Effects of cryoprotectants on fertilization and cleavage of bovine oocytes in vitro

Effects of cumulus cells, different cryoprotectants, various maturation stages and preincubation before insemination on developmental capacity of frozen-thawed bovine oocytes

To improve freezability of bovine follicular oocytes, it is necessary to minimize injury to the oocytes caused by freezing and the toxicity of cryoprotectants. The maturing ability of frozen-thawed follicular oocytes with or without cumulus complexes was tested. The proportion of frozen-thawed follicular oocytes reaching the metaphasc II (M II) stage after in vitro maturation of 24 h was significantly (P < 0.05) higher in cumulus oocyte complexes (COCs; 44%) than in denuded oocytes (30%). Oocytes were cultured for 0, 6, 12, 18 or 24 h then frozen-thawed with 1,2-propanediol (PROH) or dimethyl sulfoxide (DMSO), and cultured for 24, 18, 12, 6 or 0 h respectively. In PROH, 24:0 (67%) showed significantly (P < 0.05) higher maturation rate than 0:24 (38%), 6:18 (41%). In DMSO, 18:6 (72%) and 24:0 (61%) showed significantly (P < 0.05) higher maturation rate than 0:24 (30%), 6:18 (33%) and 12:12 (44%). In case of 18:6, DMSO (72%) showed significant (P < 0.05) higher maturation rate than PROH (52%), however in case of 0:24, 6:18, 12:12 and 24:0, there was no significant (P < 0.05) difference in the maturation rate between PROH and DMSO. The proportion of embryos developed to > or = 2 cell, > or = 8 cell, morula and blastocyst in 18:6 DMSO (35, 10, 3 and 0%) and 24:0 PROH (38, 12, 5 and 0%) was significantly (P < 0.05) lower than that of fresh oocytes (67, 38, 31 and 16%). There was no significant (P < 0.05) difference in the rate of embryos that developed to > or = 2 cells, > or = 8 cells, morulae and blastocysts between PROH and DMSO. When the frozen oocytes were grouped as rewarming culture (21:2 PROH) and control (24:0 PROH), there was no significant (P < 0.05) difference in the rate of embryos that developed to > or = 2 cells, > or = 8 cells, morulae and blastocysts between 24:0 PROH (42, 24, 11 and 1%) and 21:2 PROH (51, 29, 16 and 4%) but 21:2 PROH showed slightly higher developmental capacity than 24:0 PROH. Transferable blastocysts (4%) were obtained in 21:2 PROH when the frozen-thawed follicular oocytcs were fertilized and cultured for 8 to 9 d.

Blastocysts derived from in vitro-fertilized cat oocytes after vitrification and dilution with sucrose

Experiments were conducted to find an optimal incubation period in a sucrose solution during dilution of cryoprotectants for obtaining a higher level of survival and development of cat oocytes cryopreserved by vitrification method. In the first experiment, in vitro-matured fresh oocytes were exposed to 0.5M sucrose solution for 1 or 5 min before in vitro fertilization (IVF). The percentage of development to the blastocyst stage significantly decreased in oocytes exposed for 5 min, compared with oocytes exposed for 1 min and control oocytes without exposure to sucrose (P<0.05). In the second experiment, oocytes that had been vitrified in 40% ethylene glycol and 0.3M sucrose were liquefied and then incubated in 0.5M sucrose for 0.5, 1 or 5 min to dilute the cryoprotectant. The percentage of cleavage (>or=2-cell stage) of vitrified-liquefied oocytes incubated for 0.5 min was significantly higher (P<0.05) than that of other groups. Development of vitrified-liquefied oocytes to the morula and blastocyst stages after IVF was observed only in oocytes incubated in sucrose for 0.5 min. The present study indicates that the oocytes have sensitivity to the toxic effect of sucrose and that the incubation period during dilution of the cryoprotectant is of critical importance for developmental competence of vitrified-liquefied cat oocytes.

Effect of osmotic stress on the developmental competence of germinal vesicle and metaphase II stage bovine cumulus oocyte complexes and its relevance to cryopreservation

The effects of osmotic stress on germinal vesicle (GV) and metaphase II (MII) stage bovine cumulus oocyte complexes (COCs) were evaluated by first exposing them to various anisotonic NaCl solutions (75, 150, 600, 1200, 2400, and 4800 +/- 5 mOsm/kg) for 10 min and then returning them to isotonic TL-Hepes solution (270 +/- 5 mOsm/kg) at 20 +/- 2 degrees C. Percentages of oocyte maturation, fertilization, polyspermy, cleavage, and blastocyst formation were measured as endpoints. Exposure to anisotonic conditions had a significant (P < 0.05) effect on the developmental competence of both GV and bovine MII COCs. Oocytes at the GV stage were more sensitive to anisotonic stress than MII oocytes (P < 0.05). None of the GV oocytes developed to the blastocyst stage after exposure to hypertonic conditions (2400 or 4800 mOsm solutions), while exposure to hypotonic conditions (75 or 150 mOsm solutions) resulted in significantly lower (P < 0.05) blastocyst formation (9% and 13%, respectively) compared to the isotonic control (25%). A dramatic decrease to 4% development to blastocyst was observed for MII oocytes following exposure to a 4800 mOsm solution. Blastocyst formation of MII oocytes which were exposed to 75, 150, 600, 1200, or 2400 mOsm solutions were similar (15%, 20%, 18%, 14%, and 13%, respectively; P > 0.05), but lower (P < 0.05) than those in the control group (29%). Exposing GV oocytes to anisotonic conditions increased polyspermic fertilization (P < 0.05), although MII oocytes were not similarly affected (P > 0.05). These data support the hypothesis that osmotic stress is detrimental to bovine oocytes and must be considered when developing optimized cryopreservation procedures for these cells. Mol. Reprod. Dev. 55:212-219, 2000.

Potential use of embryo coculture with human in vitro fertilization procedures

PURPOSE

This review was designed to outline potential uses of an embryo co-culture system in human assisted reproduction programs to improve embryo quality and pregnancy rates.

RESULTS

The various cell types used in embryo co-culture were reviewed in addition to the use of co-culture for both animal and human embryos. Co-culture provides a method to enhance embryo development in an inadequate in vitro environment without compromising embryo quality. Human IVF laboratories have used various types of "helper cells" to improve rate of development, reduce cell fragmentation rate and in some instances increases pregnancy and implantation rates.

CONCLUSION

In conjunction with several assisted reproduction procedures such as IVF, microsurgical fertilization, cryopreservation and genetic evaluation, co-culture may increase the number of viable embryos for replacement and improve pregnancy rates.

Effects of different cryoprotectants and carbohydrates on freezing of matured and unmatured bovine oocytes

Cumulus cell-enclosed bovine oocytes in germinal vesicle (GV) and in metaphase II (MII) stages were cryopreserved. Different concentrations (1 M; 1.5 M) of various cryoprotectants (glycerol, PROH, DMSO) were tested. After thawing, the oocytes were exposed to various carbohydrates (sucrose, lactose, trehalose) at a concentration of 0.1 M and 0.25 M for cryoprotectant removal. Developmental capacity of the frozen-thawed oocytes was studied by in vitro maturation, fertilization and culture. We found no difference in subsequent development using glycerol or PROH for GV and MII oocytes. The DMSO treatment led to significantly better cleavage and development up to 4-cell stage in MII oocytes. Development beyond the 8-cell stage was obtained only when unmatured oocytes were frozen. No difference in the efficiency of the 3 cryoprotectants was detected in MII oocytes. However, in GV oocytes, glycerol and PROH yielded significantly better cleavage and 4-cell rate compared to DMSO (P<0.001). Influence of the concentration of a cryoprotectant on development was not observed in GV or MII oocytes. Among the 3 cryoprotectants, DMSO was less suitable, at both concentrations, than PROH and glycerol for the development of 6- to 8-cell stage embryos in the GV group. In the MII group, 1.5 M DMSO was as efficient as PROH and as glycerol at a 1.5-M concentration, and it was more efficient than 1 M glycerol. The use of carbohydrates during rehydration did not render a beneficial effect at either of the 2 concentrations, and when no carbohydrates were used in the MII group the oocytes cleaved better than GV oocytes.

Developmental capacity of bovine oocytes frozen in different cryoprotectants

Iso-osmolar concentrations of 1.6 M of 1,2-propanediol, glycerol and dimethylsulfoxide were compared for their effectiveness as cryoprotectants for in vitro-matured bovine oocytes. Survival, defined as the number of morphologically normal oocytes, after freeze-thawing in 1,2-propanediol (49.8%) was significantly higher (P<0.01) than in glycerol (28.6%) or dimethylsulfoxide (32.6%). After insemination, the fertilization rate of morphologically normal oocytes frozen-thawed in 1,2-propanediol (57.9%) was higher (P<0.05) than that of those frozen in dimethylsulfoxide (38.3%). However, the proportion of surviving oocytes developing to blastocysts was not different among the 3 groups.

Full-term development of in vitro-matured, vitrified and fertilized bovine oocytes

In vitro-matured bovine oocytes were vitrified in a mixture of 2 M-dimethyl sulphoxide (DMSO), 1 M-acetamide and 3 M-propylene glycol dissolved in mTCM199. After vitrification and thawing, the oocytes were exposed to 2-0.1M-sucrose solution in 1 or 12 steps to remove the cryoprotectants. Then the oocytes were fertilized in vitro and co-cultured with a monolayer of cumulus cells for 7 days. Nine of 88 inseminated oocytes developed to the blastocyst stage. Three blastocysts were transferred to 3 recipients, resulting in 2 pregnancies.

Developmental capacity of bovine oocytes cryopreserved after maturation in vitro and of frozen-thawed bovine embryos derived from frozen mature oocytes

The present study was conducted 1) to investigate the post-thaw developmental capacity of in vitro mature bovine oocytes (Metaphase II) frozen by 1.6 M of 1,2-propanediol and 2) to confirm the viability of frozen bovine embryos derived from frozen mature oocytes. The cleavage and developmental rates to the blastocyst stage of frozen-thawed mature oocytes were significantly lower (P<0.01) than that of nonfrozen oocytes. When mature oocytes were treated with hyaluronidase, trypsin, or base solution (solution control) before processing to remove the cumulus cells, the developmental rates to the blastocyst stage of frozen-thawed oocytes were 2.8% (5/180), 3.1% (9/295) and 1.1% (1/89), respectively. The viability and developmental capacity of frozen-thawed bovine embryos derived from frozen mature oocytes were not different from those of frozen-thawed bovine embryos derived from nonfrozen mature oocytes (control). Furthermore, nonfrozen and frozen-thawed embryos derived from frozen-thawed mature oocytes were nonsurgically transferred to recipient cows. One of the four and one of the two recipient cows became pregnant, respectively. The results of this study demonstrated the viability of embryos obtained from frozen-thawed bovine oocytes at Metaphase II followed by in vitro fertilization and culture to the blastocyst stage in vitro.