Cryopreservation of sheep embryos using ethylene glycol

The objective of the study was to evaluate the use of ethylene glycol (EG) for cryopreservation of sheep embryos. A 2 × 2 factorial treatment arrangement examining one-step vs. two-step cryoprotectant addition and removal was used. The one-step cryoprotectant addition involved placement of embryos directly into 1.5 mol EG, whereas the two-step addition utilized an intermediate 10 min exposure to 0.75 mol EG. Similarly, the one-step cryoprotectant removal involved direct placement of thawed embryos into 1.0 mol sucrose, and the two-step procedure included a 10 min exposure to 0.25 mol sucrose before placement in 1.0 mol sucrose. A total of 185 frozen-thawed embryos was placed into in vitro culture for 96 h to determine viability. No differences were observed between cryoprotectant addition or removal techniques, and overall survival was 69%. To validate the results obtained in vitro, a limited number of embryo transfers was performed. Four ewes receiving a total of 11 frozen-thawed embryos produced eight lambs (73% survival) which compared favorably with 74% survival obtained by transferring 19 non-cryopreserved embryos to eight recipients. It is concluded that one-step addition of 1.5 mol ethylene glycol followed by one-step removal in a 1.0 mol sucrose gradient is an appropriate technique for cryopreservation of sheep embryos.     

Cryopreservation of ovine embryos: slow freezing and vitrification

Different methods for the cryopreservation of ovine embryos were evaluated in vitro (survival upon culture in vitro) and in vivo (pregnancy and lambing rates after transfer in field conditions). In the first 2 experiments, slow freezing conditions were evaluated. When glycerol and ethylene glycol were compared, no differences in the overall pregnancy rate were found (40.2 vs 51.3%), but better results were obtained with ethylene glycol than with glycerol in morulae (29.7 vs 59.4%, P < 0.05). In the second experiment, 2 methods of removing ethylene glycol were compared: a 1-step procedure using 0.5-M sucrose and a 3-step process for decreasing ethylene glycol concentration. There were no differences in the overall pregnancy rate (48.0 vs 48.0%) between the 2 methods. The last series of experiments were designed to compare 2 vitrification solutions: propylene glycol--glycerol (PG) and ethylene glycol--Ficoll 70--sucrose (EFS). There were no differences between the 2 vitrification solutions, based on the overall pregnancy rate (28.1 vs 40.0%). The vitrification technique and specially with EFS solution has resulted in good pregnancy rates. The EFS solution was particularly efficacious with morulae (55.5% pregnancy). These results demonstrate that vitrification with EFS can be used successfully for the cryopreservation of ovine embryos.     

In vivo and in vitro survival of goat embryos after freezing with ethylene glycol or glycerol

The aim of the present study was to compare the survival rates of goat morulae and blastocysts after different freezing procedures. The viability of frozen-thawed embryos was assessed both in vivo and in vitro. Two cryoprotectants, ethylene glycol and glycerol, were used and three cryoprotectant removal procedures were compared: progressive dilution in 1.0, 0.5, 0.3 and 0 M of cryoprotectant in PBS; a similar progressive dilution with cryoprotectant in PBS plus 0.25 M of sucrose; or one-step transfer in PBS containing 0.25 M of sucrose. In vitro development of frozen-thawed blastocysts was always higher than that of frozen morulae irrespective of the cryoprotectant (52 129 = 40.3% vs 23 161 = 14.3% ; P< 0.001). In vivo, however, frozen-thawed morulae developed equally as well as blastocysts after an identical freezing-thawing protocol. Development both in vivo and in vitro showed ethylene glycol to be a better cryoprotectant than glycerol for goat embryos at both developmental stages (23 vs 0%, 45 vs 35% in vitro; 34.5 vs 21%, 35 vs 23% in vivo for morulae and blastocysts, respectively).     

Comparison on in vitro fertilized bovine embryos cultured in KSOM or SOF and cryopreserved by slow freezing or vitrification

The objectives of this study were to identify an improved in vitro cell-free embryo culture system and to compare post-warming development of in vitro produced (IVP) bovine embryos following vitrification versus slow freezing. In Experiment 1, non-selected presumptive zygotes were randomly allocated to four medium treatments without co-culture: (1) SOF + 5% FCS for 9 days; (2) KSOM + 0.1% BSA for 4 days and then KSOM + 1% BSA to Day 9; (3) SOF + 5% FCS for 4 days and then KSOM + 1% BSA to Day 9; and (4) KSOM + 0.1% BSA for 4 days and then SOF + 5% FCS to Day 9. Treatment 4 (sequential KSOM-SOF culture system) improved (P > 0.05) morulae (47%), early blastocysts (26%), Day-7 blastocysts (36%), cell numbers, as well as total hatching rate (79%) compared to KSOM alone (Treatment 2). Embryos cultured in KSOM + BSA alone developed slowly and most of them hatched late on Day 9, compared to other treatments. In Experiment 2, the sequential KSOM-SOF culture system was used and Day-7 blastocysts were subjected to following cryopreservation comparison: (1) vitrification (VS3a, 6.5 M glycerol); or (2) slow freezing (1.36 M glycerol). Warmed embryos were cultured in SOF with 7.5% FCS. Higher embryo development and hatching rates (P < 0.05) were obtained by vitrification at 6h (71%), 24h (64%), and 48h (60%) post-warming compared to slow freezing (48, 40, and 31%, respectively). Following transfer of vitrified embryos to synchronized recipients, a 30% pregnancy rate was obtained. In conclusion, replacing KSOM with SOF after 4 days of culture produced better quality blastocysts. Vitrification using VS3a may be used more effectively to cryopreserve in vitro produced embryos than the conventional slow freezing method.     

Quick freezing of one-cell mouse embryos using ethylene glycol with sucrose

One-cell mouse embryos were frozen by direct plunging into liquid nitrogen (LN(2)) vapor after equilibration in 3 M ethylene glycol with 0.25 M sucrose (freezing medium) for 5 to 40 minutes. After thawing, the embryos were cultured in vitro and the effects of the equilibration period and dilution method were examined. No significant difference was observed in the in vitro survival of embryos when 0.5 or 1.0 M sucrose was used for the dilution of the cryoprotectant for each equilibration period. The highest survival rate (67.2%) was obtained when the embryos were equilibrated for 10 minutes, and the cryoprotectant diluted with either 0.5 or 1.0 M sucrose after thawing. Shorter (5 minutes) or prolonged (40 minutes) equilibration of embryos in the freezing medium yielded significantly lower survival rates. Dilution by direct transfer of the frozen-thawed embryos into PB1 resulted in lower survival rates than when 0.5 or 1.0 M sucrose was used. The in vitro development to the blastocyst stage of one-cell mouse embryos frozen after 10 minutes equilibration in the freezing medium and diluted after thawing in 0.5 M sucrose was significantly lower than the control (68.0 vs 92.7%). However, transfer of the blastocysts developing from frozen-thawed one-cell mouse embryos into the uterine horns of the recipients resulted in fetal development and implantation rates similar to the control.     

Survival of IVF-derived bovine embryos frozen in glycerol or ethylene glycol

Survival of IVF-derived bovine embryos of different ages and stages of development, produced in 2 different co-culture systems and frozen in 2 different cryoprotectants, was investigated. In vitro-derived bovine embryos (n = 5,525) were utilized to study survival following exposure to cryoprotectants and after freezing. Survival of the frozen embryos was based on blastocyst re-expansion 24 h and hatching 72 h after thawing. There was no difference in survival when embryos were exposed to either glycerol (Gly) or ethylene glycol (EG) for 10 or 40 min with the cryoprotectant diluted with or without freezing. In 2 of 3 experiments in which a comparison was possible, more blastocysts frozen in 1.4 M glycerol than in 1.5 M ethylene glycol survived. Addition of 0.25 M sucrose to 1.5 M ethylene glycol in the freezing solution did not improve embryo survival. More blastocysts frozen on Day 7 of in vitro culture survived than those frozen on Day 6 or Day 8. On Days 6, 7 and 8, embryos in the most advanced stage of development survived better than those at less advanced stages. Post-thaw survival did not differ for embryos produced in co-culture with Buffalo Rat Liver (BRL) cells with either Menezo B2 Medium or Tissue Culture Medium 199 and frozen in 1.4 M glycerol.     

In vitro evaluation and pregnancy rates after vitrification of in vitro produced bovine embryos

The efficacy of different vitrification solutions to cryopreserve in vitro-produced bovine blastocysts was evaluated based on in vitro development of embryos in culture and on in vivo development of embryos transferred into recipients. In the first experiment, 2 vitrification solutions were compared: propylene glycol + glycerol (Pg + Gly) and ethylene glycol + Ficoll + sucrose (EFS). Differences in the overall development and hatching rates in favor of EFS were found (56.4 vs 33.3% and 35.4 vs 13.3%; P < 0.05). In the second experiment, 3 vitrification solutions were compared: EFS, modified EFS (EFSm) and ethylene glycol + glycerol (Eg + Gly). The vitrification solutions EFSm and Eg + Gly yield higher hatching rates than did EFS (57.7 vs 59.6 vs 35.7%; P < 0.05). The last experiment was designed to compare in vivo 2 vitrification solutions: EFSm and Eg + Gly. There were no differences between them based on the results obtained after transfer (35.2 vs 43.7%). The vitrification solutions EFSm and Eg + Gly have resulted in good pregnancy rates. These results demonstrated that vitrification can be used successfully in the cryopreservation of in-vitro produced bovine embryos, and it might be considered for use in commercial programs.     

Effect of conventional controlled-rate freezing and vitrification on morphology and metabolism of bovine blastocysts produced in vitro

This study compares the effects of conventional controlled-rate freezing and vitrification on the morphology and metabolism of in vitro-produced bovine blastocysts. Day 7 expanded blastocysts cultured in synthetic oviduct fluid with 5% fetal calf serum were frozen in 1.36 M glycerol, 0.25 M sucrose or vitrified in 25% glycerol, 25% ethylene glycol. Cell alterations and in vitro development were evaluated immediately after thawing or after 72 h. The effect of cryopreservation on inner cell mass and trophectoderm (TE) cell number as well as glucose, pyruvate, and oxygen uptakes, and lactate release by blastocysts were evaluated. Immediately after thawing, blastocysts showed equivalent cell membrane permeabilization after both cryopreservation procedures, while alterations in nuclear staining were more frequent in vitrified embryos. After culture, similar survival and hatching rates were observed. Both procedures decreased cell number immediately after thawing and after 72 h. However, the number of TE cells was lower in frozen embryos than in vitrified ones. In relation to this, frozen blastocysts showed a decrease in glucose, pyruvate, and oxygen uptake, although those parameters were not altered in vitrified embryos. An increased glycolytic activity was also observed in frozen embryos, indicating a stress response to this procedure.     

Agreement among evaluators of bovine embryos produced in vivo or in vitro

Six experienced individuals evaluated 40 embryos on videotape for stage of development and quality grade. These 40 observations comprised 15 embryos produced in vivo, 15 embryos produced in vitro, and 10 embryos that were repeated throughout the videotape. Embryos produced in vivo were recovered from uterine flushings of superovulated heifers 7 d after estrus, and embryos produced in vitro were harvested 7 d after insemination of in vitro-matured oocytes. Embryos of various stages (morulae, blastocysts, or degenerated) and quality grades (1 = excellent, 2 = good, 3 = fair, 4 = degenerated) were recorded on videotape for evaluation. After video microscopy, the embryos were stained and the number of nuclei per embryo was counted. Six evaluators reviewed the videotape and the percentage of agreement and kappa (k; agreement beyond chance) among evaluators were determined for classifications of stage and grade. Consistency of each evaluator's responses was estimated using the 10 repeated embryos. Agreement within evaluators was higher for stage of embryo development (89.2%) than quality grade (68.5%). Agreement among evaluators for stage was slightly higher with embryos produced in vivo (85.0%, k = 0.74) than in vitro (72.3%, k = 0.48). Agreement among evaluators for grade was similar with embryos from in vivo (61.0%, k = 0.46) and in vitro (57.7%, k = 0.42) production. For both sources of embryos, agreement was substantially better for Grades 1 and 4 than for Grades 2 and 3. The results of this study suggest that good to excellent agreement exists for classifying Day 7 bovine embryos by stage and by extremes of quality grade (Grades 1 and 4) but not by degree of abnormal morphology (Grades 2 and 3). Simple grading criteria of Grade 1 (highest quality), Grade 2 (morphologic defects), and Grade 3 (degenerated) maximized agreement among evaluators.     

Cryopreservation for bovine embryos in serum-free freezing medium containing silk protein sericin

Because the use of serum in the embryo cryopreservation increases the probability of animal health problems such as bovine spongiform encephalopathy (BSE) and viral infections, this study was conducted to examine the effects of sericin supplementation for serum-free freezing medium on the survival and development of bovine embryos after freezing–thawing and direct transfer to recipients. When in vitro-produced bovine embryos were frozen conventionally in the freezing medium supplemented with various concentrations (0.1%, 0.5%, and 1.0%) of sericin, the percentages of damaged zona pellucida, survival, and development of frozen–thawed embryos were similar to those of embryos frozen in freezing medium supplemented with 0.4% bovine serum albumin (BSA) and 20% fetal bovine serum (FBS) (0.4BSA/20F; control). When in vivo-derived embryos were frozen with 0.4BSA/20F (control), 0.5% sericin +20% FBS (0.5S/20F) or 0.5% sericin (0.5S) and were subsequently transferred directly to recipients, the percentages of recipients with pregnancy and normal calving in the 0.5S/20F group were higher than those in the control group (47.3% vs. 40.1% and 94.6% vs. 87.3%, respectively). Moreover, the percentages of recipients with pregnancy and normal calving (42.2% and 92.4%, respectively) in the 0.5S group were similar with those of other groups. In conclusion, these results indicate that serum-free freezing medium supplemented with sericin is available for the cryopreservation of bovine embryos and that it is beneficial for the elimination of a potential source of biological contamination by serum or BSA.     

Cryopreservation of porcine embryos by vitrification: A study of in vitro development

Until recently, attempts to preserve porcine embryos have been unsuccessful. Vitrification has been developed as a method of cryopreserving mammalian embryos by avoiding ice crystal formation, assuring a cryopreserved glass state during storage in liquid nitrogen. Vitrification may be a useful method of overcoming the deleterious effects of chilling injury when pig embryos are cryopreserved using conventional slow freezing procedures. In this study, we applied vitrification procedures for rodent and/or bovine embryos to cryopreserve porcine embryos. Following warming, survival was defined as normal development of embryos in culture, namely the formation or reexpansion of the blastocoelic cavity. Experiment 1 tested the relative toxicity of 3 vitrification procedures on Day-5, 6 and 7 porcine embryos. Embryos equilibrated in vitrification solution (VS3a) continued to develop in vitro at rates comparable to that of untreated control embryos. Experiment 2 was designed to evaluate embryonic development following cryopreservation by vitrification in VS3a. Day-5 porcine embryos did not survive cryopreservation while Day-6 and Day-7 embryos survived and continued development in vitro. In Experiment 3, we evaluated a period of culture prior to vitrification and its effect on cryosurvivability of porcine embryos. A 3-h culture period prior to vitrification had no effect on cryosurvivability over that of freshly recovered, immediately vitrified embryos. These studies indicate, for the first time, that porcine embryos can be successfully cryopreserved by vitrification based on morphology and subsequent development in vitro. However, survival following cryopreservation appears to depend upon embryonic age or stage of development.     

牛体内,外受精胚胎玻璃化冷冻保存技术的研究初报

利用３种培养液即输卵管合成液（ＳＯＦ）、ＴＣＭ１９９和ＣＲｌａａ对牛体外受精后的卵母细胞进行培养,结果卵裂率分别达８５％、６７％和７２％,囊胚发育率分别为３７％、２１％和３０％。对所获得的囊胚利用ＥＦＳ玻璃化溶液进行冷冻保存。在１０％ＥＧ中预处理５ｍｉｎ后再移入ＥＦＳ４０平衡３０ｓ二步法冷冻保存的胚胎,其１解冻后继续发育率高达８６％,与对照组９１％相比无显性差异（Ｐ＞０．０５）。而ＥＦＳ３０二步     

Cryopreservation of caprine ovarian tissue using glycerol and ethylene glycol

Cryopreservation of ovarian tissue may be a potential alternative for the conservation of genetically superior animals, including high milk- and meat-producing goat breeds. However, until now, no information was available concerning the cryopreservation of preantral follicles (PF) enclosed in caprine ovarian tissue. The objective of the present study was to evaluate the structural and ultrastructural characteristics of caprine PF after exposure to and cryopreservation of ovarian tissue in 1.5 and 3M glycerol (GLY) and ethylene glycol (EG). At the slaughterhouse, each ovarian pair from five adult mixed breed goats was divided into nine fragments and randomly distributed into treatment groups. One fragment was immediately fixed for histological examination and ultrastructural analysis, after slaughter (control). Four of the ovarian fragments were equilibrated at 20 degrees C for 20 min in 1.8 ml of MEM containing 1.5 or 3M GLY or EG for a toxicity test and the final four fragments were slowly frozen using these cryoprotectants at the concentrations above. After toxicity testing and freezing/thawing, the ovarian fragments were fixed for histological examination. Histological analysis showed that after toxicity testing and cryopreservation of the ovarian tissue in GLY or EG at both concentrations, the percentage of normal PF was significantly lower than controls. Ultrastructural analysis of PF frozen in 1.5 and 3M GLY, as well as 3M EG demonstrated that these follicles remained morphologically normal. In conclusion, we demonstrated cryopreservation of caprine PF in ovarian tissue.     

Cryopreservation of in vitro produced bovine embryos: effects of lipid segregation and post-thaw laser assisted hatching

The objective was to determine if lipid segregation, with or without post-thaw laser assisted hatching (LAH) of in vitro produced (IVP) bovine embryos, would enhance in vitro survivability and development 24 h post-thaw. On Day 6 of culture (Day 0 = IVF), in vitro produced bovine embryos were divided into three developmental stages: 32-cell (n = 78), compact morula (CM n = 223), and blastocyst (n =56). Embryos within each stage were allocated to the following treatments prior to cryopreservation in 1.5M ethylene glycol: no treatment (Control), 7.5 μg/mL Cytochalasin B for 20 min (CB), or CB with centrifugation (16,000 × g) for 20 min (CBCF). All CB treatments were extended to include embryo freezing. Immediately post-thaw, one-half of the CBCF and Control groups were subjected to zona pellucida drilling (LAH), using the XY Clone® system, creating groups CBCFLAH and LAH, respectively. All thawed embryos were cultured for 24 h and evaluated. No treatment differences were observed for either post-thaw survival or 24 h development. Within the CM stage, CBCFLAH and LAH exhibited a greater number of both total and live cells than Control (total: 69.4, 69.3, 53.0, live: 56.4, 54.7, 39.3 respectively; P < 0.05). In conclusion, LAH post-thaw alone or in combination with CBCF improved embryo viability following cryopreservation.     

Cryopreservation of immature bovine oocytes by vitrification in straws

The aim of this study was to cryopreserve by vitrification by ethylene glycol (EG) and dimethyl sulfoxide (DMSO) immature bovine oocytes in straws and to investigate the effects of vitrification on post-thaw oocyte maturation. A total of 575 cumulus oocyte complexes were obtained by follicle aspiration from 238 ovaries of cows slaughtered at a local abattoir. Following selection, oocytes with compacted cumulus cells and evenly granulated ooplasm were vitrified using one of the three different solutions with a non-vitrified group served as control. The first step vitrification solution contained 20% EG while the second step solution contained 40% EG+1M sucrose in a basic media used in group EG. Oocytes were matured in N-2-hidroxyethyl piperazine-N-2-ethanosulfonic acid (HEPES) buffered tissue culture medium (TCM) 199 for 24h at 39 degrees C in a humidified atmosphere of 5% CO2 in air. Oocytes were fixed following evaluation for polar body formation, stained with Giemsa solution and nuclear maturation was examined. The numbers of oocytes which were observed at Metaphase II (MII) stage were 41 (34.1%), 17 (14.9%), 29 (20.7%) and 78 (79.6%) in groups EG, DMSO, Mix and Control, respectively. Maturation rate distribution in group Mix was not statistically different when compared to maturation rate distributions in groups EG and DMSO (p>0.05). Differences between other groups were significant (p<0.001). However, better results were obtained in EG group compared to DMSO and mix groups. Maturation rates were lower in all treatment groups than the control group. The lowest maturation result was obtained in DMSO group. Maturation rate in group Mix was between maturation rates of EG and DMSO groups. Immature bovine oocytes can be vitrified in straws, but maturation success differs with the cryoprotectant and it seems that to obtain better maturation rates, new cryopreservation techniques specific for immature bovine oocytes are needed.     

Cryopreservation of mouse embryos by vitrification: A meta-analysis

A meta-analysis of cryopreservation studies vitrifying mouse embryos was undertaken to determine the treatment effect of vitrification. Treatment by vitrification decreased embryo viability compared with controls: the odds ratio was 9.02 (CI: 3.73-21.78; P < 0.001), a 24.90% (CI: 14.88-34.91; P < 0.001) reduction in risk was associated with embryos in the control group, and for every 4.00 (CI: 3.91-4.09) embryos treated by vitrification, one does not survive. A multiple regression analysis evaluated covariates of embryo survival. For each hour increase post-hCG treatment when embryos were cryopreserved, there was a decrease of 0.36% (SEM ± 0.01) in survival (P < 0.001). The number of embryos surviving vitrification decreased 0.25% (SEM ± 0.02) per day increase in age of the female mouse (P < 0.001), whereas there was no significant difference for control group embryos. For each 1 h increase post-hCG treatment after cryopreservation when blastocysts were assessed for viability, there was a decrease of 0.13% (SEM ± 0.01) in survival. The later interval post-hCG treatment when blastocysts were assessed, the less viable they were compared with earlier blastocysts, independent of the vitrification protocol. This effect was not observed for control embryos. A high percentage of variability in the treatment effect for vitrification was likely due to underlying heterogeneity among studies. A portion of the risk associated with vitrification could be attributed to the general effects of cryopreservation. Future research should identify effects in a cryopreservation protocol specific to vitrification that affect viability of mouse embryos.     

Vitrification of bovine embryos in a mixture of ethylene glycol and dimethyl sulfoxide

The influence of equilibration time before vitrification on the viability of vitrified morula- to blastocyst-stage bovine embryos and in vivo viability of vitrified embryos following transfer to recipients were investigated. In experiment 1, the embryos were exposed to an equilibration solution (50% VSED) containing 12.5% v/v ethylene glycol and 12.5% v/v dimethyl sulfoxide in modified Dulbecco's phosphate buffered saline with 4 mg/ml BSA (m-PBS) for 1, 2 and 5 minutes at room temperature (22 to 24 degrees C). The embryos were then placed in 15mul vitrification solution (VSED) consisting of 25% v/v ethylene glycol and 25% v/v dimethyl sulfoxide in m-PBS and were loaded into 0.25 ml plastic straws at room temperature. After 30 seconds, the straws were placed in liquid nitrogen (LN(2)) vapor for 2 minutes, plunged and stored in LN(2). To thaw, the straws were warmed in water at 20 degrees C for 15 seconds and the contents of the straws were expelled into a plastic dish. The embryos were diluted in 0.5 M sucrose + m-PBS for 5 minutes and were cultured in TCM-199 supplemented with bovine oviductal epithelial tissue. Viability of the embryos was assessed by the forming or reforming of the blastocoele after 24 hours of culture. High in vitro survival rates (73 approximately 90%) of vitrified embryos were obtained after 1 and 2 minute equilibrations, but was reduced (P<0.05) after 5 minute equilibration. In Experiment 2, morula- to blastocyst-stage embryos were vitrified after 1 minute equilibration in 50% VSED and 30 seconds of exposure to VSED. The vitrified-warmed embryos were transferred to recipient heifers at 7 days after estrus (1 embryo per recipient). Five (38%) of 13 (40%) of 10 recipients that had received blastocysts were diagnosed as pregnant using ultrasonography 60 days following transfer.     

In vitro assessment of a direct transfer vitrification procedure for bovine embryos

We developed a simple vitrification technique for bovine embryos that could permit direct transfer. Embryos were produced in-vitro by standard procedures. The base medium for cryopreservation was a chemically defined medium similar to SOF + 25 mM Hepes and 0.25% fatty acid free bovine serum albumin (FAF-BSA) (HCDM2). In experiment 1, embryos were first exposed to 3.5M ethylene glycol (V1) for 1, 2 or 3 min at room temperature (20-24 degrees C), and then moved to 7 M ethylene glycol (V2) at 4 or 20-24 degrees C and loaded in 0.25-mL straws. After 45 s in 7 M ethylene glycol, straws were placed in liquid nitrogen. Embryos that were loaded at 20-24 degrees C had higher survival rates than those loaded at 4 degrees C (P<0.05). Exposure for 1 min was best for morulae, while 3 min was best for blastocysts. In experiment 2, blastocysts were handled at 24 degrees C and exposed to two concentrations of ethylene glycol in V1 (3.5 or 5 M) followed by V2 as in experiment 1, two warming temperatures (20 or 37 degrees C) and two post-warming holding times until culture (5 or 15 min). Exposure to 5 M ethylene glycol and warming at 37 degrees C was the optimal combination of procedures, and embryos survived well after 15 min in straws if warmed at 37 degrees C. In experiment 3, ethylene glycol concentration (3, 4 or 5 M) and exposure time (0.5 or 1 min) during two-step addition of cryoprotectant were studied for bovine morulae. In experiment 4, morulae were exposed to V2 for 30 or 45 s in HCDM2 or Vigro holding medium and then held in 22-24 degrees C air or 37 degrees C water post-warming. Experiment 5 was like experiment 4 except blastocysts were used. Overall survival rates of blastocysts in experiment 5 averaged 80% of non-vitrified controls after 48 h culture. The survival rates with in vitro-produced morulae in experiments 1, 3 and 4 were unacceptable. Vitrification solutions based on Vigro tended to result in higher survival than HCDM2 for blastocysts, but not morulae. In experiment 6, the survival rate in vitro of in vivo-produced morulae and blastocysts after two-step vitrification was nearly 100%. Our vitrification technique was very effective for in vitro produced blastocysts, but not for in vitro-produced morulae.