Rapid cryopreservation of sheep embryos by direct transfer into liquid nitrogen vapour at -180 degrees C

Recent advances of oocyte/embryo vitrification in mammals from rodents and large animals

Vitrification is a valuable technology that enables semipermanent preservation and long-distance or international transportation of genetically modified and native animals. In laboratory mice, vitrification maintains and transports embryos, and many institutions and companies sell vitrified embryos. In contrast, despite numerous papers reporting on vitrification in livestock over the past decade, practical implementation has yet to be achieved. However, with advances in genome editing technology, it is anticipated that the number of genetically modified domestic animals will increase, leading to a rise in demand for vitrification of oocytes and embryos. Here, we provide an objective overview of recent advancements in vitrification technology for livestock, drawing a comparison with the current developments in laboratory animals. Additionally, we explore the future prospects for vitrification in livestock, focusing on its potential benefits and drawbacks.

Cryopreservation Methods and Frontiers in the Art of Freezing Life in Animal Models

The development in cryobiology in animal breeding had revolutionized the field of reproductive medicine. The main objective to preserve animal germplasm stems from variety of reasons such as conservation of endangered animal species, animal diversity, and an increased demand of animal models and/or genetically modified animals for research involving animal and human diseases. Cryopreservation has emerged as promising technique for fertility preservation and assisted reproduction techniques (ART) for production of animal breeds and genetically engineered animal species for research. Slow rate freezing and rapid freezing/vitrification are the two main methods of cryopreservation. Slow freezing is characterized by the phase transition (liquid turning into solid) when reducing the temperature below freezing point. Vitrification, on the other hand, is a phenomenon in which liquid solidifies without the formation of ice crystals, thus the process is referred to as a glass transition or ice-free cryopreservation. The vitrification protocol applies high concentrations of cryoprotective agents (CPA) used to avoid cryoinjury. This chapter provides a brief overview of fundamentals of cryopreservation and established methods adopted in cryopreservation. Strategies involved in cryopreserving germ cells (sperm and egg freezing) are included in this chapter. Last section describes the frontiers and advancement of cryopreservation in some of the important animal models like rodents (mouse and rats) and in few large animals (sheep, cow etc).

Vitrified embryo transfer in Merino sheep under extensive conditions

The aim was to evaluate pregnancy success after transfer of embryos vitrified in micropipette tips in Merino sheep under extensive conditions. A second objective was to evaluate the influence of embryo stage in such pregnancy rate. One hundred and twenty-seven embryos were rewarmed and transferred into recipient ewes. On rewarming, the embryos were placed into three-step cryoprotectant dilutions. Finally, prior to transfer to recipient females, embryos were maintained in Basic Medium for 5 min at 25ºC and were re-evaluated by morphological criteria; all degenerated embryos were eliminated. Recipient ewes (n = 150) were treated for estrus with sponges placed for 14 days and 300 IU of eCG. At embryo transfer, three experimental groups were defined: morulae transferred on Day 7, blastocysts transferred on Day 7 and blastocysts transferred on Day 8 after sponge removal. In all groups, semi-laparoscopic transfer of one rewarmed embryo per recipient was performed. Pregnancy was diagnosed by ultrasonography on day 28 after embryo transfer. The embryo selection rate after rewarming was higher for blastocysts (89.3% - 67/75) compared to morulae (65.9% - 60/91) (P < 0.05). Pregnancy diagnosis showed a 38.3% (23/60) of success after morula transfer on Day 7 post progestagen removal. The day of transfer showed a significant influence on pregnancy rate after blastocyst transfer (Day 8, 55.9% - 19/34 vs Day 7, 21.2% - 7/33) (P < 0.05). Blastocysts transfer on Day 8 showed the highest global efficiency (pregnancies/total embryos after rewarming) (47.5% - 19/40) (P < 0.05). In conclusion, reproductive efficiency obtained by vitrified embryo transfer allows its recommendation for embryo transfer programs under extensive conditions. The importance of considering the synchrony between the embryo age and the recipient uterus stage is emphasized.

Assisted reproductive technologies in the reproductive management of small ruminants

In modern agriculture, assisted reproductive technologies are being used for out of season oestrus induction, enhancement of reproductive performance and genetic improvement. In addition, they can have substantial contribution in preservation of endangered species or breeds, as well as in eradication programs of various diseases. While their applications are widespread in cattle, in small ruminants it is almost restricted to artificial insemination. The main limitations of a wider application in small ruminants are the naturally occurring anoestrus period, the variability of response to superovulatory treatments, the fertilisation failure and the need of surgery for collection and transfer of gametes and embryos. Nonetheless, during the last 30 years, considerable progress has been made in sheep and goat embryo technologies, especially in the fields of oestrus synchronisation, superovulation and in vitro embryo production. This paper reviews the status of assisted reproductive technologies in sheep, analysing the prospects offered by recent advances in in vivo and in vitro embryo production from mature and juvenile lambs.

Vitrification of in vitro produced ovine embryos at various developmental stages using two methods

This study was conducted to evaluate the effects of developmental stage of in vitro produced (IVP) ovine embryos and the type of vitrification procedure used on embryo cryotolerance. The IVP embryos were vitrified at five different developmental stages: 4-, 8- and 16-cell, morula, and blastocyst. For each stage, half of the embryos were vitrified in either 30 microl 3.4M glycerol+4.6M ethylene glycol in straw (method 1) or in <0.1 microl 2.7 M ethylene glycol+2.1 M Me(2)SO+0.5M sucrose placed on the inner surface of a straw (method 2) of vitrification solution, based on two different procedures. After warming embryo viability was determined by assessing the rates of re-expansion, survival, and blastocyst formation. The quality of surviving embryos was evaluated by their hatching rate and blastocyst cell numbers. In both vitrification methods, embryo survival progressively increased as the developmental stage progressed. In method 1 few of the early cleavage stage embryos (4-, 8- and 16-cell) could reach to the blastocyst stage following warming. There was no significant difference in blastocyst cell numbers (total, ICM, and trophectoderm cells) or hatching rate of blastocysts derived from vitrified embryos at different developmental stages. The number of dead cells in vitrified blastocysts in method 1 was higher than for non-vitrified blastocysts (P<0.05). The number of apoptotic cells in vitrified blastocysts was higher than for non-vitrified counterparts (P<0.05). In conclusion, both the developmental stage of IVP ovine embryos and the method of vitrification have a significant effect on the viability and developmental competence of sheep embryos.

Aseptic technology of vitrification of human pronuclear oocytes using open-pulled straws

BACKGROUND

The aim of this study was to compare the viability of human pronuclear oocytes subjected to vitrification using cooling by direct submerging of open-pulled straws in liquid nitrogen versus vitrification by cooling of open-pulled straws located inside a closed 0.5 ml straw (aseptic system).

METHODS

Two- and three-pronuclei stage oocytes (n=114) were cryopreserved in super-open-pulled straws by vitrification in 20% ethylene glycol +20% dimethylsulphoxide (DMSO) + osmotic active and neutral non-permeable cryoprotectants with a four-step exposure in 20, 33, 50 and 100% vitrification solution for 2, 1 and 1 min, and 30-50 s, respectively at room temperature, and plunging into liquid nitrogen. Oocytes of group 1 (n=42) were rapidly cooled at a speed of 20,000 degrees C/min by direct plunging of open-pulled straws into liquid nitrogen. Oocytes of group 2 (n=44) were first located in 0.5 ml straws, which were closed at both sides by metal balls, and then plunged into liquid nitrogen. This method resulted in a cooling speed of 200 degrees C/min. For both groups, oocytes were thawed rapidly at a speed of 20 000 degrees C/min using an identical protocol. Oocytes subsequently were expelled into a graded series of sucrose solutions (1.0, 0.75, 0.5, 0.25 and 0.12 mol/l) at 2.5 min intervals.

RESULTS

Oocyte development up to expanded blastocyst stage after in vitro culture was 15% in group 1, 14% in group 2 and 29% in an untreated control group.

CONCLUSION

The deposition of human pronuclear oocytes in open-pulled straws which are placed inside a hermetically closed container guarantees a complete isolation of oocytes from liquid nitrogen and avoids potential contamination by pathogenic microorganisms. The combination of direct plunging of this container into liquid nitrogen and rapid warming makes this process as efficient as conventional vitrification.

Comparison of different vitrification protocols on viability after transfer of ovine blastocysts in vitro produced and in vivo derived

We compare different vitrification protocols on the pregnancy and lambing rate of in vitro produced (IVP) and in vivo derived (IVD) ovine embryos. Ovine blastocysts were produced by in vitro maturation, fertilization and culture of oocytes collected from slaughtered ewes or superovulated and inseminated animals. Embryos were cryopreserved after exposure at room temperature either for 5 min in 10% glycerol (G), then for 5 min in 10% G + 20% ethylene glycol (EG), then for 30 s in 25% G + 25% EG (glycerol group), or for 3 min in 10% EG + 10% dimethyl sulphoxide (DMSO), then for 30s in 20% EG + 20% DMSO + 0.3 M sucrose (DMSO group). One group of in vitro produced embryos was cryopreserved similarly to the DMSO group, but with 0.75 M sucrose added to the vitrification solution (DMSO 0.75 group). Glycerol group embryos were then loaded into French straws or open pulled Straws (OPS) while the DMSO group embryos were all loaded into OPS and directly plunged into liquid nitrogen. Embryos were warmed with either a one step or three step process. In the one step process, embryos were placed in 0.5 M sucrose. The three-step process was a serial dilution in 0.5, 0.25 and 0.125 M sucrose. The embryos of DMSO 0.75 group were warmed directly by plunging them into tissue culture medium-199 (TCM-199) + 20% foetal bovine serum (FBS) in the absence of sucrose (direct dilution). Following these manipulations, the embryos were transferred in pairs into synchronised recipient ewes and allowed to go to term. The pregnancy and the lambing rate within each group of IVP and IVD embryos indicated that there was no statistical difference among the vitrification protocols.

Vitrification of in vivo and in vitro produced ovine blastocysts

Although cryopreservation of bovine embryo has made great progress in recent years, little achievement was obtained in ovine embryo freezing, especially in vitro produced embryos. However, a simple and efficient method for cryopreservation of sheep embryos will be important for application of ovine embryonic techniques such as in vitro fertilization, transgenic, cloning and etc. In this study ovine blastocysts, produced in vivo or in vitro, were cryopreserved by vitrification in EFS40 (40% ethylene glycol (EG), 18% ficoll and 0.5 M sucrose) or GFS40 (40% glycerol (GL), 18% ficoll and 0.5 Mol sucrose). In vitro produced, early blastocysts were directly plunged into liquid nitrogen (LN2) after preparation by one of the following procedures at 25 degrees C: (A) equilibration in EFS40 for 1 min; (B) equilibration in EFS40 for 2 min; (C) equilibration in EFS40 for 30 s following pretreatment in 10% EG for 5 min; (D) equilibration in EFS40 for 30 s following pretreatment in EFS20 for 2 min (E) equilibration in GFS30 for 30 s following pretreatment in 10% GL for 5 min. The survival rates observed after thawing and in vitro culture for 12 h were A 78.0% (39/50), B 50.0% (26/52), C 93.3% (70/75), D 92.0% (46/50) and E 68.0% (34/50). Survival rates were not significantly different for treatments C and D (p>0.05), but those for groups C and D were significantly higher than for A, B and E (p<0.05). After 24 h in vitro culture, hatched blastocyst rates were A 28.0% (14/50), B 21.1% (11/52), C 49.3% (37/75), D 48.0% (24/50), E 32.0% (16/50) and control 54.0% (27/50). The hatching rates for groups A, B and E were significantly lower than the control (p<0.05) in which early IVF blastocysts were cultured in fresh SOFaaBSA medium following treatment in PBS containing 0.3% BSA for 30 min, but for groups C and D it was similar to the control (p>0.05). The freezing procedures A, B and C were used to vitrify in vivo produced, early blastocysts recovered from superovulated ewes. The survival rates of frozen-thawed in vivo embryos were A 94.7% (72/76), B 75.0% (45/60) and C 96.4% (54/56) and for group B was significantly lower than for the other two treatment groups (p<0.05). Hatched blastocyst rates were A 46.0% (35/76), B 26.6% (16/60), C 51.8% (29/56) and the control 56.7% (34/60) in which early blastocysts from superovulation were cultured in fresh SOFaaBSA medium following treatment in PBS containing 0.3% BSA for 30 min. The hatching rate for treatment B was significantly lower than for the control (p<0.05) but did not differ between groups A, C and the control (p>0.05). Frozen-thawed embryos vitrified by procedure C were transferred into synchronous recipient ewes. Pregnancy and lambing rates were similar for embryos transferred fresh or frozen/thawed for both in vivo and in vitro produced embryos. These rates did not differ between in vivo and in vitro embryos transferred fresh (p>0.05). However, for frozen-thawed embryos, both rates were significantly lower for in vitro than for in vivo produced embryos (p<0.05).

Successful direct transfer of vitrified sheep embryos

The use of a simple cryopreservation method, adapted to direct transfer of thawed embryos may help to reduce the costs of embryo transfer in sheep and increase the use of this technique genetic improvement of this species. Two experiments were made to test a vitrification method that is easy to apply in field conditions. All embryos were collected at Day 7 of the estrous cycle of FSH-stimulated donor ewes and were assessed morphologically, washed in modified PBS and incubated for 5 min in 10% glycerol, for 5 min in 10% glycerol and 20% ethylene glycol and were transferred into the vitrification solution (25% glycerol and 25% ethylene glycol). All solutions were based on mPBS. Embryos were loaded in straws (1 cm central part, the remaining parts being filled with 0.8 M galactose in mPBS) and plunged into liquid N2 within 30 sec of contact with the vitrification solution. The straws were thawed (10 sec at 20 degrees C) and the embryos were either transferred directly or after 5 min of incubation in the content of the straw (followed by washing in PBS) into the uterus of a recipient ewe. In Trial 1, the pregnancy rates at term (72 vs. 72%) as well as the embryo survival rates (60 vs 50% respectively) were not different between fresh (n = 48 embryos) and vitrified (n = 50) embryos. In a second trial no difference was observed between vitrified embryos transferred after in vitro removal of the cryoprotectant (n = 86 embryos) or directly after thawing (n = 72) both in terms of lambing rate (67 vs. 75%, respectively) and embryo survival rate (lambs born/embryos transferred; 49 vs. 53%). This method of sheep embryo cryopreservation provided high pregnancy and embryo survival, even after direct transfer of the embryos.

Cryopreservation of embryos of farm animals

This review contains two parts. The first part is devoted to the significant steps in cryopreservation of mammalian embryos with emphasis on cattle and sheep that serve as models of reference. These steps are: (1) shortening of cooling and warming processes; (2) addition and dilution of cryoprotectant in one step; (3) introduction of plastic straw as a freezing and dilution container; (4) the choice of ethylene glycol as the quite universal cryoprotectant because of its low toxicity and high permeability; (5) vitrification, a cryopreservation method which enable passage from the liquid to the solid state by extreme elevation of viscosity due to high concentration of cryoprotectants and very rapid cooling. There are several vitrification solutions which contain dimethyl sulphoxide, glycerol, ethylene glycol, or a mixture of them, as basic cryoprotectants. The second part considers some factors affecting the efficiency of cryopreservation concerning (i) the origin of embryos and (ii) the stage of development and species. The origin of embryos (in vivo versus in vitro): in vitro embryos show a chilling and freezing sensitivity associated with their lipid content which can be modified by the culture conditions. Both conventional freezing and vitrification have been used and it seems that vitrification is more adapted to in vitro embryos when some modifications of initial protocols are carried out, particularly the rate of cooling. Thus considerable progress has been achieved by using the open pulled straw method of Vajta which enables the use of a minimum volume of freezing medium (0.5 microl) and a very high cooling rate that permits rapid traversal of the damaging temperature zone, corresponding to chilling sensitivity. The stage of development and species: not only are there differences between species at the same stage of development but in the same species all stages of development do not survive equally under the same freezing protocol. In cattle for example, oocytes and early stages of development in vivo or in vitro do not survive whereas compacted morulae and blastocysts survive very well. In the pig hatched blastocysts survive better than the other stages. Horse embryos have special characteristics that pose problems for successful freezing. In conclusion, a lot of work remains to be done to define fundamental characteristics of embryos of certain species (pig, horse) and of embryos of some stages or of oocytes.

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.

Polyvinyl alcohol as a defined substitute for serum in vitrification and warming solutions to cryopreserve ovine embryos at different stages of development

The purpose of this study was to assess the viability of ovine embryos after replacing fetal calf serum (FCS) with polyvinyl alcohol (PVA) in vitrification and warming solutions. Ovine embryos were obtained from superovulated Sardinian breed ewes at 4, 5, 6, and 7 days after insemination. All vitrification and warming solutions were prepared using buffered saline solution with 20% FCS (group a) or 0.1% PVA (group b). Embryos were vitrified in 20 microliters of glycerol 3.4 M + ethylene glycol 4.6 M and loaded into the centre of 0.25 ml straws between two columns of sucrose solution (0.5 M), and plunged immediately into liquid nitrogen. After being warmed in a water bath at 35 degrees C for 10 s, the vitrified embryos were moved to 0.25 M sucrose solution for 3 min. Embryos were cultured in TCM-199 after washing with 10% FCS and sheep oviductal epithelial cells up to hatching or re-expansion of the blastocoelic cavity. No significant difference in the viability rates was observed between embryos vitrified/warmed in PVA or FCS solutions. In both groups, the rate of in vitro viability was (P < 0.01) lower at the precompacted and compacted morula stages than at the expanded, hatching or hatched blastocyst stage. In both groups, early blastocysts were less viable than expanded (P < 0.01), hatching or hatched blastocyst (P < 0.05). There was no significant difference in survival rates at days 14 (79 and 76%) and 45 (63 and 59%) after transfer into sychronised recipients between vitrified expanded blastocysts of groups a and b, respectively. These results suggest that it is possible replace serum with PVA in vitrification and warming solutions without reducing in vivo and in vitro viability.

Cryopreservation of kunming mouse oocytes using slow cooling, ultrarapid cooling and vitrification protocols

The cryopreservation of oocytes has been only marginally successful with any of the current protocols, including slow cooling, rapid cooling and vitrification. We wished to test the hypothesis that oocytes from a single mouse strain would freeze successfully by 1 of the 3 mentioned protocols. Unfertilized Kunming mouse oocytes obtained 14 h after PMSG/hCG administration were randomly assigned to be cryopreserved after slow cooling, ultra rapid cooling and vitrification. Oocytes were thawed by straws being placed into 37 degrees C water, and their morphological appearance and in vitro fertilization capability were compared with that of oocytes that had not undergone cryopreservation. Survival of oocytes was indicated by the absence of darkened ooplasm or by broken membranes or zona pellucida. Functional integrity was evaluated by the formation of a 2-cell embryo after IVF. Survival rate of slow cooled oocytes did not differ from that seen in vitrified oocytes (55.1 vs 65.9%) but was significantly lower in the rapidly cooled oocytes (24.2%; P < 0.01). The results of IVF of slow cooled and vitrified oocytes were similar to those of the control group (72 and 73 vs 77%; P > 0.05). It appears that Kunming mouse oocytes can be successfully cryopreserved using the slow cooling method with 1,2-propanediol and vitrification, which contains both permeating and nonpermeating cryoprotectants.

Effect of biopsy and vitrification on in vitro survival of ovine embryos at different stages of development

The objective of the present study was to assess the in vitro viability of ovine embryos at different stages of development after combining cell sampling and vitrification. Precompacted morulae, compacted morulae and blastocysts were obtained from superovulated Sarda ewes at 4, 5 or 6 d following insemination. Embryo cell biopsy was carried out in a 100-microl drop of PBS + 10% fetal calf serum (FCS) with 10 micromol nocodazole and 7.5 microg/ml cytochalasin-b by aspiration (3-5 cells). Embryos were cryopreserved at room temperature after exposure of 2 solutions for 5 min, transferred into a vitrification solution, loaded into the center of 0.25-ml straws separated by air bubbles from 2 columns of sucrose 0.5 M and plunged immediately into liquid nitrogen. In Experiment 1, the in vitro viability of manipulated or vitrified embryos after in vitro co-culture in TCM 199 medium with 10% FCS and sheep oviductal epithelial cells (SOEC) in 5% CO2 humidified atmosphere in air at 39 degrees C was significantly lower (P < 0.05 and P < 0.01, respectively) at precompacted morula (60 and 30%) and compacted morula (62 and 39%) stages than intact embryos at the same stages (87 and 88%). No differences were found at the blastocyst stage. In Experiment 2, the in vitro survival rate of precompacted morulae which were manipulated and immediately vitrified was lower (P < 0.05) than in those manipulated and, after a temporary period of culture, vitrified at blastocyst stage (21 vs 48%); while no differences were found at compacted morula and blastocyst stages. The results show that 1) the stage of development influences the subsequent in vitro viability of manipulated and vitrified ovine embryos, 2) temporary culture after manipulation and before vitrification improves the in vitro viability of embryos, and 3) the hole in the zona pellucida resulting from biopsy does not affect blastocyst survival after subsequent vitrification.

Survival of vitrified sheep embryos in vitro and in vivo

The effects of the composition of vitrification media, the duration of exposure to the media and the stage of development were examined on the survival of vitrified Day-6 sheep embryos. Vitrification media that contained two cryoprotectants in equal molar concentrations were used. In Experiment 1, the effects of the types (glycerol + propylene glycol or glycerol + ethylene glycol) and concentrations (3.5 + 3.5 or 4.5 + 4.5 M) of cryoprotectants and the level of BSA supplementation (0.4 or 20%) were investigated in a 2 x 2 x 2 design. The embryos were exposed to vitrification media for 30 sec at 18 to 24 degrees C before vitrification. The in vitro survival rate was not affected by the level of BSA supplementation, but there was an interaction between the types and concentrations of cryoprotectants used (P<0.01). Embryos cryopreserved in mixtures of glycerol + propylene glycol survived better when the concentration of cryoprotectants was 3.5 M while the survival of embryos cryopreserved in mixtures of glycerol + ethylene glycol was higher at 4.5 M cryoprotectant concentration. In Experiments 2 and 3, the effect of the duration of exposure (15, 30, 60 or 120 sec) to vitrification media at 4 to 12 degrees C was investigated on the survival rate in vivo. Vitrification media contained 3.5 M glycerol + 3.5 M propylene glycol or 4.5 M glycerol + 4.5 M ethylene glycol in Experiments 2 and 3, respectively. The survival rate in vivo, increased when the duration of exposure to vitrification media was increased from 15 to 30 sec, but the viability declined when the duration of exposure was further increased to 60 (Experiment 3) or to 120 sec (Experiment 2). The effect of the stage of development was significant only in Experiment 1 (P = 0.032), but in all three experiments the rate of survival increased with advancing stages of development from late morulae to late blastocysts. The best result was achieved in Experiment 2, when embryos were exposed to a mixture of 3.5 M glycerol + 3.5 M propylene glycol for 30 or 60 sec. Under these conditions 52% (22 42 ) of rapidly cryopreserved sheep embryos developed into lambs. This result shows that a simple rapid procedure for the cryopreservation of sheep embryos can produce a survival rate comparable to that obtained using more complex traditional procedures.

Sheep embryo cryopreservation by vitrification and conventional freezing

The survival of ovine embryos (morulae and blastocysts) either frozen by a conventional method or vitrified was investigated in culture. In Experiment I, embryos were vitrified using a solution containing 25% propylene glycol and 25% glycerol. A group of embryos (simulated control) was processed without freezing to evaluate the toxicity of the vitrification solution. In Experiment II, embryos were exposed to a solution of PBS containing 10% glycerol and 0.25 M sucrose placed horizontally in a programmable freezer. Automatic seeding was applied at -7 degrees C in 2 positions on straws and cooled at -0.3 degrees C/min to -25 degrees C and then stored in liquid nitrogen. In vitro development rates of vitrified embryos were 12% (morulae) and 19% (blastocysts). Simulated embryos showed a higher rate of survival than embryos cryopreserved by vitrification (67 and 63%, morulae and blastocysts respectively). In conventional cooling, the blastocysts showed the highest viability percentage (67%) of all the experimental groups but these values decreased significantly in morulae (31%). Differences in temperature between straws placed in distinct positions in the freezing chamber and thermic deviation were observed when automatic seeding was applied. Embryo viability differed from 51 to 75% according the relative position of the embryos within the chamber. Survival was higher when automatic seeding was applied on the meniscus of the embryo column versus the central point of this column (65 vs 21%). The damage of both cryopreservation methods on zona pellucida integrity (27 and 35% in vitrified and conventionally frozen embryos, respectively) had no effect on the in vitro survival.

Cryopreservation of mammalian ova and embryos by vitrification

Vitrification is a new approach to oocyte and embryo cryoconservation. It consists in the solidification of a solution caused not by crystallization, but by a drastic increase in viscosity during cooling. The application of this approach to cryoconservation of oocytes and embryos of different species depends upon the development of proper procedures and non-toxic media. From the technical point of view, the vitrification method is simple and relatively easily applicable under field conditions. The authors review the current procedures applied to oocytes and embryos of laboratory and farm animals.

Factors affecting viability of fresh and frozen-thawed sheep demi-embryos

The addition of 0.1 M sucrose to the medium in which sheep embryos were bisected had no effect (39.5 vs 36.4%) on the survival rate of demi-embryos transferred (one per ewe) to recipients. There was a trend to greater survival of demi-blastocysts (44.7%) compared to demi-morulae (30%), and all the surviving twins were derived from the demi-blastocysts. It is suggested that the survival of demi-morulae is enhanced by the transfer of two demi-morulae to one uterine horn. In three experiments demi-embryos were frozen after the addition of 1.5 M glycerol in three or six steps or after the addition of 1.5 M ethylene glycol in six steps. No treatment resulted in acceptable survival rates of the demi-embryos transferred to recipients after thawing and step-wise removal of the cryoprotectant. Overall, 8 of 142 (5.6%) cryopreserved demi-embryos survived as 50-day fetuses or term lambs compared with 14 of 31 (45.2%) whole embryos.

Factors affecting the survival of bisected sheep embryos in vivo

Two experiments were carried out to examine the effects of different factors on the survival of split sheep embryos. In Experiment 1, embryos collected on Day-6, Day-7 or Day-8 were bisected and transferred into recipient ewes in pairs. The proportions of Day-6, Day-7 and Day-8 demi-embryos developing to lambs were 26% (14/54), 30% (31/102) and 32% (24/74), respectively. Replacement of bisected late morula to expanded late blastocyst stage embryos into zonae did not affect their survival rate (P>0.5). The proportion of demi-embryos developing to lambs in recipients with two or more ovulations was higher (35%, 53/152) than in recipients with a single ovulation (21%, 16/78 ; P<0.05). In Experiment 2, Day-6 embryos were split with or without exposure to 0.25 M of sucrose and were transferred into recipients in pairs or singly. Exposure to 0.25 M of sucrose decreased the proportion of split embryos developing to lambs compared with that of the controls (31%, 22/70 vs 49%, 34/70 ; P<0.05). The effects of the number of demi-embryos transferred or the stage of development on the survival rate were not significant (P>0.05). The number of lambs born per original embryo was the highest when the embryos were split without exposure to sucrose and transferred into recipients singly (106%, 17/16).