Detrimental effects of sodium during mouse oocyte cryopreservation

Cryopreservation of oocyte and ovarian tissue

Cryopreservation of reproductive cells (i.e., oocytes, spermatozoa) and tissues (i.e., ovarian and testicular tissue) is a developing technology that has tremendous implications for rapid advancement of biomedical research in general. Since the early 1980s, advances have been made in establishing optimal conditions for in vitro oocyte maturation, fertilization, and culture of resulting embryos. These in vitro systems have contributed significantly to the utilization of these cells and tissues after thawing and have made it possible to evaluate protocols designed to cryopreserve such biomaterials more effectively. Although cryopreservation of preimplantation embryos from various species including mouse, human, and farm animals has been successful, cryopreservation of oocytes from most mammalian species has been more challenging due to their extreme sensitivity to suboptimal conditions during the cryopreservation process. Cryopreservation on mouse oocytes have been well documented and have resulted in greater success than studies with other mammalian species. Ovarian tissue cryopreservation and transplantation techniques have recently received much scientific and public attention due to their great potential use in human infertility treatment, in safeguarding the reproductive potential of the endangered species, and in genome banking of genetically important lab animal strains. A review of past and current research in the field of oocyte and ovarian tissue cryopreservation and transplantation and discussion of possible strategies for oocyte and ovarian tissue banking are provided.

Vitrification of mouse oocytes using a nylon loop

Cryopreservation of mouse oocytes was improved by the use of ultra-rapid vitrification using a nylon loop of 0.5 mm diameter. Oocytes that were vitrified using the loop survived at high rates and were fertilized following a small hole being made in the zona pellucida (69.8%) and developed to the blastocyst stage in culture (67.4%) at similar rates to that of oocytes that were not cryopreserved. Blastocysts resulting from oocytes vitrified using the nylon loop had similar development of the inner cell mass and trophectoderm as blastocysts from non-cryopreserved oocytes. In contrast, oocytes that were cryopreserved using a slow-freezing protocol where most of the Na+ is replaced with choline had lower rates of fertilization (39.5%), reduced development to the blastocyst stage (25.7%), and blastocysts had reduced development of the inner cell mass. Blastocysts derived from oocytes that were vitrified with the nylon loop were able to implant (88.0%) and develop into fetuses (56.5%) at significantly higher rates compared to blastocysts derived from oocytes that were slow-frozen (52.4 and 26.2%, respectively). Vitrification of mouse oocytes using the nylon loop results in the retention of viability of the oocytes and subsequent embryos.

Use of fluorescence in situ hybridization to assess the chromosomal status of embryos obtained from cryopreserved oocytes

OBJECTIVE

To analyze the chromosomal status of human embryos obtained from frozen-thawed oocytes.

DESIGN

Fluorescence in situ hybridization analysis of embryos obtained after oocyte cryopreservation.

SETTING

Department of Obstetrics and Gynecology at the University of Perugia, Italy, and the Instituto Valenciano de Infertilidad, Spain.

PATIENT(S)

Oocyte donors (n = 43). Fertilization, development, and chromosomal status of the embryos were compared with a control group (n = 18) of patients undergoing preimplantation genetic diagnosis for sex chromosome-linked diseases.

INTERVENTION(S)

Collection of oocytes after conventional ovarian stimulation and cryopreservation using propanediol as the cryoprotectant and a slow freezing procedure. Microinjection of surviving metaphase II oocytes and evaluation of fertilization and embryo development up to blastocyst stage. Chromosomal analysis after embryo biopsy.

MAIN OUTCOME MEASURE(S)

Survival, fertilization, and blastocyst rates. Embryo chromosomal analysis employing specific probes for chromosomes 13,18,21, X and Y.

RESULT(S)

The overall survival rate was 59.4%. There was no difference between cryopreservation and control groups in fertilization rates (76.5% vs. 90.5%) or blastocyst development (29.6% vs. 35%). The percentage of blastocysts from the original number of cryopreserved oocytes was only 5.6%, comparable to the 5.9% obtained in the control group. The percentage of embryos with abnormal number of chromosomes in the cryopreservation group (28.6%) was comparable to the 26% observed in the controls.

CONCLUSION(S)

Fertilization and cleavage rates after oocyte freezing are acceptable. Survival is, however, still poor, leading to overall results that make the technique clinically inefficient. There is no increase in the rate of chromosomal abnormalities, indicating that the technique is, nevertheless, safe enough to be further explored and improved.

Mechanisms for managing cellular and homeostatic stress in vitro

The ability to maintain embryo development in culture depends upon the ability of the embryo to maintain cellular homeostasis. Disruptions in the ability to regulate cellular homeostasis such as pH, calcium levels and osmotic pressure result in perturbed development and a reduced ability to establish and maintain a pregnancy following transfer. Therefore, it is important that in vitro conditions are designed to minimize stress on the embryo and maximize the ability of the embryo to maintain cellular homeostasis. While embryos do exhibit a degree of plasticity and can adapt to their environment, this requires expenditures of extra energy which negatively impacts viability. Therefore, reducing stress by taking into account the physiology of the embryo is essential for the maintenance of developmental competence in vitro.

Piglets born after vitrification of embryos using the open pulled straw method

Morulae and unhatched blastocysts from Large White hyperprolific (LWh) and Meishan (MS) gilts were selected to test an ultrarapid open pulled straw (OPS) vitrification method with two media. The viability of vitrified/warmed embryos was estimated by the percentage of embryos that developed to the hatched blastocyst stage in vitro or by birth after transfer. In Experiment 1, two cryoprotectant dilution media were compared for cryopreservation of MS and LWh blastocysts: TCM was a standard Hepes-buffered TCM199 + 20% NBCS medium and PBS was a PBS + 20% NBCS medium. After a two-step equilibration in ethylene glycol, dimethyl sulfoxide, and sucrose, 2-5 blastocysts were loaded into OPS and plunged into liquid nitrogen. Embryos were warmed; a four-step dilution with decreasing concentrations of sucrose was applied. In PBS, LWh blastocysts (27%) had a lower viability in vitro than MS blastocysts (67%; P = 0.001). In TCM, no significant difference was observed between genotypes (41% for LWh and 43% for MS blastocysts) and both viability rates were lower than that of the control groups. In Experiment 2, morula-stage LWh and MS embryos were vitrified and warmed using PBS. The viability rate was low and did not differ between LWh (11%) and MS (14%). In Experiment 3, 200 MS and 200 LWh blastocysts were vitrified/warmed as described in Experiment 1 (PBS). In each of 20 MS recipients, 20 embryos were transferred. The farrowing rate was 55% and recipients farrowed four and five piglets (median) for MS and LWh blastocysts, respectively. The OPS method is therefore appropriate for cryopreservation of unhatched porcine blastocysts.

Cryopreservation of human germinal vesicle stage and in vitro matured M II oocytes: influence of cryopreservation media on the survival, fertilization, and early cleavage divisions

OBJECTIVE

To study the influence of low-sodium cryopreservation media (CPM) on the survival and development of frozen-thawed germinal vesicle (GV) stage and in vitro matured human oocytes.

DESIGN

Prospective experimental study.

SETTING

Academic hospital-based fertility center.

PATIENT(S)

Experimental groups: Oocytes cryopreserved at the GV (group A, n = 63 and group B, n = 64) or M II stage (group C, n = 62) with use of conventional (group A) or low-sodium CPM (groups B and C). Control groups: Sibling GV stage oocytes subjected to in vitro maturation (IVM; control group A, n = 64; control group B, n = 64).

INTERVENTION(S)

IVM, intracytoplasmic sperm injection and subsequent culture.

MAIN OUTCOME MEASURE(S)

Rates of survival, maturation, fertilization, and cleavage.

RESULT(S)

The postthaw survival was significantly lower in groups A (57.1%) and B (48.4%) compared to C (84.4%). In group A, maturation and cleavage rates were significantly lower, and fertilization rate was similar to controls (GVBD: 72.2% vs. 90.6%; progression to M II: 33.3% vs. 76.6%; cleavage: 42.9% vs. 88.2%; and fertilization: 58.3% vs. 69.4% in group A vs. control group A, respectively). There was no such difference in group B. In group C, despite a slight but significant lowering of the rate of 2 PN and an increase in that of 3 PN (2 PN: 47.4% vs. 70.2% and 3 PN: 15.8% vs. 3.2% in group C vs. total controls, respectively), embryonic cleavage per GV oocyte was significantly higher (25.8%) compared to group A (4.8%) but not to group B (15.6%). The rate of maturation and cleavage per surviving GV oocyte was significantly higher in group B than group A.

CONCLUSION(S)

Low-sodium-based CPM is beneficial for in vitro matured M II stage oocytes and is significantly better than the conventional sodium-based media for the GV stage oocytes.

Cryopreservation of mouse oocytes using a medium with low sodium content: effect of plunge temperature

The effect of various combinations of plunge temperature and thawing protocol on the survival and viability of mouse oocytes was examined. The oocytes were frozen either in a standard freezing medium (ETFM, embryo transfer freezing medium) or in a low-sodium, choline-based freezing medium (CJ2), with 1.5 M 1,2-propanediol and 0.1 M sucrose, and using a conventional slow cooling method. The criteria used to assess survival were morphological state after thawing (intact or lysed), ability to become fertilized, and ability to develop to the two-cell, morula, and blastocyst stage in vitro. Oocytes frozen in CJ2 and plunged into liquid nitrogen (LN(2)) from -10, -20, or -33 degrees C remained intact and developed to the blastocyst stage at significantly higher rates than oocytes frozen in ETFM. For oocytes plunged into LN(2) from -33 degrees C, very rapid thawing (10 s in 30 degrees C water) was more detrimental than rapid or slow thawing (holding in air at room temperature for 10 or 30 s, respectively, prior to submersion in water at 30 degrees C for 10 s). By contrast, oocytes plunged into LN(2) from -10 or -20 degrees C survived better when thawing was very rapid or rapid than when thawing was slow. With the current protocol CJ2 was very effective over a wide range of plunge temperatures (-20 to -33 degrees C), although the optimal thawing protocol depended on the particular plunge temperature. Over 90% of oocytes surviving after slow cooling in CJ2 to -33 degrees C could be plunged to -196 degrees C with little or no further damage.

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.

Cryopreservation of unfertilized mouse oocytes: the effect of replacing sodium with choline in the freezing medium

Although embryo cryopreservation has become commonplace in many species, effective methods are not available for routine freezing of unfertilized eggs. Cryopreservation-induced damage may be caused by the high concentration of sodium ions in conventional freezing media. This study investigates the effect of a newly developed low-sodium choline-based medium (CJ2) on the ability of unfertilized, metaphase II mouse eggs to survive cryopreservation and develop to the blastocyst stage in vitro. Specifically, the effects of cooling to subzero temperatures, thawing rate, LN2 plunge temperature, and equilibration with a low-sodium medium prior to freezing are examined. In contrast to cooling to 23, 0, or -7.0 degreesC in a sodium-based freezing medium (ETFM), cooling in CJ2 had no significant negative effect on oocyte survival or development. Oocytes frozen in CJ2 survived plunging into LN2 from -10, -20, or -33 degreesC at significantly higher rates than oocytes frozen in ETFM. With the protocol used (1.5 M PrOH, 0.1 M sucrose, -0.3 C/min, plunging at -33 degreesC) rapid thawing by direct submersion in 30 degreesC water was more detrimental to oocyte survival than holding in air for 30 or 120 s prior to transfer to water. Equilibration of unfertilized oocytes with a low-sodium medium prior to cryopreservation in CJ2 significantly increased survival and blastocyst development. These results demonstrate that the high concentration of sodium in conventional freezing media is detrimental to oocyte cryopreservation and show that choline is a promising replacement. Reducing the sodium content of the freezing medium to a very low level or eliminating sodium altogether may allow oocytes and other cells to be frozen more effectively.