Effects of cryopreservation of pronuclear-stage rabbit zygotes on the morphological survival, blastocyst formation, and full-term development after DNA microinjection

The objectives of this study were to examine the freezing sensitivity of pronuclear-stage rabbit zygotes and to produce transgenic rabbits using the cryopreserved zygotes. Zygotes were cryopreserved either by one of two vitrification protocols or by one of the two conventional freezing protocols. The morphological survival rates of zygotes subjected to two-step freezing in 1.5 M ethylene glycol and 0.1 M sucrose (74%) or to vitrification in 7.2 M ethylene glycol and 1.0 M sucrose (81%) were higher than those subjected to freezing in 1.5 M DMSO (46%) or to vitrification in a mixture of 2.0 M DMSO, 1.0 M acetamide, and 3.0 M propylene glycol (41%). But the in vitro development into blastocysts of zygotes cryopreserved by vitrification (17%) or to a lesser extent by freezing (52%) was impaired, when compared to that of fresh control zygotes (89%). Next, a fusion gene composed from bovine aS1-casein promoter and a human GH structural gene (2.8 kb) was microinjected into the pronucleus of rabbit zygotes frozen-thawed in ethylene glycol and sucrose. Then, the presence of exogenous DNA in the genome of newborn offspring was determined by PCR. The post-injection survival of frozen zygotes (97%) was the same as that of fresh control zygotes (96%). However, of 18 offspring derived from 414 frozen-thawed and DNA-injected zygotes, no transgenic rabbits were produced. Of 52 offspring derived from 403 DNA-injected fresh zygotes, 3 transgenic rabbits were found. Here we report the first rabbit offspring resulting from zygotes cryopreserved at the pronuclear-stage, although the cryopreservation procedure employed must be improved if zygotes are to be used for systematic production of transgenic rabbits.     

Timing of the first cleavage post‐insemination affects cryosurvival of in vitro–produced bovine blastocysts

The time of the first cleavage of bovine zygotes during in vitro culture can affect the rate of development and cell number of the blastocysts. The aim of this work was to study the effect of the timing of first cleavage on the cryosurvival of the resulting blastocysts. Following standard IVM and IVF, zygotes were cultured in modified synthetic oviduct fluid (SOF), with 10% fetal calf serum (FCS) added 48 hr post insemination, in a humidified atmosphere of 5% CO_2, 5% O₂ and 90% N₂. Embryos which cleaved by 24, 27 30, 33, or 36 hr after insemination (IVF) were harvested and further cultured to the blastocyst stage (day 7 or day 8 post IVF). All developing blastocysts on days 7 and 8 were classified into three groups and were cryopreserved by vitrification. Group A consisted of blastocysts (<150 μm, small blastocysts); group B consisted of expanded or hatching blastocysts (>150 μm, large blastocysts); and group C consisted of morphologically poor quality blastocysts. The vitrification solution consisted of 6.5 M glycerol and 6% bovine serum albumin in PBS (VS3a). Thawed embryos were cultured further and survival was defined as the re‐expansion and maintenance of the blastocoel over 24, 48, and 72 hr, respectively. Overall survival and hatching at 72 hr post‐thawing was higher in blastocysts formed by day 7 than those formed by day 8 (60% vs. 40% survival; 63% vs. 45% hatching). Large blastocysts from day‐7 and day‐8 groups survived significantly better than small or poor quality blastocysts (76% vs. 63% and 31%; 72% vs. 30% and 26%, respectively; P < 0.05). Day‐7 blastocysts from the 27‐ and 30‐hr cleavage groups survived significantly better than those from the 36‐hr group (63% and 66% vs. 25%, P < 0.05). Day‐8 blastocysts from later cleaved (30 hr) zygotes had a higher survival than the 27‐hr cleavage groups (52% vs. 26%, P < 0.05). These results indicate that the day of blastocyst appearance, developmental stage, and timing of the first cleavage post‐insemination can influence the cryosurvival of bovine blastocysts following vitrification. Mol. Reprod. Dev. 53:318–324, 1999. © 1999 Wiley‐Liss, Inc.     

Efficient cryopreservation of bovine blastocysts derived from nuclear transfer with somatic cells using partial dehydration and vitrification

Preservation by vitrification of Day 7 and Day 8 bovine blastocysts derived from nuclear transfer with cumulus cells was compared with preservation of in vitro fertilized blastocysts. In Experiment 1, embryos were vitrified in PBS containing 60% ethylene glycol. In Experiment 2, they were vitrified in combination with partial dehydration using a solution of 39% ethylene glycol + 0.7 M sucrose and 8.6% Ficoll. In Experiment 1, survival and hatching rates were 44 and 95% for nuclear transferred embryos, and 78 and 55% for in vitro fertilized embryos, respectively. In Experiment 2, survival and hatching rates were 93 and 95% for nuclear transfer embryos, and 77 and 85% for in vitro fertilized embryos, respectively. It is concluded that Day 7 and Day 8 bovine blastocysts derived from cumulus cells could be cryopreserved without the loss of viability by a simple and efficient method using a combination of partial dehydration and vitrification.     

Usefulness of polyethylene glycol for cryopreservation by vitrification of in vitro-derived bovine blastocysts

A series of five experiments measured the high survival of bovine blastocysts produced in vitro after cryopreservation by vitrification. The vitrification solution (designated VS) contained 40% (v/v) ethylene glycol, 6% (w/v) polyethylene glycol and 0.5 M sucrose in phosphate-buffered saline. Embryos developed in vitro at Days 7 and 8 (Day 0 = insemination day) were exposed in one step to VS for 1 min or two steps with 10% ethylene glycol for 5 min and then VS for 1 min. In both cases, the embryos were finally cryopreserved in liquid nitrogen. After the embryos were warmed rapidly and the VS solution diluted, the survival rates were assessed by monitoring hatching rate in vitro. They were 13.0% for the one-step and 72.7% for the two-step procedures (P < 0.001). When embryos were exposed to individual solutions containing 6% (w/v) of each of 4 macromolecules (polyethylene glycol, BSA, polyvinylpyrrolidone or Ficoll) in the two-step protocol and then cryopreserved, the survival rates were 79.3, 34.8, 41.4 and 57.1%, respectively. After embryos had been exposed to the VS in two steps and then cryopreserved, there were no significant differences in survival rates when the solutions were diluted with or without sucrose. These results indicated that a vitrification solution containing polyethylene glycol can be used for cryopreservation of bovine blastocysts produced in vitro, and that a two-step addition of VS improved the in vitro survival of post-warming embryos. It was also shown to be possible to dilute post-warming embryos directly without the use of sucrose solution.     

Vitrification of ICSI- and IVF-derived bovine blastocysts by minimum volume cooling procedure: effect of developmental stage and age

The objective was to investigate the effects of developmental stage (fully-expanded or expanding blastocysts) and/or age (harvested on Days 7 or 8) on post-vitrification in vitro survival of bovine blastocysts derived from intracytoplasmic sperm injection (ICSI) or in vitro fertilization (IVF). Post-warming survival (re-expansion of blastocoele within 24 h) of ICSI-derived fully-expanded blastocysts (80%) was similar to that of their IVF-derived counterparts (88%). However, the ability of ICSI-derived expanding blastocysts to survive vitrification procedures (61%) was lower than that of IVF-derived blastocysts (85%; P < 0.05), although the ICSI- and IVF-derived fresh blastocysts were of similar quality. The age of the blastocysts before vitrification did not affect cryotolerance for either ICSI-derived (73 and 59% for Days 7 and 8 embryos, respectively) or IVF-derived blastocysts (86% for both Days 7 and 8 embryos). At 24 h of post-warming culture, ICSI-derived blastocysts surviving vitrification contained a higher proportion of dead cells than their IVF-derived counterparts (5-13% vs. 2-4%; P < 0.05), but these proportions were not different from those of fresh control embryos. There was an adverse effect of vitrification on the ability of blastocysts to hatch within 72 h of culture only in IVF-derived Day 8 blastocysts (41 and 70% in vitrified and fresh control groups, respectively). In conclusion, the proportion of blastocysts that survived vitrification procedures was similar for ICSI- and IVF-derived bovine blastocysts if the former were cultured to the fully-expanded stage prior to vitrification, with no significant difference between embryos harvested on Day 7 versus Day 8.     

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

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

Factors affecting survival rates of in vitro produced bovine embryos after vitrification and direct in-straw rehydration

The aim of this work was to investigate the possibilities of simplification, and to outline the limits of application, of a vitrification method for cow embryos. Morulae and blastocysts were produced by in vitro fertilization of slaughterhouse-derived, in vitro matured oocytes with frozen-thawed bull semen, and subsequent culture on a granulosa cell monolayer. Vitrification was performed by equilibration of embryos with 12.5% ethylene glycol and 12.5% dimethylsulphoxide at 20–22°C for 60 s, then with 25% ethylene glycol and 25% dimethylsulphoxide at 4°C for another 60 s. Embryos were then loaded in straws, placed in liquid nitrogen vapour for 2 min, and then plunged. Straws were thawed in a 22°C water-bath, the embryos were directly rehydrated and further incubated in straw, and were then expelled and cultured in vitro for 72 h. In the first experiment, embryos of different age and developmental stage (Day 5 compacted morulae, Day 6 early blastocysts, Days 6 and 7 blastocysts, Day 7 expanded blastocysts and Day 8 hatched blastocysts) as well as Days 7 and 5 blastocysts previously subjected to partial zona dissection were vitrified. After thawing, the re-expansion rates of blastocysts and zona-dissected embryos did not differ (67 and 87%, respectively), and hatching was more frequent for blastocysts frozen in advanced developmental stages (34, 47 and 63% for early blastocysts, blastocysts and expanded blastocysts, respectively). The re-expansion rate of morulae was lower (10%) and no hatching of these embryos was observed. In the second experiment, Day 7 expanded blastocysts were vitrified using PBS, PBS + albumin, TCM199 and TCM199 + calf serum as holding media. No differences in re-expansion and hatching rates were seen. However, when incubation with the concentrated cryoprotectant solution was performed at 20–22°C, the embryo survival rate decreased (PBS + albumin) or no embryo survived (TCM199 + calf serum) the vitrification procedure. In the third experiment, Day 7 expanded blastocysts were vitrified, thawed, cultured for 1 day, and then re-expanded embryos were again vitrified and thawed. Out of the 87% that survived the first cycle, 73% re-expanded and 47% hatched following the second vitrification and thawing. These observations prove that the vitrification procedure described is relatively harmless, that it can be used for blastocysts of different developmental stages and that an intact zona is not required to obtain high survival rates.     

Successful vitrification of pronuclear-stage rabbit zygotes by minimum volume cooling procedure

Rabbit zygotes at the pronuclear-stage were cryopreserved by vitrification using a gel-loading tip (GL-tip), Cryoloop or Cryotop. In GL-tip and Cryoloop methods, zygotes were first exposed to 10% ethylene glycol (EG)+10% DMSO in TCM199+20% fetal bovine serum (FBS) for 2 min, and then equilibrated for 30 s in a vitrification solution composed of 20% EG+20% DMSO+0.6 M sucrose in TCM199+20% FBS. In Cryotop method, zygotes were first exposed to 7.5% EG+7.5% DMSO+20% FBS in TCM199 for 3 min, and then equilibrated for 1 min in a vitrification solution composed of 15% EG+15% DMSO+0.5 M sucrose+20% FBS in TCM199. In vitro culture of vitrified-warmed zygotes using GL-tip and Cryoloop resulted in low cleavage rates (2 and 5%, respectively) and no development into blastocysts. In contrast, zygotes vitrified-warmed using Cryotop exhibited higher proportions of cleavage (58%) and development into blastocysts (24%). When compacted morulae or early blastocysts were vitrified by these three procedures, 80-93% of them exhibited blastocoele expansion or zona hatching during the subsequent 48 h of culture. Use of Cryotop instead of GL-tip or Cryoloop for zygote vitrification, without changing conditions of solutions and periods for exposure, equilibration and post-warm dilution, resulted in cleavage and blastocyst development rates of 88 and 45%, respectively. A longer exposure time (10 min) of zygotes to 7.5% EG+7.5% DMSO+20% FBS in TCM199 resulted in higher proportions of zygotes cleaving (94%) and developing into blastocysts (51%) after Cryotop vitrification. Proportions of post-warm zygotes (10-min exposure group) and fresh control zygotes developing into newborn offspring were 36 and 53%, respectively. Pronuclear-stage rabbit zygotes were successfully cryopreserved by vitrification using the Cryotop method.     

Viability of bovine blastocysts obtained after 7, 8 or 9 days of culture in vitro following vitrification and one-step rehydration

This study examined morphological appearance, viability and hatching rates in relation to the total cell number following vitrification of in vitro produced bovine blastocysts and expanded blastocysts. In Experiment 1, embryos obtained after 7, 8 or 9 d of culture were pooled and equilibrated in either 10% ethylene glycol (EG) or 10% EG plus 0.3M trehalose in Dulbecco's phosphate buffered saline (DPBS) supplemented with 10% calf serum and 0.6% BSA for 5 min each, at room temperature, and then vitrified together in precooled vitrification solutions consisting of 40% EG (Treatment 1), 40% EG plus 0.3M trehalose (Treatment 2), 40% EG plus 0.3M trehalose and 20% polyvinylpyrrolidone (PVP, Treatment 3) in DPBS. The embryo viability and hatching rates of Treatment 1 (19 and 3%) differed significantly (P < 0.05) from those of Treatment 2 (56 and 31%) and Treatment 3 (70 and 43%). There was a significant difference (P < 0.05) in embryo viability between Treatment 2 (31%) and Treatment 3 (43%). In Experiment 2, Day 7, 8 and 9 embryos were vitrified separately, with higher viability and hatching rates in Experiment 1 than in Experiment 2. The viabilities of Day 7 (87%), 8 (71%) and 9 (46%) embryos differed significantly (P < 0.05). Again, there were significant differences (P < 0.01) among the hatching rates of Day 7 (75%), 8 (38%) and 9 (9%) embryos. The total cell number of hatched blastocysts was then determined by differential fluorochrome staining. The total cell number of Day 7, 8 and 9 embryos differed significantly (P < 0.05).     

Delipidating in vitro-produced bovine zygotes: effect on further development and consequences for freezability

To study the effect of partial removal of intracytoplasmatic lipids from bovine zygotes on their in vitro and in vivo survival, presumptive zygotes were delipidated by micromanipulation and cocultured with Vero cells in B2+10% FCS. Blastocyst rates of delipidated (n=960), sham (centrifuged but not delipidated, n=830) and control embryos (n=950) were 42.1, 42.3 and 39.9% respectively (P > 0.05). Day 7 blastocysts derived from delipidated zygotes had a mean of 123.9 +/-45.6 nuclei compared to 137.5+/-32.9 for control blastocysts (P > 0.05). The full-term development of delipidated blastocysts after single transfer to recipients was similar to that of control IVF blastocysts (41.2% vs 45.4% respectively). To assess the effect of delipidation on the embryo tolerance to freezing/thawing, delipidated (n=73), control (n=67) and sham (n=50) Day 7 blastocysts were frozen in 1.36 M glycerol + 0.25 M sucrose in PBS. After thawing, embryos were cocultured for 72 h with Vero cells in B2+10% FCS. Survival rates at 24 h were not significantly different between groups. However, in the delipidated group, the survival rate after 48 h in culture was significantly higher than in the control group (56.2 vs 39.8, P < 0.02), resulting in a higher hatching rate after 3 days in culture (45.2 vs 22.4, P < 0.02). Pregnancy rates for delipidated and control frozen/thawed embryos were respectively 10.5 and 22.2% (P > 0.05). Electron microscopic observations showed much fewer lipid droplets (and smaller) in delipated blastocysts than in controls. Taken together, our data show that delipidation of one cell stage bovine embryos is compatible with their normal development to term and has a beneficial effect on their tolerance to freezing and thawing at the blastocyst stage. This procedure, however, alters the developmental potential of such blastocysts, suggesting that maternally inherited lipid stores interfere with metabolic recovery after thawing.     

Evaluation of different culture systems on the in vitro production of bovine embryos

The objective of this study was to determine the development potential and quality of in vitro produced bovine embryos cultured individually or in groups. After IVM and IVF, presumptive zygotes were cultured in groups or individually, either in drops or in the modified "well of the well" (mWOW) system. In Experiment 1, four culture systems were utilized: T1: drop in group (control); T2: mWOW in groups; T3: mWOW individually; and T4: drop individually. Cleavage and blastocyst rates at Days 6, 7 and 8 and total cell number of Day 6 blastocysts were similar (P > 0.05) for all treatments. However, in Day 7 blastocysts, total cell number was lower (P < 0.05) in embryos cultured individually in a small drop than those cultured in the mWOW. In Experiment 2, blastocysts of T1, T2 and T3 were allocated into two groups, control and vitrified. After warming, the vitrified embryos were cultured for 72 h. At 48 h, the development of the Days 6 and 7 embryos was similar (P > 0.05) for all treatments in the control group. For the vitrified embryos, lower hatching rates (P < 0.05) were observed in the T3 group. In conclusion, embryos cultured in groups in the mWOW system had the same blastocyst rates but better quality (measured by their survival after vitrification) than those cultured individually in the mWOW system.     

Optimization of cryopreservation of buffalo (Bubalus bubalis) blastocysts produced by in vitro fertilization and somatic cell nuclear transfer

The objective of this study was to optimize cryopreservation conditions for buffalo in vitro produced (IVP) embryos. The in vitro fertilized (IVF) and somatic cell nuclear transferred (SCNT) blastocysts were vitrified with either 40% ethylene glycol (EG), 25% EG + 25% dimethylsulfoxide (DMSO), or 20% EG + 20% DMSO + 0.5 m sucrose, and the IVF blastocysts produced from abattoir-derived ovaries were also slow-frozen with either 10% EG or 0.05 m trehalose dehydrate + 1.8% EG + 0.4% BSA. Cryosurvival rates of blastocysts harvested on various days or at various developmental stages were also examined. In this study: (1) vitrification with 20% EG + 20% DMSO + 0.5 m sucrose had the best cryopreservation efficiency; (2) IVF and SCNT blastocysts had similar cryotolerance (P > 0.05); (3) after thawing, slow-frozen blastocysts reexpanded earlier than the vitrified blastocysts (P < 0.01); (4) cryosurvival rate of expanded blastocysts was higher than that of early blastocysts (P < 0.05); (5) cryosurvival rates of Days 5 to 7 blastocysts (Day 0 = day of IVF or SCNT) were higher than those of Days 8 to 9 blastocysts (P < 0.01); and (6) after embryo transfer, pregnancy rates for fresh and cryopreserved blastocysts were not different (P > 0.05). In conclusion, vitrification of Days 6 to 7 expanded blastocysts with 20% EG + 20% DMSO + 0.5 m sucrose was optimal for cryopreservation of buffalo IVP embryos.     

Higher survival rate of vitrified and thawed in vitro produced bovine blastocysts following culture in defined medium supplemented with beta-mercaptoethanol

The present study was conducted to compare bovine embryo developmental quality, after culture in different defined culture media, up to blastocyst stage, and subsequently cultured in media supplemented with beta-mercaptoethanol (beta-ME) following blastocyst vitrification and thawing. In part one of this study, presumptive zygotes were randomly allocated into the following media: (1) CR1, (2) KSOM, (3) SOF, and (4) sequential KSOM-SOF. In the second part of the study, blastocysts derived from four different culture media were subjected to a solid surface vitrification (35% (v/v) ethylene glycol+0.5M Sucrose+5% (w/v) Polyvinylpyrrolidone (PVP), and tested for the effect of beta-ME on their post-vitrification survival. Following thawing, blastocysts were cultured with or without beta-ME. Culture medium had no effect on cleavage rates; however, a significantly greater number of zygotes cultured in KSOM, KSOM-SOF, or SOF developed to the 8-cell stage, compared with those cultured in CR1. A greater proportion of the zygotes cultured in SOF or KSOM-SOF reached blastocysts, than did those cultured in CR1 or KSOM. The use of sequential KSOM-SOF significantly increased total cell numbers of Day 7 expanded-blastocysts when compared to those cultured in CR1, KSOM, or SOF. Addition of beta-ME into culture media after vitrification and thawing improved blastocyst survival, hatching rates, and total cell numbers of blastocysts. In conclusion, supplementation of beta-ME into culture medium after vitrification and thawing significantly increased blastocyst survival, hatching rates, and their total cell numbers. These results suggest that vitrified IVF embryos should be thawed and briefly cultured in beta-ME medium prior to embryo transfer.     

Vitrification of bovine IVF blastocysts in an ethylene glycol/sucrose solution and heat-stable plant-extracted proteins

The use of heat-stable plant proteins in an ethylene glycol-based solution for the vitrification of in vitro-derived embryos was examined. Day 7, 8 and 9 bovine in vitro matured, fertilized and cultured (IVMFC), full and expanded blastocysts were vitrified in solutions composed of 40% ethylene glycol (EG) plus 0.3 M sucrose supplemented with 20% Ficoll and 0.3% BSA (VF-1), 25 mg/ml heat-stable plant proteins (HSPP; VF-2), or with no supplement (VF-3). In Experiment 1, embryos were expelled from the straw after thawing, and EG was diluted from embryos with 0.5 M sucrose. There were no differences in post-thaw embryo survival rates or in hatching/hatched rates after 24 h of culture between the VF-1, VF-2 and VF-3 solutions (40.1, 54.1 and 50.8% and 10.7, 16.4 and 17.5%, respectively). Transfer of 12 frozen/thawed embryos to 6 recipients (2 recipients per treatment) resulted in 2 pregnancies from the VF-2 group and 1 pregnancy from the VF-3 group. In Experiment 2, EG was diluted from embryos after thawing within the straw with 0.5 M sucrose. There were no differences in post-thaw survival or hatching/hatched rates after 24 h of culture (19.0, 13.6 and 23.8% and 9.5, 9.0 and 14.4% for VF-1, VF-2 and VF-3, respectively). Transfer of 6 frozen/thawed embryos to 3 recipients (1 recipient per treatment) resulted in no pregnancies. The post-thaw histology of Day 7, 8 and 9 IVMFC blastocysts showed typical ultrastructure with well preserved cell-to-cell contacts. There were no major differences in the fine structure of blastocysts regardless of treatment. The use of HSPP at a concentration of 25 mg/ml in the vitrification medium did not affect the post-thaw embryo survival over that of no protein supplementation. The presence of macro molecules in a 40% EG/sucrose vitrification solution also did not improve post-thaw viability of IVMFC-derived blastocysts.     

Serum free embryo culture medium improves in vitro survival of bovine blastocysts to vitrification

The aim of this study was to examine the effects of co-culture with Vero cells during the in vitro maturation (IVM) and three culture media, B2+5% fetal calf serum (FCS) on Vero cells, synthetic oviduct fluid (SOF)+5% FCS, and SOF+20 gL(-1) bovine serum albumin (BSA), on the developmental competence of the embryos and their ability to survive vitrification/warming. We also tested the effect of morphological quality and the age of the embryo on its sensitivity to vitrification. The IVM system neither affects the embryo development up to Day 7 nor survival rates after vitrification. The culture of embryos in SOF+FCS and in Vero cells+B2 allowed obtaining more Day 6 and Day 7 blastocysts, and a higher % of Day 7 blastocysts vitrified than culture in SOF+BSA. Contrarily, on Day 8, more blastocysts were vitrified in SOF+BSA than in SOF+FCS. Blastocysts quality affected development after vitrification/warming, and Day 7 embryos showed higher survival rates than their Day 8 counterparts. Day 7 blastocysts produced in Vero cells or in SOF+BSA survived at higher rates than those produced in SOF+FCS at 24 and 48 h after warming. Embryo culture with BSA allows obtaining hatching rates after vitrification/warming higher than those obtained after co-culture with Vero cells in B2 and FCS. Moreover, this system provides hatching rates from Day 8 blastocysts comparable to those obtained on Day 7 in Vero cells. Further studies, including embryo transfer to recipients, are needed to clarify factors affecting the freezability of in vitro produced bovine embryos.     

Cryopreservation of bovine in vitro produced embryos using ethylene glycol in controlled freezing or vitrification

In this study, the cryoprotectant ethylene glycol (EG) was tested for its ability to improve and facilitate the cryopreservation of in vitro produced (IVP) bovine embryos. Embryos were cryopreserved in EG solutions supplemented with either newborn calf serum (NBCS) or polyvinyl alcohol (PVA). To assess EG toxicity, the embryos were equilibrated in EG concentrations from 1.8 to 8.9 M at room temperature for 10 min and then cultured for 72 h on a cumulus cell monolayer. The hatching rate was highest for day 7 blastocysts frozen in 3.6 M EG (98%) and was not different from the control group (85%). The controlled freezing (0.3 degrees C/min to -35 degrees C) of expanded day 7 blastocysts resulted in a hatching rate of 81%, which was similar to that of the nonfrozen controls (76%). Differential staining revealed only very few degenerate blastomeres attributed to freezing and thawing. Upon direct nonsurgical transfer of day 7 expanded blastocysts frozen in 3.6 M EG, a pregnancy rate of 43% was achieved, while the pregnancy rate after transfer of other developmental stages was significantly lower (22% with expanded day 8 blastocysts). When bovine IVP embryos were incubated at room temperature in 7.2 M EG preceded by preequilibration in 3.6 M EG, the hatching rate of day 7 expanded blastocysts reached 93%. Upon vitrification of IVP day 7 and day 8 blastocysts and expanded blastocysts in 7.2 M EG, the latter showed a higher hatching rate (42%) than blastocysts (12%). Overall, PVA as supplement to the basic freezing solution instead of NBCS had deleterious effects on survival after controlled freezing or vitrification. The simple cryopreservation protocol employed in this study and the low toxicity of ethylene glycol highlight the usefulness of this approach for controlled freezing of IVP embryos. However, further experiments are needed to improve the pregnancy rate following embryo transfer and to enhance survival after vitrification.     

Pregnancies following transfer of equine embryos cryopreserved by vitrification

The objective of this study was to investigate the in vitro and in vivo developmental abilities of equine embryos cryopreserved by vitrification. Twenty-eight embryos were recovered from Native pony and Thoroughbred mares at Days 5 to 7 by nonsurgical uterine flushing (detection of ovulation=Day 0). The vitrification solution contained 40% ethylene glycol, 18% Ficoll, and 0.3 M sucrose in PBS. The embryos were placed for 1 to 2 min in vitrification solution (Group 1) or following exposure to 20% ethylene glycol in PBS for 10 to 20 min (Groups 2 and 3). Single embryos were loaded in 0.25-ml straws, cooled for 1 min in liquid nitrogen vapor and immersed in liquid nitrogen. Straws were warmed in water (20 degrees C, 20 sec), and the contents were expelled with 0.5 M sucrose in PBS. Then the sucrose was diluted in 1-step (Groups 1 and 2) or 4-steps (Group 3). Embryos (n=21) were cultured for 120 h in TCM199 supplemented with 10% fetal bovine serum at 37 degrees C in 5% CO(2) in air and evaluated morphologically. Development to the hatching or hatched blastocyst stage was obtained in 0 7 , 4 7 and 4 7 embryos in Groups 1, 2 and 3, respectively. An additional 7 embryos were vitrified-warmed according to the treatment of Group 2 (4 embryos) and Group 3 (3 embryos). Five embryos were selected after in vitro culture for 4 h and were transferred nonsurgically into the uterine horn of Day-4 recipient mares. Transfer of 2 embryos (both Day-6 blastocysts: Group-2 treatment) resulted in pregnancies with a viable fetus at Day-60 of the gestation period.     

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.     

Effect of culture system on the yield and quality of bovine blastocysts as assessed by survival after vitrification

The aim of this study was to assess the effect of a bovine in vitro culture system on blastocyst yield and quality after vitrification. In Experiment 1, IVM/IVF zygotes were cultured in either synthetic oviduct fluid (SOF) in 5% CO2, 5% O2, 90% N2; or TCM199-granulosa cells (TCM199-GCM) in 5% CO2 in air. In vivo blastocysts were used as a control. Culture in SOF resulted in a significantly higher blastocyst yield on both Day 7 (31.3 vs 13.2%, P < 0.001) and 8 (36.8 vs 23.7%, P < 0.001) than did culture in TCM199-GCM. After vitrification, survival at 72 h of in vivo blastocysts was significantly higher than both in vitro groups, while significantly more blastocysts produced in TCM199-GCM survived compared to those produced in SOF (0, 43.5, 78.3% for SOF, TCM199-GCM and in vivo, respectively P < 0.01). In Experiment 2, SOF-GCM proved to be the best post-warming culture system of those tested and was adopted as the post-warming medium for all subsequent experiments. In Experiment 3, zygotes were cultured in SOF or SOF-GCM, in either 5% CO2 in air, or 5% CO2, 5% O2, 90% N2. In agreement with Experiment 1, culture in SOF in 5% O2 resulted in significantly more blastocysts at Day 7 (26.4 vs 17.3%, P < 0.01) and Day 8 (31.5 vs 23.2%, P < 0.01) than did culture in SOF-GCM. However, survival at 72 h post vitrification was significantly higher for SOF-GCM (44 vs 8.3%, P < 0.001). Increasing the O2 concentration to 20% significantly reduced the blastocyst eld from SOF (31.5 vs 17.3%, P < 0.001). In addition, the quality of blastocyst produced was reduced in terms of survival post vitrification (8.3 vs 0%, P < 0.05). In contrast, there was no difference in blastocyst yield (23.2 vs 25.2%) or survival (44.0 vs 36.9%) in SOF-GCM, irrespective of O2 concentration. Experiment 4 examined the duration of exposure to GCM necessary to acquire improved blastocyst quality. Zygotes were cultured in SOF; SOF until Day 3, followed by SOF-GCM for the remainder of the culture; SOF until Day 5, followed by SOF-GCM for the remainder of the culture; or SOF-GCM for the entire culture. Survival at 72 h post vitrification was significantly higher (P < 0.05) in Groups 2 (50.0%, 13/26) and 4 (55.3%, 26/47) than in Groups 1 (21.7%, 10/46) and 3 (10.8%, 4/37). In conclusion, culture system can affect blastocyst yield and quality and crytolerance is a useful indicator of blastocyst quality.     

Factors affecting in vivo viability of DNA-injected bovine blastocysts produced in vitro

In vitro matured and fertilized bovine ova were microinjected with pBL1, which consisted of the bovine beta-casein gene promoter, human lactoferrin cDNA and SV40 polyadenylation signal. Of the 2931 zygotes injected, 2505 (85.5%) survived 1 h after DNA injection and were cultured in 50-microl drops of CR1aa medium containing 3 mg/ml BSA under mineral oil at 39 degrees C, 5% CO2 in air. Cleaved (2- to 8-cell) embryos were selected at approximately 48 h after DNA injection and then cultured further in 50-microl drops of CR1aa medium supplemented with 10% (v/v) FBS. Blastocysts were classified into 4 quality grades and 3 developmental stages by morphological criteria. Then all but poor quality blastocysts were nonsurgically transferred to the uterus of heifers 7 to 8 d after natural estrus. Following transfer, the recipients were observed for signs of estrus, and pregnancy was confirmed by palpation per rectum at approximately 60 d of gestation. Although 72.0% (1804/2505 ) of the DNA-injected zygotes reached 2- to 8-cell stages only 5.2% (131/2505) developed to blastocysts. A total of 75 DNA-injected, in vitro cultured blastocysts were transferred to 59 recipients. When 2 blastocysts were transferred to a single recipient, only the better quality embryo was counted. The overall pregnancy rate was 30.5% (18/59 ) and reflected 1) an apparent correlation between the quality of embryos and the pregnancy rate. However, the difference was not statistically significant. 2) expanded blastocysts had a higher pregnancy rate (50.0%, 11/22 ) than early (13.3%, 2 15 ) or mid (22.7%, 5/22 ) blastocysts with a significant difference between expanded and early blastocysts (P < 0.05). 3) the pregnancy rate of DNA-injected blastocysts was higher when they were transferred at Day 7 (34.5%, 10/29 ) or 8 (36.8%, 7/19 ) than at Day 6 (9.0%, 1/11 ). The results indicate that the developmental stage of DNA-injected bovine embryos may be one of contributing factors in improving the pregnancy rate after transfer, although the effects of the quality and culture period of the embryos may not be inconsequential.