Cellular alteration after dilution of cryoprotective solutions used for the vitrification of in vitro-produced bovine embryos

Birth of a Live Cria After Transfer of a Vitrified-Warmed Alpaca (Vicugna pacos) Preimplantation Embryo

The alpaca (Vicugna pacos) is an important species for the production of fiber and food. Genetic improvement programs for alpacas have been hindered, however, by the lack of field-practical techniques for artificial insemination and embryo transfer. In particular, successful techniques for the cryopreservation of alpaca preimplantation embryos have not been reported previously. The objective of this study was to develop a field-practical and efficacious technique for cryopreservation of alpaca preimplantation embryos using a modification of a vitrification protocol originally devised for horses and adapted for dromedary camels. Four naturally cycling non-superovulated Huacaya females serving as embryo donors were mated to males of proven fertility. Donors received 30 μg of gonadorelin at the time of breeding, and embryos were non-surgically recovered 7 days after mating. Recovered embryos (n = 4) were placed individually through a series of three vitrification solutions at 20°C (VS1: 1.4 M glycerol; VS2: 1.4 M glycerol + 3.6 M ethylene glycol; VS3: 3.4 M glycerol + 4.6 M ethylene glycol) before loading into an open-pulled straw (OPS) and plunging directly into liquid nitrogen for storage. At warming, each individual embryo was sequentially placed through warming solutions (WS1: 0.5 M galactose at 37°C; WS2: 0.25 M galactose at 20°C), and warmed embryos were incubated at 37°C in 5% CO₂ in humidified air for 20-22 h in 1 ml Syngro® holding medium supplemented with 10% (v/v) alpaca serum to perform an initial in vitro assessment of post-warming viability. Embryos whose diameter increased during culture (n = 2) were transferred individually into synchronous recipients, whereas embryos that did not grow (n = 2) were transferred together into a single recipient to perform an in vivo assessment of post-warming viability. Initial pregnancy detection was performed ultrasonographically 29 days post-transfer when fetal heartbeat could be detected, and one of three recipients was pregnant (25% embryo survival rate). On November 13, 2019, the one pregnant recipient delivered what is believed to be the world's first cria produced from a vitrified-warmed alpaca embryo.

Cryoprotectant agents and cooling effect on embryos of Macrobrachium amazonicum

SummaryThere are few reports of cryopreservation and injuries in Macrobrachium amazonicum embryos. Thus, the aim of this study was to analyze the effects of cryoprotectants agents and cooling on stage VIII of this species. Fertilized eggs from ovigerous females were removed from the incubation chamber, then placed in 10 ml Falcon tubes with a cryoprotectant solution and saline-free calcium solution. Thus, the embryos underwent a cooling curve of 1°C per min until reaching 5°C, and then were stored for 2 h. The tubes containing the embryos were washed to remove the cryoprotectant, acclimated for 5 min and then transferred to 50 ml incubators. At the end of the 24-h period, living embryos from each tube were counted and tabulated. A pool of embryos was fixed with 4% formaldehyde and then subjected to histology using 3-mm thick sections and stained with haematoxylin/eosin. Another pool was used for biometric analysis in which length, width and volume were analyzed. The cryoprotectants agents used were: dimethylsulfoxide (DMSO), methyl alcohol, ethylene glycol at 1, 5 and 10% and sucrose (0.5 M). Variance analysis was performed followed by Tukey's honest significant difference (HSD) test at 5% significance level. DMSO cryoprotectant affected embryo survival the least with rates of 71.8, 36.2 and 0% for concentrations of 1, 5 and 10%, respectively. Ethylene glycol caused 100% mortality at all the concentrations used. It was not possible to observe the interference of cooling and cryoprotectants on embryonic structures in this study.

Vitrification of bovine embryos followed by in vitro hatching and expansion

The objective of this study was to assess the effects of bovine embryo vitrification by applying three different vitrification solutions containing ethylene glycol (EG) and dimethylsulphoxide (DMSO) at different concentrations (10, 20 or 25% each) combined with 1.0 M glucose or 1.0 M sucrose, on the in vitro hatching and expansion rates. Healthy oocytes were selected for in vitro maturation and fertilization from 200 bovine ovaries, and subsequently cultured up to the blastocyst stage (n = 800). Control (n = 200) and vitrified cells (n = 100 per treatment; 600 in total) were cultured for an extra 24 or 48 h to evaluate hatching and expansion, respectively. Vitrification significantly decreased embryonic re-expansion and hatching rates independently of the tested solution when compared with control embryos, but solutions with 25% EG + 25% DMSO resulted in the highest re-expansion (75%) and hatching (70%) rates, independently of the added sugar. The addition of sucrose resulted in higher rates of re-expanded and hatched embryos when compared with glucose addition. We concluded that the combination of 25% EG + 25% DMSO and 1.0 M sucrose allowed hatching and expansion of vitrified-warmed bovine embryos produced in vitro.

Optimal timing of blastocyst vitrification after trophectoderm biopsy for preimplantation genetic screening

Is the timing of vitrification after trophectoderm (TE) biopsy associated with successful implantation and pregnancy after the embryo transfer of blastocysts subjected to preimplantation genetic screening (PGS)? In this retrospective cohort study, 1329 blastocysts from 223 patients were subjected to TE biopsy for performing array comparative genomic hybridization (CGH) tests. The PGS and frozen blastocyst transfer (FET) cycles were performed from December 2012 to May 2015. Only the good quality and expanded blastocysts on day 5 or 6 were selected for biopsy. After TE biopsy, the re-expansion grades relative to the original blastocoel were (1) collapsed blastocysts (CB), (2) 3/4 re-expansion but not full expansion (RE), and (3) full re-expansion or hatching (FE). All biopsied blastocysts were subjected to vitrification within 0.5-6 h after biopsy; the time intervals between TE biopsy and vitrification and the expansion grades at the time of vitrification were recorded. By combining two factors, namely the expansion grades and culture intervals between biopsy and vitrification, the patients were further divided into four groups, namely CB with a < 3 h culture interval (n = 34 cycles, Group I), RE and FE blastocysts with a < 3 h culture interval (n = 10 cycles, Group II); CB blastocysts with a ≥ 3 h culture interval (n = 6 cycles, Group III); and RE or FE blastocysts with a ≥ 3 h culture interval (n = 173 cycles, Group IV). The implantation (63.7%, 179/281) and clinical pregnancy (74.0%, 128/173) rates in Group IV were significantly higher than those in Group I (45.3%, 24/53; 50.0%, 17/34; P = 0.012 and 0.005, respectively). According to our findings, optimal vitrification timing > 3 hours to enable blastocysts to reach RE or FE provides improved implantation and pregnancy rates after FET.

TRIAL REGISTRATION

ClinicalTrials.gov NCT03065114.

Ultrastructure of vitrified rabbit transgenic embryos

The aim of the study was to determine the viability of rabbit transgenic (enhanced green fluorescent protein (EGFP)-positive) embryos cultured in vitro and compare with gene-microinjected (Mi) non-transgenic (EGFP-negative) embryos following vitrification. Non-microinjected and non-vitrified embryos were used as the control. Morphological signs of injury to embryo organelles were determined at the ultrastructural level using transmission electron microscopy (TEM). Morphometric evaluation was performed on cellular organelles using microphotographs obtained by TEM. Intact and Mi embryos recovered from in vivo fertilized eggs at 19-20 hours post coitum (hpc) were cultured for up to 72 hpc (morula stage), evaluated for the EGFP gene integration and then vitrified in 0.25 ml insemination straws in modified EFS (40% ethylene glycol + 18% Ficoll 70 + 0.3 M sucrose) vitrification solution. After 1-3 days the embryos were devitrified, a representative selection of embryos was analyzed by TEM and the remaining embryos were subjected to additional in vitro culture. Observations by TEM showed that the vitrified/warmed EGFP-positive and EGFP-negative embryos had a slight accumulation of cellular debris and lipid droplets compared with the control intact embryos. More severe changes were detected in the membrane structures of the treated embryos, mostly in the cytoplasmic envelope, trophoblastic microvilli, junctional contacts and mitochondria. We suggest that the higher proportion of deteriorated cell structures and organelles in the treated embryos may be due to the vitrification process rather than to mechanical violation (the gene-microinjection procedure), as a detailed inspection of ultrastructure revealed that most damage occurred in the cell membrane structures.

Effects of different cryopreservation methods on post-thaw culture conditions of in vitro produced bovine embryos

The aim of this work was to evaluate the effect of cryopreservation protocols on subsequent development of in vitro produced bovine embryos under different culture conditions. Expanded in vitro produced blastocysts (n = 600) harvested on days 7-9 were submitted to controlled freezing [slow freezing group: 10% ethylene glycol (EG) for 10 min and 1.2°C/min cryopreservation]; quick-freezing [rapid freezing group: 10% EG for 10 min, 20% EG + 20% glycerol (Gly) for 30 s]; or vitrification [vitrification group: 10% EG for 10 min, 25% EG + 25% Gly for 30 s] protocols. Control group embryos were not exposed to cryoprotectant or cryopreservation protocols and the hatching rate was evaluated on day 12 post-insemination. In order to evaluate development, frozen-thawed embryos were subjected to granulosa cell co-culture in TCM199 or SOFaa for 4 days. Data were analyzed by PROC MIXED model using SAS Systems for Windows®. Values were significant at p < 0.05. The hatching rate of the control group was 46.09%. In embryos cultured in TCM199, slow freezing and vitrification group hatching rates were 44.65 ± 5.94% and 9.43 ± 6.77%, respectively. In embryos cultured in SOFaa, slow freezing and vitrification groups showed hatching rates of 11.65 ± 3.37 and 8.67 ± 4.47%, respectively. In contrast, the rapid freezing group embryos did not hatch, regardless of culture medium. The slow freezing group showed higher hatching rates than other cryopreservation groups. Under such conditions, controlled freezing (1.2°C/min) can be an alternative to cryopreservation of in vitro produced bovine embryos.

Factors affecting the outcome of human blastocyst vitrification

With single blastocyst transfer practice becoming more common in ART, there is a greater demand for a convenient and reliable cryostorage of surplus blastocysts. Vitrification has emerged in the last decade as an alternative promising substitute for slow freezing. Blastocysts represent a unique challenge in cryostorage due to their size, multicellular structure and presence of blastocoele. The continuous acquisition of experience and introduction of many different technological developments has led to the improvement of vitrification as a technology and improved the results of its application in blastocyst cryostorage. The current information concerning safety and efficacy of the vitrification of blastocysts will be reviewed along with the variables that can impact the outcome of the procedure.

Vitrification of In Vitro-produced Bovine Embryos by Addition of Ethylene Glycol in One-step

The objective of this study was to simplify two-step addition of cryoprotectant for vitrification of bovine embryos by developing a one-step procedure. Survival was calculated as a percentage of non-vitrified controls developed from the same batch of oocytes. In experiment 1, bovine blastocysts were vitrified following one- or two-step addition of cryoprotectant. Exposure of embryos to cryoprotectant in one-step resulted in survival rates not significantly lower (p > 0.1) than those obtained by two-step addition (85% vs 98%, respectively). Based on these results, experiments 2-4 were designed to test one-step addition of cryoprotectant more rigorously. Experiment 2 exposed day 7 blastocysts to 6, 7 or 8 M ethylene glycol for 2.5 or 3.5 min. At 24 h post-vitrification, survival of embryos was similar, irrespective of ethylene glycol concentration or exposure time (6 M 38%, 7 M 51%, 8 M 59%; 2.5 min 54%, 3.5 min 45%). In experiment 3, blastocysts were exposed to 7 M ethylene glycol for shorter times (30 or 60 s); 30 s exposure resulted in decreased survival (8% vs 31%, p < 0.05). Experiment 4 concerned one-step addition of cryoprotectant to day 6 bovine morulae, exposed to 7 M ethylene glycol for 1 or 1.5 min. There was no difference in survival between exposure times of 1 or 1.5 min (28% vs 45%, respectively; p > 0.1). It is unclear why many embryos survive vitrification with one-step addition of cryoprotectant, but others do not. Although, one-step addition of cryoprotectant simplifies the vitrification procedure, survival rates were inadequate for routine cryopreservation of in vitro-produced bovine embryos.

Freezing of in vitro produced bovine embryos in animal protein-free medium containing vegetal peptones

Successful cryopreservation is essential for a large-scale dispersal of bovine in vitro produced (IVP) embryos that have been shown to be more sensitive to cryopreservation than their in vivo counterparts. On the other hand, the use of animal proteins in freezing media increases sanitary risks. We first replaced animal proteins, such as bovine serum albumin (BSA) in the freezing medium by plant-derived peptides (vegetal peptones). A batch of wheat peptones was selected after a preliminary experiment showing the absence of toxicity of concentrations<18 mg/mL on in vitro bovine blastocysts. Increasing concentrations of peptones were then added in the freezing medium. The surviving and hatching rates were not affected by comparison with those observed with BSA. No significant difference was observed between groups either for the total number of cells or for the ratio ICM/Total cell, nor for the rate of apoptosis in surviving embryos. When embryos were cryopreserved in 1.8 mg/mL peptone, the hatching rate and embryo quality as assessed at 48 h post-thawing were not significantly different from those of unfrozen embryos. In a second experiment two additives were added in this animal protein-free freezing medium containing 1.8 mg/mL peptones. No beneficial effect of adding 1 mg/mL sodium hyaluronate or 100 microM beta-mercaptoethanol was observed on embryo survival or quality. In conclusion, we have demonstrated that vegetal peptones can replace BSA in freezing media without affecting blastocyst survival and quality.

Effect of estrous cow serum during bovine embryo culture on blastocyst development and cryotolerance after slow freezing or vitrification

The present study investigated the effect of estrous cow serum (ECS) during culture of bovine embryos on blastocyst development and survival after cryopreservation by slow freezing or vitrification. Embryos were derived from in vitro maturation (IVM) and in vitro fertilization (IVF) of abbatoir-derived oocytes. At Day 3, embryos were cultured in three different media: Charles Ronsenkrans medium + amino acids (CR1aa; without bovine serum albumin (BSA)) + 5% estrous cow serum (CR1-ECS), CR1aa + 3 mg/mL BSA (CR1-BSA) or CR1aa + 5% ECS + 3 mg/mL BSA (CR1-ECS-BSA). At 7.5 d post-insemination (PI), blastocyst yield and quality were evaluated; blastocysts and expanded blastocysts from each media were cryopreserved by Open Pulled Straw (OPS) vitrification method or slow freezing (1.5 M ethylene glycol, EM). Total blastocyst yield did not differ among CR1-ECS, CR1-BSA and CR1-ECS-BSA (30.9, 33.1 and 32.9%, respectively, P < 0.05). Embryo survival (hatching rate) was higher in vitrified versus slow-frozen embryos (43% versus 12%, respectively, P < 0.01), and in embryos cultured in CR1-BSA (40.3%) compared with those cultured in serum-containing media (CR1-ECS, 21.5% and CR1-ECS-BSA, 19.8%; P < 0.01).

IN CONCLUSION

(a) it was possible to produce in vitro bovine embryos in serum-free culture medium without affecting blastocyst yield and quality; (b) serum-free medium produced the best quality embryos (in terms of post-cryopreservation survival); and (c) vitrification yielded the highest post-cryopreservation survival rates, regardless of the presence of serum in the culture medium.

Interferon tau secretion in cattle embryos in vitro fertilized before and after cryopreservation

The effect of cryopreservation in IFN-tau, from bovine embryos produced in vitro was evaluated. Two treated groups (G1= fresh bovine embryos, n=59 and G2= freezed embryos, n=84) were used to study the effect of cryopreservation on IFN-tau secretion. After reaching the blastocyst phase, the embryos were kept on individual culture for additional period of 7 days. On days 3 and 7 after the beginning of embryos cultivation, samples of the media culture were taken for IFN-tau secretion titration. Oocysts taken from follicles ranging from 3 to 5mm in diameter were obtained from ovaries of females at slaughterhouse. The embryos were frozen, after being dehydrated with ethylene glycol (1.8m), conditioned on 0.5ml palletes and frozen. Frozen embryos secreted lower IFN-tau than fresh embryos (P<0.05). At day 7 it was registered higher IFN-tau secretion from trophoblast than at day 3 (P<0.05). The increasing of IFN-tau secretion was observed when the blastocyst began to longed and it was directly related to the embryos development. The synthesis of IFN-tau is related to the capability of development of the blastocyst. Cryopreservation is a method that affects the maternal recognition of pregnancy and the post-freezing embryo development.

FSH different regimes affect the developmental capacity and cryotolerance of embryos derived from oocytes collected by ovum pick-up in donor sheep

The objective of the present study was to compare the developmental capacity of sheep oocytes obtained by OPU after two different ovarian stimulations, and cryotolerance to vitrification procedures of in vitro derived embryos after in vitro maturation, fertilisation and culture of these oocytes. Sheep were divided into three groups: (A) no treatment (control); (B) constant doses of FSH (FSH-c); (C) decreasing doses of FSH (FSH-d). Ovine groups FSH-c and FSH-d were synchronised by the insertion of intravaginal sponges left in situ for 7 days; FSH (total dose: 96IU) was administered in four doses given every 12h starting on Day 5. Twelve hours after the last FSH administration oocytes were collected by OPU technique. The control group showed a significantly lower number ( P<0.05 ) of follicles (166) than FSH-c (294) and FSH-d (317) groups, while the number of follicles >5mm was significantly higher ( P<0.01 ) in FSH-d group, showing that this protocol stimulates the growth of a different follicle population compared to FSH-c group. The control group showed a higher number of <2mm follicles ( P<0.01 ). We did not find any difference in oocyte quality between the three groups and therefore the percentage of discarded oocytes was similar. No significant differences were found between control, FSH-c and FSH-d groups in terms of maturation (90.9, 85.7 and 87.7%, respectively) and fertilisation rates (75.2, 80.9 and 83.7%, respectively) while a significantly higher ( P<0.01 ) blastocyst rate was observed in the FSH-c group than in the FSH-d and control groups (20.4% versus 11.8 and 13.7%, respectively). After vitrification, warming and 72 h in vitro culture, the hatching rate was significantly higher ( P<0.01 ) in the control (87.5%) and FSH-c (90.5%) groups than in the FSH-d group (66.7%). Control and FSH-c groups showed a significantly higher ( P<0.001 ) number of total cells than FSH-d group ( 217.6+/-26.5 and 203.0+/-33.2 versus 147.5+/-20.2 ), while no differences were observed in ICM cell rates in the control ( 35.6+/-3.8 ), FSH-c ( 37.1+/-4.6 ) and FSH-d ( 36.6+/-6.7 ) groups. These results indicate that donor sheep stimulated with FSH-c produced better quality oocytes and blastocysts showing better cryotolerance than ewes given the decreasing doses treatment.

An overview of oocyte cryopreservation

The ability to cryopreserve human oocytes and store them indefinitely would be beneficial for cancer patients at risk of becoming sterile after therapy, allow women to delay reproduction, and alleviate religious concerns associated with embryo storage. In 1986, Chen was the first to report a pregnancy originating from a frozen-thawed human oocyte. Although over 100 babies have been born from oocyte storage since then, pregnancy rates remain unacceptably low. Adapting embryo cryopreservation techniques to oocyte storage has had limited success and new reproducible methods are needed. Problem areas other than intracellular ice formation and osmotic effects need to be identified. A broad approach of critical analysis should be conducted regarding the entire cryopreservation process from pre-equilibration and cooling, to thawing and stepout. All established facets deserve reanalysis in order to assess which aspects can be optimized or changed so that cellular demise can be avoided and cellular viability enhanced. New methods, including the use of choline-based media and vitrification have proven useful in increasing survival and pregnancy rates in some clinics. Other methods yet untested, such as injection of complex carbohydrates into the oocyte, deserve further studies. Vitrification research has led to the formulation of new ideas and has demonstrated the flexibility of cells to survive cryopreservation. Although successful, vitrification protocols are potentially harmful and technically challenging, due to elevated cryoprotectant concentrations and rapid cooling rates. Bovine embryo vitrification methods have been used to store human oocytes and embryos, particularly blastocysts with some success. Vitrification solutions containing high molecular weight polymers have also proved beneficial by reducing solution toxicity. In general, further advances are needed to improve human oocyte storage before widespread routine clinical use.

Resumption of metabolic activity of vitrified/warmed ovine embryos

To evaluate resumption of metabolic activity of vitrified ovine embryos during a short time immediately after warming, blastocysts collected from superovulated Sarda ewes were incubated with (35)S-methionine. In vitrified/warmed embryo groups the protein secretion significantly (P < 0.05) increased from 0 to 24 hr of culture, reaching significantly (P < 0.01) higher activity at 18-24 hr and dropping to values similar to the control nonvitrified embryo group at 29-35 hr. Within the control group at 29-35 hr there was a significantly (P < 0.01) higher level of protein secretion compared to the other interval times. The electrophoretic pattern showed a 48-50 kDa secreted protein identified as urokinase-type plasminogen activator (PA). The caseinolytic assay of PA activity showed a course similar to protein secretion in both vitrified and control groups. During 29-35 hr of culture, we did not observe any improvement in PA activity as seen for secreted proteins. At this time, we observed the secretion of a new 20 kDa protein that was not present in vitrified/warmed embryos. Analysis of BrDU incorporation in newly synthesised DNA showed a significant (P < 0.01) improvement in positive cell number from 3 to 9 hr after warming, reaching a value similar to that of the control group at 12 hr of culture. Our results suggest that vitrified/warmed embryos require 9-12 hr of culture to complete resumption of DNA synthesis and 29-35 hr to re-acquire the full capacity of protein secretion but not the qualitative secretion pattern.

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.

Cryoloop vitrification yields superior survival of Rhesus monkey blastocysts

BACKGROUND

Vitrification using the cryoloop procedure was evaluated for preservation of non-human primate blastocysts by comparing survival results from two different cryoprotectant mixtures with prior results from controlled rate cooling.

METHODS

Rhesus monkey blastocysts were produced by intracytoplasmic sperm injection of mature oocytes from cycling females stimulated with recombinant human hormones. Morphologically well-formed blastocysts were divided between Procedure A (2.8 mol/l dimethylsulphoxide and 3.6 mol/l ethylene glycol with 0.65 mol/l sucrose and 25 micromol/l Ficoll in TALP-HEPES with 20% fetal bovine serum (TH20)) and Procedure B (3.4 mol/l glycerol and 4.5 mol/l ethylene glycol in TH20). After >48 h in liquid nitrogen, the removal of cryoprotectants was accomplished in the presence of a 3-step series of decreasing sucrose concentrations in TH20. Surviving embryos were co-cultured on buffalo rat liver cells.

RESULTS

Of 16 blastocysts vitrified via Procedure A, 38% survived with minimal lysis and only one hatched in culture; in contrast, of 33 blastocysts vitrified by Procedure B, 85% survived and 71% hatched. Of 22 blastocysts cryopreserved by conventional slow cooling, 36% survived and 6% hatched. Transfer into three recipients, each with two embryos vitrified with Procedure B, resulted in a successful twin-term pregnancy.

CONCLUSION

Modified cryoloop vitrification with a final solution of 3.4 mol/l glycerol and 4.5 mol/l ethylene glycol is a promising procedure for preserving Rhesus monkey blastocysts that is simple, rapid, and inexpensive.

Blastocyst evaluation by means of differential staining: a practical approach

Techniques for in vitro production of embryos have been developed world-wide in different species, with promising results in human and ruminants. Thousands of human IVF-babies have been born during the last 20 years and thousands of in vitro-produced calves have been born since the late 1980s. With current methods for bovine in vitro fertilization, about 30-40% of in vitro-fertilized bovine oocytes develop further to the blastocyst stage and can be used for transfer. A proper evaluation of blastocyst quality remains however, an important challenge for every researcher involved in embryology and for every clinician who wants to select the best embryos for transfer. This review attempts to summarize the different methods available for estimation of blastocyst quality with a special emphasis upon differential staining.

Osmotic behavior of in vitro produced bovine blastocysts in cryoprotectant solutions as a potential predictive test of survival

The osmotic behavior of bovine blastocysts produced in vitro was filmed during exposure to and dilution of cryoprotectant solutions used for vitrification. The relationship between the changes in the diameter of embryos and their subsequent survival was assessed. Embryos collected on Day 6 and Day 7 postinsemination were exposed to 10% glycerol (GLY) for 5 min, 10% GLY + 20% ethylene glycol (EG) for 5 min, and 25% Gly + 25% EG for 30 s, before dilution in 0.85 M galactose and finally in embryo transfer freezing medium (ETF). Embryos that had a higher probability of survival behaved as perfect osmometers, shrinking, reexpanding, or swelling according to an identical pattern, whereas embryos that deviated from this standard usually did not survive. The initial embryo diameter, duration of shrinkage and expansion in 10% glycerol, duration of reexpansion in ETF, and final embryo diameter were clearly predictive of the ability to hatch after culture in vitro. On a given day postinsemination, larger blastocysts were more likely than smaller blastocysts to survive and hatch after exposure to cryoprotectants with or without vitrification.