rate of re-expansion and hatching was influenced by the type and concentration of the cryoprotectant. A significantly higher re-expansion and hatching rate was achieved at 7.0 mol of both cryoprotectants compared with 4.5 and 6.0 mol of the respective cryoprotectants. When comparing 2 cryoprotectants, a higher (P < 0.05) rate of hatching was obtained with ethylene glycol at 7.0 mol compared with a similar concentration of propylene glycol. The highest re-expansion (91%) and hatching (86%) of expanded blastocysts was achieved with pre-exposure of embryos to a low concentration of ethylene glycol followed by freezing in the same cryoprotectant at 7.0 mol. The transfer of embryos frozen using this protocol resulted in the development of live fetuses. The proportion of live fetuses in the pregnant recipients with frozen-thawed embryos were not different from those transferred nonfrozen embryos (49 vs 57%). It may be concluded that simple rapid-freezing with dehydration in ascending sucrose concentrations or pre-equilibration in a low concentration of ethylene glycol or propylene glycol followed by exposure to the respective cryoprotectant at 7.0 mol resulted in high survival and development of expanded blastocysts. Ethylene glycol at 7.0 mol with pre-equilibration is, however, most effective for cryopreservation of this stage in the mouse.

High survival rate of one-cell mouse embryos cooled rapidly to -196 degrees C after exposure to a propylene glycol-dimethylsulfoxide-sucrose solution

We investigated the effect of duration of exposure at 22 degrees C to a solution containing dimethylsulfoxide (Me2SO), propylene glycol (PROH), and sucrose on the survival rate of one-cell mouse embryos cooled rapidly to -196 degrees C. The cryoprotectant solutions were made up on a molar basis and six different times of exposure were tested (1, 2.5, 5, 10, 15, and 20 min). After rapid thawing and dilution in a 1 M sucrose solution the survival rate was calculated as the number of hatching or hatched blastocysts formed per frozen-thawed and recovered one-cell embryo. We demonstrated that the optimum time of exposure depends on the type of cryoprotectant solution used. The optimum time of exposure was 1 min when a 4.5 M PROH-0.25 M sucrose solution was used, 2.5 min when a 2.25 M PROH-2.25 M Me2SO-0.25 M sucrose solution was used, and 5 min when a 4.5 M Me2SO-0.25 M sucrose solution was used. For prolonged exposure times beyond the optimum, survival rates were the highest when solutions containing 2.25 M PROH-2.25 M Me2SO-0.25 M sucrose were used. To identify factors that may influence the survival rates, we carried out further experiments on one-cell embryos in the different cryoprotectant solutions. We evaluated (i) the survival rates after exposure without freezing and (ii) the volume changes during exposure. It is suggested that the optimum time of exposure to a cryoprotectant solution depends on cryoprotectant permeation and that detrimental effects after prolonged exposure are a consequence of chemical toxicity. When one-cell mouse embryos were exposed for 3 min to a solution containing 2.25 M PROH-2.25 M Me2SO-0.25 M sucrose before rapid cooling to -196 degrees C, 98% of the zygotes were morphologically intact after rapid thawing. There was no further loss in viability during in vitro culture on after transfer in vivo as compared to unfrozen control embryos.