Simple and efficient methods for vitrification of mammalian embryos

A Cryoprotectant-Gel Composite Designed to Preserve Articular Cartilage during Frozen Osteoarticular Autograft Reconstruction for Malignant Bone Tumors: An Animal-Based Study

OBJECTIVE

We designed a highly adhesive cryoprotectant-gel composite (CGC), based on regular liquid-form cryoprotectant base (CB), aiming to protect cartilage tissue during frozen osteoarticular autograft reconstruction for high-grade sarcoma around the joint. This study aimed to evaluate its effectiveness in rat and porcine distal femur models.

DESIGN

Fresh articular cartilage samples harvested from distal rat and porcine femurs were divided into 4 test groups: untreated control group, liquid nitrogen (LN) freezing group, LN freezing group pretreated with CB (CB group), and LN freezing group pretreated with CGC (CGC group). Microscopic and macroscopic evaluation of cartilage condition, TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) assay, and apoptotic protein analysis of chondrocytes were performed to confirm our results.

RESULTS

In the rat model, CGC could prevent articular cartilage from roughness and preserve more proteoglycans when compared with the LN freezing and CB groups. Western blot analysis showed CGC could prevent cartilage from LN-induced apoptosis supported by caspase-3/8 apoptotic signaling cascade. Macroscopically, we observed CGC could reduce both articular clefting and loss of articular luminance after freezing in the porcine model. In both models, CGC could reduce articular chondrocytes from degeneration. Fewer TUNEL-positive apoptotic and more viable chondrocytes in cartilage tissue were observed in the CGC group in our animal models.

CONCLUSION

Our study proved that CGC could effectively prevent cartilage surface and chondrocytes from cryoinjury after LN freezing. Freezing articular cartilage surrounded with high concentration of CGC can be a better alternative to preserve articular cartilage during limb salvage surgery for malignant bone tumor.

Archiving genetically altered animals: a review of cryopreservation and recovery methods for genome edited animals

With the ever-expanding numbers of genetically altered (GA) animals created in this new age of CRISPR/Cas, tools for helping the management of this vast and valuable resource are essential. Cryopreservation of embryos and germplasm of GA animals has been a widely used tool for many years now, allowing for the archiving, distribution and colony management of stock. However, each year brings an array of advances, improving survival rates of embryos, success rates of in-vitro fertilisation and the ability to better share lines and refine the methods to preserve them. This article will focus on the mouse field, referencing the latest developments and assessing their efficacy and ease of implementation, with a brief note on other common genetically altered species (rat, zebrafish, Xenopus, avian species and non-human Primates).

Effect of warming method on embryo quality in a simplified equine embryo vitrification system

Equine embryo vitrification is still not a well-established technique in equine practice. Notably, little work has been done on the effect of the warming system on viability of vitrified embryos. Our goal was to evaluate the effect of warming without cryoprotectants on in vitro - produced (IVP) embryo viability in culture, quality assessment parameters, and pregnancy after transfer. Equine IVP blastocysts were vitrified using commercial embryo vitrification media and a semi-closed vitrification device. In Exp. 1, we evaluated two warming temperatures (room temperature, RT, ∼22 °C; and 38 °C) for each of three warming systems: commercial warming solution (Kit); commercial embryo holding medium (EHM) with decreasing concentrations of sucrose (EHM + SS); or EHM alone without added sucrose. Embryos (n = 9 to 14 per treatment) were cultured in vitro for 24 h, stained with DAPI, TUNEL, and fluorophore-labelled phalloidin, and evaluated for nucleus number, mitotic rate, apoptotic rate, and actin filament distribution. In Exp. 2, to survey embryo viability in vivo, vitrified IVP blastocysts were shipped to an embryo transfer facility, then warmed immediately before transfer to recipient mares, using the warming treatments associated with the nominally best (Kit-RT, Kit-38, EHM-RT) and poorest (EHM + SS-38) assessed embryo quality in Exp. 1 (n = 7 to 8 per treatment). Subsequently, IVP blastocysts produced as part of our clinical program were vitrified and shipped, then warmed in embryo holding medium at an embryo transfer facility before transfer to recipient mares; fresh IVP embryos were shipped and transferred as controls. In Exp. 1, embryos increased significantly in diameter after culture (P < 0.01), with no difference among treatments. There was no difference (P > 0.05) in the number of viable nuclei, apoptotic rate, or microfilament distribution among treatments, or between vitrified-warmed and Control embryos. The mitotic rate was higher (P = 0.021) for Kit-RT (3.6%) when compared with the other treatment groups (1.5-2.0%). In Exp. 2, there was no difference (P > 0.05) in initial pregnancy (71.4-87.5%) or heartbeat (57.1%-85.7%) rates among warming treatments. In the clinical trial, there was no difference (P > 0.05) between vitrified-warmed and Control embryos in initial pregnancy (90.9% and 66.6%, respectively) or heartbeat (81.8% and 66.6%, respectively) rates. These results indicate that a semi-closed vitrification system using commercially-available media, and incorporating warming in the field in a single step using commercial embryo holding medium without cryoprotectants, can provide high pregnancy rates with IVP equine embryos.

Ethylene Glycol and Dimethyl Sulfoxide Combination Reduces Cryoinjuries and Apoptotic Gene Expression in Vitrified Laying Hen Ovary

Successful cryopreservation of avian gonads is important not only for avian breeding but is also crucial for preservation of species, especially of endangered birds. The aim of this study was to evaluate the effect of vitrification by several cryoprotectants on the ovarian tissues of laying hens. Ovarian tissues were randomly divided into six groups: control (nonvitrified: C), dehydrated using ethylene glycol (EG), dehydrated with propylene glycol (PROH), dehydrated using dimethyl sulfoxide (DMSO), and two combined groups, EG+DMSO and EG+PROH. The composition of vitrification solutions was as follows: EG group: V1 = 7.5% EG and V2 = 15% EG +0.5 M sucrose, DMSO group: V1 = 7.5% DMSO and V2 = 15% DMSO +0.5 M sucrose, PROH group: V1 = 7.5% PROH and V2 = 15% PROH +0.5 M sucrose, EG+DMSO group: V1 = 7.5% EG +7.5% DMSO and V2 = 15% EG +15% DMSO +0.5 M sucrose and EG+PROH group: V1 = 7.5% EG +7.5% PROH and V2 = 15% EG +15% PROH +0.5 M sucrose. Ovarian tissues of each group were dehydrated for 10 minutes with V1 solution and 2 minutes with V2. Among the vitrified groups, intact primordial and primary follicles showed significant increase in EG+DMSO, but follicular attrition had the highest rate in the PROH group (p < 0.05). Immunohistochemical analysis showed that the percentage of active caspase 3-positive cells was lower (p < 0.05) when using EG+DMSO versus PROH. Further gene expression of caspase 3, 8, and 9 was highest in the PROH group (p < 0.05). Vitrification of ovaries of laying hens using EG+DMSO can afford effective protection of primordial and primary follicles during preservation and may therefore be successfully used for storing avian gonadal tissues.

Expression of the T85A mutant of zebrafish aquaporin 3b improves post-thaw survival of cryopreserved early mammalian embryos

While vitrification has become the method of choice for preservation of human oocytes and embryos, cryopreservation of complex tissues and of large yolk-containing cells, remains largely unsuccessful. One critical step in such instances is appropriate permeation while avoiding potentially toxic concentrations of cryoprotectants. Permeation of water and small non-charged solutes, such as those used as cryoprotectants, occurs largely through membrane channel proteins termed aquaporins (AQPs). Substitution of a Thr by an Ala residue in the pore-forming motif of the zebrafish (Dario rerio) Aqp3b paralog resulted in a mutant (DrAqp3b-T85A) that when expressed in Xenopus or porcine oocytes increased their permeability to ethylene glycol at pH 7.5 and 8.5. The main objective of this study was to test whether ectopic expression of DrAqp3b-T85A also conferred higher resistance to cryoinjury. For this, DrAqp3b-T85A + eGFP (reporter) cRNA, or eGFP cRNA alone, was microinjected into in vivo fertilized 1-cell mouse zygotes. Following culture to the 2-cell stage, appropriate membrane expression of DrAqp3b-T85A was confirmed by immunofluorescence microscopy using a primary specific antibody directed against the C-terminus of DrAqp3b. Microinjected 2-cell embryos were then cryopreserved using a fast-freezing rate and low concentration (1.5 M) of ethylene glycol in order to highlight any benefits from DrAqp3b-T85A expression. Notably, post-thaw survival rates were higher (P<0.05) for T85A–eGFP-injected than for -uninjected or eGFP-injected embryos (73±7.3 vs. 28±7.3 or 14±6.7, respectively). We propose that ectopic expression of mutant AQPs may provide an avenue to improve cryopreservation results of large cells and tissues in which current vitrification protocols yield low survival.

Cryopreservation of Day 8 equine embryos after blastocyst micromanipulation and vitrification

Pregnancy rates after cryopreservation of large equine blastocyst stage embryos have remained lower than other domesticated livestock species. It is generally accepted that the embryonic capsule is the primary barrier to cryoprotectant entry into the embryo proper and techniques need to be developed to circumvent this obstacle. Therefore, the objective of this study was to develop an efficient Day 8 equine embryo cryopreservation protocol through blastocyst micromanipulation and vitrification. Grade 1 and 2 embryos recovered from mares (n = 15) 8 days after ovulation were used in these experiments. In experiment 1, the effect of either one- or two-puncture treatments before aspiration of blastocoel fluid and exposure to vitrification solutions was evaluated. No difference was detected in mean embryo volume across treatment groups after exposure to vitrification solutions or after 1, 24, 48, and 72 hours of culture. Percent of embryos re-expanding at 24 hours and percent of embryos showing diameter increase at 48 and 72 hours during in vitro culture were 100%, 83%, and 75% compared with 93%, 67%, and 50% for one- and two-puncture treatment groups, respectively. Capsule loss was 25% for one-puncture and 50% for two-puncture treatment groups. In experiment 2, no difference was detected in mean embryo volume for indirect introduction (aspiration of blastocoel fluid + equilibration) and direct introduction (injection of cryoprotectant into blastocoel cavity) treatment groups, after exposure to dilution solution or to culture medium. There was no difference in mean embryo volume for the indirect and direct introduction treatment groups after 1, 24, 48, and 72 hours of culture. Percent of embryos re-expanding at 24 hours and percent of embryos showing diameter increases at 48 and 72 hours during in vitro culture were 100%, 76.9%, and 69.2%, respectively, for both treatment groups. Those embryos subjected to the direct introduction treatment had a higher (P = 0.05) percent capsule loss (70%) compared with the indirect introduction treatment group (31%). The pregnancy rate after transfer of vitrified expanded Grade 1 blastocysts using the indirect introduction method was 83% (5/6). Three pregnancies were allowed to continue to term and resulted in the birth of three healthy foals. The vitrification protocol used in this study has the potential to become a key tool for the successful cryopreservation of equine expanded blastocysts.

Vitrification of oocytes, embryos and blastocysts

In assisted reproductive technology, cryopreservation of human oocytes and embryos has been significantly improved by refined slow-cooling and the new vitrification method. The slow-cooling method requires a programmed cryo-machine, and usually takes several hours. It is, however, difficult to eliminate injuries resulting from ice formation completely. Vitrification has become a reliable strategy because it is simple, can lead to high survival rates and viability, and has better clinical outcome. Vitrification transforms cells into an amorphous glassy state inside and outside the vitrified cell with ultra-rapid cooling and warming steps by plunging the oocytes and embryos into liquid nitrogen, instead of ice-crystal formation. Over the past decade, several advances in vitrification technologies have improved clinical efficiency and outcome. In this chapter, we focus on vitrification technologies for cryopreservation in human assisted reproductive technology.

Injuries in pacu embryos (Piaractus mesopotamicus) after freezing and thawing

Although the sperm cryopreservation of freshwater and marine teleosts has been feasible for years, the cryopreservation of some fish embryos still remains elusive. Thus, the objective of this experiment was to analyze the embryo morphology after freezing and thawing 40 embryos of Piaractus mesopotamicus immersed into methanol and ethylene glycol, both at 7, 10 and 13% plus 0.1 M sucrose for 10 min. Soon after thawing, three embryos were treated with historesin, stained with hematoxylin–eosin and analyzed under an optical microscope. From every treatment, one palette containing embryos was thawed and incubated, but none of the eggs hatched. Samples containing two embryos were immersed into 10% methanol or 10% ethylene glycol both in association with sucrose, and embryos immersed into only water or sucrose solution were frozen, processed and analyzed using scanning electron microscopy (SEM). In both cases, the control group was immersed into only water. Although the embryos had the chorion, vitello, yolk syncytial layer and blastoderm, all of them were found altered under the optical microscope and by SEM. The chorion was irregular and injured; there was no individuality in the yolk granules; the yolk syncytial layer had an irregular shape, thickness and size; the blastoderm showed injuries in the nucleus shape and sometimes was absent; the blastoderm was located in atypical areas and absent in some embryos. In conclusion, no treatment was effective in preserving the embryos, and none of the embryos avoided injury from intracellular ice formation. These morphological injuries during the freezing process made the P. mesopotamicus embryos unfeasible for hatching.

Reliability of ultrasound for early sexing of goat fetuses derived from natural mating and from fresh, frozen and vitrified embryo transfer

The aim of the present study was to identify the migration period of the genital tubercle and its later differentiation into external genital structures in fetuses derived from natural mating and fetuses from fresh, frozen and vitrified embryo transfer. A transrectal ultrasound with a double-frequency linear transducer (6.0 and 8.0 MHz) was used to monitor 123 goat fetuses, which were allocated to one of four groups: fetuses originating from controlled natural mating (G1, n = 32) and fetuses derived from fresh (G2, n = 34), frozen (G3, n = 30) and vitrified (G4, n = 27) embryo transfer. The transferable embryos were collected 7 days after mating by laparoscopy. Migration of the genital tubercle occurred significantly earlier (P < 0.05) in G1 than in G2, G3 and G4. The visualisation of the scrotum, prepuce and vulva occurred significantly earlier (P < 0.05) in G1 than in G2, G3 and G4. Our results show that fetal sexing is feasible after 55 days for fetuses from natural mating and after 60 days in fetuses from fresh and cryopreserved embryos. Thus, real-time ultrasonography is a reliable tool for fetal sex determination in goats after Day 50 of pregnancy, taking into account both the location of the genital tubercle and the identification of external genital structures.

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.

Effect of macromolecules in solutions for vitrification of mature bovine oocytes

This study was designed to evaluate vitrification procedures for in vitro matured bovine oocytes for efficient blastocyst production after warming, IVF and culture. A second goal was to replace serum as the macromolecular component of the vitrification solution, without compromising efficacy. The first experiment compared two containers, open pulled straws (OPS) versus cryoloops, and two vitrification protocols: short equilibration (H-TCM-199+10% EG+10% DMSO+20% FCS for 30s, followed by H-TCM-199+20% EG+20% DMSO+20% FCS+0.48M galactose for 20s) versus long equilibration (H-TCM-199+3% EG+20% FCS for 10min, followed by H-TCM-199+31% EG+20% FCS+1M galactose for 20s). Subsequent experiments used only cryoloops and the short equilibration protocol to evaluate the effect of replacing FCS with defined macromolecules (BSA, Ficoll, PVP, and PVA) in vitrification solutions. Cryoloops were superior to OPS for vitrification of oocytes as determined by blastocyst production (P<0.05). The short and long vitrification protocols gave similar results. The presence of macromolecules in vitrification solutions for bovine oocytes was necessary for acceptable post-warming developmental capacity; 20% FCS, 1% and 2% BSA, 6% and 18% Ficoll, 6% and 20% PVP, 1% PVA, and the combinations of 18% Ficoll+1% BSA, and 6% PVP+1% BSA provided similar protection during vitrification of oocytes; development ranged from 14.8% to 23.0% blastocysts/oocyte, which was not different (P>0.05) from non-vitrified controls (26.9-34.0% blastocysts/oocyte). Too much (6%) and too little (0.3%) BSA, and 0.3% PVA for vitrification resulted in lower blastocyst production (P<0.05) relative to unvitrified oocytes.

Vitrification of Boer Goat Morulae and Early Blastocysts by Straw and Open-Pulled Straw Method

The aim of this study was to investigate the effects of different vitrification solutions [EFS30 or EFS40 contains 30% (v/v) ethylene glycol (EG), 40% (v/v) EG; EDFS30 or EDFS40 contains 15% (v/v) EG and 15% (v/v) dimethyl sulfoxide (DMSO), 20% (v/v) EG and 20% (v/v) DMSO], equilibrium time during vitrification (0.5-2.5 min) and vitrification protocols [one-step straw, two-step straw and open-pulled straw (OPS)] on in vivo development of vitrified Boer goat morulae and blastocysts after embryo transfer. In the one-step straw method, the lambing rates of vitrified embryos in EFS30 (37.5%), EFS40 (40.5%) or EDFS30 (38.2%) group were similar to that of fresh embryos (57.5%) and conventional freezing method (46.7%) when the equilibrium time was 2 min. In the two-step straw method, the highest lambing rate was obtained when embryos were pretreated with 10% EG for 5 min and then exposed to EFS40 for 2 min (51.4%), showing similar lambing rates compared with fresh embryos (56.1%) or the embryos cryopreserved by conventional freezing method (45.2%). In the OPS method, the lambing rate in EFS40, EDFS30 or EDFS40 groups were similar to that (57.1%) of fresh embryos, or to that (46.0%) of embryos cryopreserved by conventional freezing method. The highest lambing rate (51.4%) of the group of OPS was obtained when the embryos were vitrified with EDFS30. In conclusion, either the two-step straw method in which embryos were pretreated in 10% EG for 5 min and then exposed to EFS40 for 2 min, or the OPS method in which embryos were pretreated in 10% EG + 10% DMSO for 30 s and then exposed to EDFS30 for 25 s was a simple and efficient method for the vitrification of Boer goat morulae and blastocysts.

Effect of In-Straw Thawing on In Vitro- and In Vivo-Development of Vitrified Mouse Morulae

For the purpose of ascertaining parameters to embryo transfer on some domestic animals, mouse morulae were used as a model to investigate the effect of in-straw thawing on in vitro and in vivo-development of vitrified embryos. Embryos were vitrified in 0.25 ml straws preloaded with dilution solution (0.5 M Sucrose) and thawed in the straw by mixing the vitrification solution (Ethylene glycol + Ficoll 70 + Sucrose) and the dilution solution at 25 degrees C. The embryos were randomly divided into six groups and expelled from the straws after they had been suspended in the in-straw mixture for 3 min, 5 min, 8 min, 12 min, 16 min, and 20 min, respectively, and then they were collected under a microscope for in vitro culture or direct transfer. The in vitro developmental rates of the embryos were 92.3% to 98.4% and hatching rates were 64.1% to 75.6% for the groups of 3 min to 16 min, showing no significant differences with those of nonfrozen controls (100%, 76.2%; P > 0.05). While embryos were suspended in the straw for 20 min, the developmental rate (86.6%) and hatching rate (52.4%) were significant lower than those of the control (100%, 76.2%; P < 0.01). When the 168 frozen-thawed embryos (in-straw thawing for 5 min) and 168 fresh embryos were transferred, respectively, the proportion of live fetuses in the pregnant recipients between them (58.7% vs. 54.5%) showed no significant difference (P > 0.05). The data indicate that vitrification with EFS30 and suspension in the in-straw mixture for 3 min to 16 min, when thawing, did not affect the in vitro developmental rate and hatching rate. Moreover, the in vivo developmental rate between vitrified embryos and fresh embryos did not differ significantly. It can be concluded that this method is fit for nonsurgical embryo transfer in some domestic animals with a suggestion that the operation of embryo transfer should be accomplished within 16 min.

Improved vitrification method allowing direct transfer of goat embryos

The aim of this study was to design a vitrification method suited to field embryo transfer experiments in goat. In a first experiment, a standard vitrification protocol, previously designed for sheep embryos was compared to slow freezing of goat embryos. No significant difference was observed on kidding rate (48% versus 69%, respectively), nor on embryo survival rate (35% versus 45%). Second experiment: all embryos were vitrified. After warming, embryos were either transferred directly (direct transfer), or after in vitro dilution of the cryoprotectants (conventional transfer). The kidding rate was not affected by the transfer method (38% versus 23%, respectively). However, embryo survival rate tended to be higher after direct transfer (26% versus 14%). Third experiment: OPS vitrification was compared to standard vitrification. The kidding rate was not affected (22% versus 39%, respectively), but the embryo survival rate was lower after OPS (14% versus 28%). Fourth experiment: 0.4M sucrose was added with cryoprotectants in vitrification. The kidding rate after direct transfer was significantly enhanced after addition of sucrose (56% versus 27%, respectively), whereas embryo survival rate was not significantly affected (32% versus 18%). Fifth experiment: vitrification with sucrose supplementation was compared to slow freezing. No significant difference was observed after direct transfer on kidding rate (52% versus 31%, respectively), but embryo survival rate tended to be higher after vitrification (34% versus 21%). In conclusion, our results indicate that addition of 0.4M sucrose in association with direct transfer improves significantly the viability of goat vitrified embryos.

Are programmable freezers still needed in the embryo laboratory? Review on vitrification

The predictable answer to the provocative question of whether programmable freezers are still needed in the embryo laboratory is an even more provocative 'no'. However, such a radical statement needs strong support. Based on the extensive literature of the past 5 years, the authors collected arguments either supporting or contradicting their opinion. After an overview of the causes of cryoinjuries and strategies to eliminate them, the evolution of vitrification methods is discussed. Special attention is paid to the biosafety issues. The authors did not find any circumstance in oocyte or embryo cryopreservation where slow freezing offers considerable advantages compared with vitrification. In contrast, the overwhelming majority of published data prove that the latest vitrification methods are more efficient and reliable than any version of slow freezing. Application of the proper vitrification methods increases the efficiency of long-term storage of stem cells and opens new perspectives in cryopreservation of oocytes, both for IVF and somatic cell nuclear transfer. However, lack of support from regulatory authorities, and conservative approachs regarding novel techniques can slow down the implementation of vitrification. The opinion of the authors is that vitrification is the future of cryopreservation. The public have the final say in whether they want and allow this future to arrive.

Analysis of DNA Fragmentation of Porcine Embryos Exposed to Cryoprotectants

The chemical toxicity of cryoprotectants to porcine embryos was examined by the evaluation of survival and DNA damage after exposure to cryoprotectants. Porcine blastocysts were exposed to 10% of ethylene glycol (EG), 1,2-propanediol (PD) or glycerol (GLY) for 1 h at room temperature (23-25 degrees C) and then cultured in vitro for 24 h. The survival rates of blastocysts exposed to PD and GLY were significantly lower than those of control blastocysts in which the embryos were exposed to carrier solution without cryoprotectants. Significantly more DNA-fragmented nuclei occurred in the cryoprotectant-exposed blastocysts, compared with the control blastocysts. Moreover, the indices of DNA-fragmented nuclei in the blastocysts without blastocoele re-formation after culture were significantly higher than those with blastocoele re-formation, irrespective of the exposure treatment. These results indicate that the exposure of porcine blastocysts to cryoprotectant decreases the survival rates and increases the DNA-fragmented nuclei in embryos.

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.

Permeability of mouse oocytes and embryos at various developmental stages to five cryoprotectants

To assess the permeability of mouse oocytes and embryos, matured oocytes and embryos at various stages of development were placed in five cryoprotectant solutions at 25 C for 25 min. From the cross-sectional areas of the oocytes/embryos, the relative change in volume was analyzed. In oocytes, shrinkage was least extensive and recovery was quickest in the propylene glycol solution, showing that propylene glycol permeates the oocytes most rapidly. Dimethyl sulfoxide, acetamide, and ethylene glycol permeated the oocytes slightly more slowly than propylene glycol. The oocytes in glycerol shrunk extensively and then expanded marginally, indicating slow permeation. The volume changes of 1-cell and 2-cell embryos were similar to those of oocytes, showing little change in permeability. In 8-cell embryos, the volume recovered much faster than in the earlier stages especially in glycerol and acetamide. In morulae, the volume recovery was much faster in glycerol and in ethylene glycol; in ethylene glycol, the extent of shrinkage was small and the recovery was fast, indicating an extremely rapid permeation. Although the permeability of oocytes/embryos generally increased as embryo development proceeded, the degree of increase varied greatly among the cryoprotectants. Interestingly, the volume change in propylene glycol was virtually unaffected by the stage of development. Such information will be valuable for determining a suitable protocol for the cryopreservation of oocytes/embryos at different stages of development.

Survival of mammalian cells under high vacuum condition for ion bombardment

An ion beam has been used to irradiate various organisms and its effects have been studied. Because of the poor tolerance that mammalian cells have for vacuum, such studies have not been carried out on living mammalian cells until now. However, this work is important both for elucidating the mechanism of mutation in response to low-energy ions and in exploring possible new applications of ion beam technology. The current paper describes an investigation of the survival of mammalian cells (the A(L) cell line) in a high-vacuum chamber in preparation for ion bombardment studies. The ion beam facility is described and the actual vacuum profile that the cells endured in the target chamber is reported. Cells were damaged immediately following vacuum exposure; the injury was characterized by alteration of the membrane permeability, loss of firm adhesion to the dish, and increased fragility. Three cryoprotective agents were tested (glycerol, propylene glycol, and trehalose) and of these, glycerol showed the highest potency for protecting cells against vacuum stress. This was revealed by an increase in the cell survival level from <1 to >10% with a glycerol concentration of 15 and 20%. Two glycerol-based protocols were investigated (freezing-vacuum vs. non-freezing-vacuum), but there was no significant difference (P > 0.1) in their ability to improve cell survival, the values being 10.31 +/- 4.5 and 12.7 +/- 3.37%, respectively with 20% glycerol concentration. These cells had a normal growth capability, and also retained integrity of the cell surface antigen CD59. These initial experiments indicate that mammalian cells can withstand vacuum to the degree that is needed to study the effect of the ion beam. In addition to the improvements made in this study, other factors are discussed that may increase the survival of mammalian cells exposed to a vacuum in future studies.

Comparison of the survival of human biopsied embryos after cryopreservation with four different methods using non-transferable embryos

BACKGROUND

The standard embryo cryopreservation method is still less than optimal for biopsied embryos. The aim of this study was to compare the survival of biopsied embryos cryopreserved with four different methods using non-transferable embryos.

METHODS

Abnormal embryos from one or three pronuclei and spare embryos of grade 3 and 4 were used for this study. Non-biopsied embryos were cryopreserved using the standard method as control. Biopsied embryos were cryopreserved using four methods as follows: standard method, modified freezing method, modified thawing method and vitrification. Blastomere survival and blastulation of frozen-thawed embryos were compared between the different methods.

RESULTS

The proportion of embryos with > or = 50% blastomere survival and total blastomere survival rate of biopsied embryos were significantly higher with vitrification than the other three methods. Both the modified freezing and modified thawing methods had significantly higher embryo survival and total blastomere survival rates than standard methods. However, there was no significant difference in blastulation of surviving embryos in all the five groups.

CONCLUSIONS

Non-transferable embryos derived from clinical IVF/ICSI are useful for evaluation of the optimal freezing procedures for biopsied embryos. Vitrification increases the survival rate of human biopsied embryos above standard and modified cryopreservation methods.

Cryopreservation of animal and human embryos by vitrification

Vitrification is a method in which not only cells but also the whole solution is solidified without the crystallization of ice. For embryo cryopreservation, the vitrification method has advantages over the slow freezing method. For example, injuries related to ice is less likely to occur, embryo survival is more likely if the embryo treatment is optimized, and embryos can be cryopreserved by a simple method in a short period without a programmed freezer. However, solutions for vitrification must include a high concentration of permeating cryoprotectants, which may cause injury through the toxicity of the agents. Since the development of the first vitrification solution, which contained dimethylsulphoxide, acetamide, and propylene glycol, numerous solutions have been composed and reported to be effective. However, ethylene glycol is now most widely used as the permeating component. As supplements, a macromolecule and/or a small saccharide are frequently added. Embryos of various species, including humans, can be cryopreserved by conventional vitrification using insemination straws or by ultrarapid vitrification using minute tools such as electron microscopic grids, thin capillaries, minute loops, or minute sticks, or as microdrops. In the ultrarapid method, solutions with a lower concentration of permeating cryoprotectants, thus having a lower toxicity, can be used, because ultrarapid cooling/warming helps to prevent ice formation.

Aquaporin proteins in murine trophectoderm mediate transepithelial water movements during cavitation

Mammalian blastocyst formation is dependent on establishment of trophectoderm (TE) ion and fluid transport mechanisms. We have examined the expression and function of aquaporin (AQP) water channels during murine preimplantation development. AQP 3, 8, and 9 proteins demonstrated cell margin-associated staining starting at the 8-cell (AQP 9) or compacted morula (AQP 3 and 8) stages. In blastocysts, AQP 3 and 8 were detected in the basolateral membrane domains of the trophectoderm, while AQP3 was also observed in cell margins of all inner cell mass (ICM) cells. In contrast, AQP 9 was predominantly observed within the apical membrane domains of the TE. Murine blastocysts exposed to hyperosmotic culture media (1800 mOsm; 10% glycerol) demonstrated a rapid volume decrease followed by recovery to approximately 80% of initial volume over 5 min. Treatment of blastocysts with p-chloromercuriphenylsulfonic acid (pCMPS, > or =100 microM) for 5 min significantly impaired (P < 0.05) volume recovery, indicating the involvement of AQPs in fluid transport across the TE. Blastocysts exposure to an 1800-mOsm sucrose/KSOMaa solution did not demonstrate volume recovery as observed following treatment with glycerol containing medium, indicating glycerol permeability via AQPs 3 and 9. These findings support the hypothesis that aquaporins mediate trans-trophectodermal water movements during cavitation.

Comparison of slow- and rapid-cooling protocols for early-cleavage-stage Macaca fascicularis embryos

Cryostorage of nonhuman primate embryos by time-consuming slow-cooling methods is often limited to early cleavage stages. Effective rapid-cooling methods have been developed for many species and represent valuable tools for laboratory- and field-based studies of nonhuman primate reproductive biology. However, few rapid-cooling protocols have been applied to nonhuman primate embryos in terms of comparing various developmental stages. Here we compare slow cooling vs. two- and three-step rapid cooling of two-, four-, and eight-cell Macaca fascicularis (Mf) embryos. Rapid cooling was conducted in open pulled straws (OPS) using cooling solutions containing reduced quantities of ethylene glycol (EG) and supplemented with either of two high-molecular-weight polymers, ficoll and dextran. The survival of the slow-cooled embryos, but not the rapid-cooled embryos, was independent of embryonic stage at cryostorage. Slow cooling was associated with greater cell survival (82%) post thaw compared to warming following rapid cooling (18-29%). Slow cooling resulted in a high proportion of embryo survival (18/20; 90%) and cleavage (15/18; 83%) post thaw. Rapid cooling resulted in significantly reduced percentages of embryo survival (26-32%) and embryo cleavage in culture (29-38%) after warming. Conventional slow cooling was more effective than the rapid-cooling protocols employed in this study for cryopreservation of early-cleavage-stage Mf 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.

Overcoming a permeability barrier by microinjecting cryoprotectants into zebrafish embryos (Brachydanio rerio)

The goal of this research was to examine the developmental effects on zebrafish embryos (Brachydanio rerio) when cryoprotectants were directly microinjected into the yolk. Our objectives were to: (i) determine the final concentration of propylene glycol (PG) and dimethyl sulfoxide (Me(2)SO) that the embryos could tolerate without causing teratogenic effects; (ii) determine if the toxicity of Me(2)SO could be reduced by the simultaneous presence of various proportions of amides; and (iii) examine whether this intracellular cryoprotectant incorporation could reduce the cryodamage to the yolk syncytial layer (YSL) after vitrification trials. The rationale for conducting these microinjection experiments was to overcome the permeability barrier of the YSL. Intracellular PG produced better survival than Me(2)SO (P < 0.05). Embryos tolerated both 10- and 30-nl microinjections of PG, yielding final concentrations of 2.3 and 5.0 M within the yolk, resulting in 70 +/- 3 and 35 +/- 4% survival at day 5, respectively. In similar experiments with Me(2)SO, survival was lower than PG at 60 +/- 4 and 14 +/- 4% at 2.4 and 5.2 M. Unlike other cellular systems, the presence of amides, specifically acetamide or formamide, did not reduce the toxicity of Me(2)SO in zebrafish embryos (P > 0.05). During vitrification trials, we estimated a 25% dehydration of the yolk, yielding an effective PG concentration of 5.9 M. However, the incorporation of this vitrifiable concentration of PG was not sufficient to improve the postthaw morphology of the YSL (P > 0.05). Clearly, other factors need to be examined in establishing a successful vitrification protocol for zebrafish embryos.

Vitrification of the oocytes and embryos of domestic animals

After the first successful application of vitrification for embryo cryopreservation 15 years ago, a rapid application of the method in domestic animal embryology was presumed. However, although the advantages of vitrification (simplicity, cost efficiency, speed of the procedure) were widely acknowledged, its use has been mainly restricted to experimental studies. For commercial embryo transfer purposes, the traditional slow-rate or equilibrium freezing has been used. This review attempts to explain the reasons for this phenomenon and discusses the theoretical and practical differences between the two technologies as well as their commercial prospects. Recent developments that improve the efficiency of vitrification and applications to other reproductive technologies are also summarized. These advances may result in considerable advantage and could lead to widespread application of vitrification in certain areas of domestic animal embryology.

Expression of mRNAs of the aquaporin family in mouse oocytes and embryos

The molecular basis of water and cryoprotectant permeability in mammalian oocytes and embryos is poorly understood. Therefore, we investigated the expression of mRNAs of water channel proteins (aquaporins) in mouse oocytes and embryos by RT-PCR. The total RNA of mouse oocytes at metaphase II and embryos at the 4-cell, morula, and blastocyst stages was isolated, reverse-transcribed, and subjected to nested PCR amplification. Aquaporins were expressed in both oocytes and embryos, but the types were different among the developmental stages: aquaporins 3 and 7 were expressed in oocytes and embryos at all stages examined, but aquaporins 8 and 9 were expressed only in blastocysts. On the other hand, aquaporins 1, 2, 4, 5, and 6 were not detected in any of the stages examined. The present study shows for the first time that aquaporins are expressed in mammalian oocytes and embryos. These aquaporins may play a role in water transport and conceivably also in cryoprotectant transport across the plasma membrane in these cells.

Effects of vitrification medium composition on the survival of bovine in vitro produced embryos, following in straw-dilution, in vitro and in vivo following transfer

This study examined the effects of adding a macromolecule, polyvinylpyrrolidone (10% PVP) and a sugar (0.3 M trehalose) to vitrification solutions (VS) containing either one (40% ethylene glycol [EG], two (25% EG+25% DMSO) or three (20% EG+20% DMSO+10% 1, 3-butanediol [BD]) permeable cryoprotectants on the survival and hatching of IVP bovine embryos, following vitrification, warming and in-straw cryoprotectant dilution. Grade 1 and 2 compact morulae and blastocysts were selected on Day 7 (Day 0=IVF) of culture in SOFaaBSA and equilibrated for 10 min at room temperature in 10% EG. Following exposure, for up to 1 min at 4 degrees C, to one of the above VS (with or without PVP+trehalose), the embryos were loaded into straws and immersed in liquid nitrogen. Following warming and in-straw cryoprotectant dilution, the embryos were cultured for 48 h to assess hatching. There was no effect of VS on the survival of embryos after 24 h, however fewer compact morulae than blastocysts survived after 24 h (24% vs. 75%; P<0.001) or hatched after 48 h (15% vs. 59%; P<0.001). When blastocysts only were considered, an interaction between VS and additional PVP+trehalose was also observed (P<0.01). Hatching was reduced when they were added to 25% EG+25% DMSO (70% vs. 45%) but was not affected for either 40% EG (44 and 49%) or to 20% EG+20% DMSO+10% BD (72 and 72%). Pregnancy rates (Day 90 ultrasound) of recipients that were transferred either two non-vitrified or two vitrified (20% EG+20% DMSO+10% BD) blastocysts, did not differ (3/6 [50%] and 11/20 [55%]). However, significantly (P<0.02) fewer recipients that received compact morulae maintained pregnancy to Day 90 although this was not affected by vitrification (fresh vs. vitrified; 1/5 [20%] vs. 3/18 [17]). These data demonstrate that a VS comprising three cryoprotectants, rather than one, enables more embryos to hatch during post-thaw culture and that the survival, following direct transfer of these vitrified embryos, is not different to non-vitrified embryos.

The effect of high concentrations of cryoprotectants on the passage of bovine viral diarrhea virus through the zona pellucida of in vitro fertilized embryos

The effect of high concentrations of cryoprotectants on the passage of bovine viral diarrhea virus (BVDV) through the zona pellucida (ZP) of intact bovine embryos during the pre-freezing step of cryopreservation was investigated in a series of experiments. In vitro fertilized (IVF) embryos at the blastocyst stage were exposed to 10(6) TCID50 BVDV (non-cytopathic NY-1 strain) in a 30% suspension of either ethylene glycol, glycerol, DMSO, or 2 M sucrose in physiological saline for 10 min at 20 degrees C. Subsequently, the embryos were washed free of residual unbound viral particles, and the ZP of some embryos were removed by micromanipulation. Groups of ZP-intact embryos, ZP-free embryonic cells and their respective ZP were then tested separately for the presence of virus. The infectious virus was detected in association with 81% (17/21) of samples containing non-micromanipulated ZP-intact embryos which were exposed to the virus and cryoprotectants and then washed 10 times and in 83% (43/53) of the samples containing only ZP from micromanipulated embryos (P > 0.05). The virus was not found in the samples containing the corresponding embryonic cells of embryos exposed previously to the virus and cryoprotectants. It was concluded that the transfer of embryos from the isotonic PBS solution into a highly hypertonic cryoprotectant solution did not cause the passage of BVDV through ZP and its entry to embryonic cells.

Vitrification of two-cell rat embryos derived from immature hypothyroid rdw rats by in vitro fertilization in ethylene glycol-based solutions

Two-cell embryos derived from immature rdw rats by in vitro fertilization (IVF) were vitrified in ethylene glycol-based solutions. Embryos exposed to EFS20 before being vitrified in EFS40 exhibited improved viability in vitro. All embryos exposed to EFS20 for 1-3 min before vitrification in EFS40 were morphologically normal. However, 2-3 min of exposure to EFS20 increased the number of embryos that developed beyond the four-cell stage. More embryos exposed to EFS20 for 2-3 min developed to morulae (63-64%) and blastocysts (34-38%) than those exposed for 1 min (35 and 10%, respectively). After transfer, 33% of embryos exposed to EFS20 for 3 min and vitrified in EFS40 developed to term compared to 29% of fresh embryos. Fifteen (47%) of live young were homozygous rdw and all of the others were heterozygous rats. The present study demonstrated that vitrification in EFS solution can be routinely used to cryopreserve rat two-cell IVF-embryos with no loss of viability.

Vitrification of in vitro produced bovine blastocysts: methodological studies and developmental capacity

Methodological studies were undertaken to test the validity of a three-step vitrification procedure for bovine in vitro produced embryos using glycerol and ethylene glycol as cryoprotectants. Embryos were produced in a low-phosphate culture system (medium VT1 + 10% foetal calf serum) and vitrified at day 7 post-insemination either in a mixture of 25% glycerol--25% ethylene glycol or a mixture of 10% glycerol--40% ethylene glycol. In the first mixture 67% (n = 283) of blastocysts were re-expanded after 72 h of culture and 53% were hatched while in the second one (n = 65) only 5% survived. The mean number of cells of the surviving blastocysts was correlated with the rate of survival (R2 = 0.47; P = 0.0024). Embryo size (diameter < or > to 180 microm) did not influence blastocyst survival or cell number, but hatching rate was higher for embryos > 180 microm. Embryo survival, hatching rate and cell number 72 h post-warming were not affected by the mode of vitrification (direct plunging into nitrogen liquid or vitrification into nitrogen liquid vapour), the mode of preparation of the vitrification solutions (molar or molal basis) or by the concentration of galactose used as a diluent (0 to 0.85 M). Only one calf was born after transfer of 22 vitrified blastocysts. These results confirm the apparent lack of correlation for cryopreserved embryos between in vitro survival or hatching and viability after transfer.

Cryopreservation of mature bovine oocytes by vitrification in straws

Experiments were conducted to determine optimal conditions for vitrification of in vitro matured bovine oocytes in straws. In the first series of experiments, the effects of stepwise addition before exposure of oocytes to vitrification solution consisting of 30% (v/v) ethylene glycol (EG) with 0.35 M sucrose was tested. The rates of morphological survival and cleavage of oocytes vitrified by the three-step addition procedure were higher than those vitrified by the one-step addition procedure. In the second series, the effect of the concentration of the vitrification solution was tested (20, 30, 40, and 50% EG, all with 0.35 M sucrose). The survival rates of oocytes vitrified in 20 and 30% EG were lower than those in 40 and 50% EG. The rates of cleavage and development to blastocysts of oocytes vitrified in 40% EG were the highest among the four groups. In the third series, the effect of duration of exposure of oocytes to 0.5 M sucrose (1, 5, and 10 min) at the first step during the three-step dilution was tested. Although there were no significant differences among the groups with respect to developmental competence of oocytes vitrified in 40% EG, the highest development rate (10%) to blastocysts was observed when oocytes were exposed to sucrose for 5 min. These data demonstrate that improvements to the concentration of cryoprotectant and addition procedures have critical effects on the developmental competence of oocytes vitrified in straws.

Effects of hypotonic stress on the survival of mouse oocytes and embryos at various stages

To examine the sensitivity of mammalian oocytes and embryos to osmotic swelling, which can occur during the removal of cryoprotectant from cryopreserved cells, the effect of hypotonic stress on the survival of fresh and vitrified mouse oocytes/embryos at various stages was examined. Oocytes and embryos were suspended in phosphate-buffered saline (PBS) media of various hypotonicities for 30 min at 25 degrees C. They were then returned to isotonic PBS medium, and the survival was assessed by the apparent integrity of the blastomeres and/or the developmental potential during culture. The survival of stressed embryos at one- to eight-cell stages assessed by the appearance was close to that assessed by the developmental ability, suggesting that hypotonic stress causes physical damage in the cell membrane. Fresh oocytes and embryos were almost totally unaffected by exposure to a 0.5x isotonic solution at all developmental stages examined. However, the extent of injury resulting from exposure to 0.3 to 0.2x isotonic solutions varied and depended on the developmental stage of the embryos. For example, zygotes were the least sensitive and morulae were the most sensitive to the hypotonic stresses. Except for morulae, vitrified cells were more sensitive to hypotonic stresses than were fresh ones. However, in many cases, the sensitivity was reduced or eliminated when the oocytes and embryos were cultured for a short period before exposure to the hypotonic stress. Furthermore, the survival rate of some stressed embryos which had been equilibrated in vitrification solution without cooling was higher than the survival of embryos stressed immediately following vitrification. These results show that sensitivity to osmotic swelling is variable among oocytes and embryos. The results also show that cryopreserved cells just after warming are more sensitive to osmotic swelling than are fresh ones, and even swelling corresponding to that in 0.5x solution may decrease survival in some stages.

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.