Cryopreservation of embryos and ova

Does the developmental stage at freeze impact on clinical results post-thaw?

The value of cryopreserving prezygotes, pre-embryos or blastocysts for future thaw and transfer is an important consideration of every IVF program. The convergence of two factors, a higher pregnancy rate and a lower multiple gestation rate, can be managed effectively through the establishment of a successful cryopreservation programme. In this article, freezing and thawing results from pronuclear oocytes, pre-embryos, and blastocysts are compared.

Permeability characteristics and osmotic sensitivity of rhesus monkey (Macaca mulatta) oocytes

BACKGROUND

Permeability characteristics and sensitivity to osmotic shock are principal parameters that are important to derive procedures for the successful cryopreservation of mammalian oocytes.

METHODS AND RESULTS

The osmotically inactive volume of rhesus monkey oocytes was determined by measuring their volumes in the presence of hypertonic solutions of sucrose from 0.2 to 1.5 mol/l, compared with their volume in isotonic TALP-HEPES solution. Boyle-van't Hoff plots at infinite osmolality indicated that the non-osmotic volumes of immature and mature oocytes were 20 and 17% respectively. Osmotic responses of oocytes exposed to 1.0 mol/l solutions of glycerol, dimethylsulphoxide (DMSO) and ethylene glycol (EG) were determined. Rhesus monkey oocytes appeared to be less permeable to glycerol than to DMSO or to EG. Sensitivity of oocytes to osmotic shock was determined by exposing them to various solutions of EG (0.1 to 5.0 mol/l) and then abruptly diluting them into isotonic medium. Morphological survival, as measured by membrane integrity, of oocytes diluted out of EG depended significantly on the concentration of EG (P < 0.01).

CONCLUSION

Determination of permeability characteristics and sensitivity to osmotic shock of rhesus oocytes will aid in the derivation of procedures for their cryopreservation.

Fetal development of mouse oocytes and zygotes cryopreserved in a nonconventional freezing medium

This study (1) analyzed fetal development of mouse embryos after oocyte cryopreservation in CJ2, a choline-based medium, (2) examined the effect of culture duration in vitro on subsequent fetal development, and (3) compared survival and fetal development of zygotes frozen in embryo transfer freeze medium (ETFM; sodium-based medium) or CJ2. Unfertilized oocytes and zygotes were cryopreserved using a slow-cooling protocol. After thawing, oocytes were inseminated after drilling a hole in their zona, cultured in vitro either to the two-cell or blastocyst stage, and transferred to the oviducts or uterine horns of recipient mice. In parallel experiments, frozen-thawed zygotes were similarly cultured and transferred. Implantation rates for transferred embryos were high (range 66-88%), regardless of whether they had been frozen as oocytes or zygotes and whether they had been transferred to the oviduct or uterus. However, fetal development was significantly higher when two-cell embryos were transferred. With blastocyst transfer, control embryos implanted and produced a greater proportion of fetuses than did oocytes frozen in CJ2, whereas transfer at the two-cell stage resulted in similar proportions of implantation sites and fetuses. Blastocyst transfer of zygotes cryopreserved in ETFM or CJ2 produced similar fetal development rates (23.6% vs 20.0%), but when frozen-thawed zygotes were transferred at the two-cell stage the fetal development rates were higher in the ETFM group (53.3%) than in the CJ2 group (32.0%). A high proportion (46.7%) of oocytes frozen in CJ2 in a nonprogrammable freezer and plunged at -20 degrees C developed into live offspring. This study shows that in the mouse (1) oocytes frozen in CJ2 can develop into viable fetuses, (2) prolonging culture in vitro has a detrimental effect on embryo transfer outcome, and (3) CJ2 offers no advantage for zygote cryopreservation.

Beneficial effect of microinjected trehalose on the cryosurvival of human oocytes

OBJECTIVE

To determine the effectiveness of trehalose as an intracellular cryoprotectant for the cryopreservation of human oocytes.

DESIGN

In vitro comparative study.

SETTING

Clinical and academic research environment at a medical school teaching hospital.

PATIENT(S)

Women undergoing in vitro fertilization (IVF).

INTERVENTION(S)

Discarded human oocytes, obtained from IVF patients, were randomly distributed into three groups: control group (no trehalose), extracellular trehalose group (0.5 M extracellular trehalose), and intracellular trehalose group (0.15 M intra- and 0.5 M extracellular trehalose). Trehalose was introduced into oocytes by microinjection. The oocytes in each group were cooled to different temperatures (i.e., -15 degrees C, -30 degrees C, and -60 degrees C) at rate of 1 degrees C/minute and thawed at ambient air temperature. Survival was examined after overnight culture.

MAIN OUTCOME MEASURE(S)

Survival of human oocytes cryopreserved in the presence and absence of trehalose.

RESULT(S)

The majority of oocytes in the intracellular trehalose group survived cooling to -15 degrees C (63%), -30 degrees C (53%), and -60 degrees C (66%). In contrast, only a small number of oocytes in both the control (13%) and extracellular trehalose group (22%) survived cooling to -15 degrees C, while all oocytes degenerated when cooled to -30 degrees C and -60 degrees C.

CONCLUSION(S)

Small amounts of intracellular trehalose in the absence of any other cryoprotectant provide a significant protection against freeze-associated stresses. Our results suggest that sugars such as trehalose should be considered as intracellular cryoprotectants for cryopreservation of human oocytes.

Starfish oocytes form intracellular ice at unusually high temperatures

Starfish oocytes, eggs, and embryos are popular models for studying meiotic maturation, fertilization, and embryonic development. Their large (170- to 200-microm) oocytes are obtainable in copious amounts and are amenable to manipulations that mammalian oocytes are not. The most formidable obstacle to working with marine oocytes is their seasonal availability, yet a successful means of preserving them for use during the nonreproductive season has not been reported. The aim of this study was to investigate the response of starfish oocytes to freezing with rapid and slow cooling rates under a variety of conditions to develop a cryopreservation protocol for these cells. Cryomicroscopic observation revealed that starfish oocytes in isotonic medium undergo intracellular ice formation (IIF) at very high subzero temperatures, such that the mean difference between the temperature of extracellular ice formation (T(EIF)) and IIF (TI(IF)) was less than 3 degrees C and the average T(IIF) was approximately between -4 and -6 degrees C. Neither partial cellular dehydration nor addition of the cryopreservative dimethyl sulfoxide significantly depressed the T(IIF). Under some conditions, we observed ice nucleation at multiple locations within the cytoplasm, suggesting that several factors contribute to the unusually high T(IIF) during controlled-rate freezing and thus vitrification may be a more suitable method for cryopreserving these cells.

Dimethyl sulfoxide suppresses NMDA- and AMPA-induced ion currents and calcium influx and protects against excitotoxic death in hippocampal neurons

Dimethyl sulfoxide (DMSO) is widely used in neuroscience research as a solvent for various pharmacological agents in both cell culture and in vivo studies and is also used in humans to treat musculoskeletal problems and pain. We now report that concentrations of DMSO to which neurons are typically exposed in experimental studies and in human patients (0.5-1.5%) inhibit glutamate responses in hippocampal neurons. DMSO suppresses, in a rapidly reversible manner, electrophysiological responses and calcium influx induced by glutamate, N-methyl-d-aspartate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate. Moreover, DMSO can prevent excitotoxic death of the neurons. These findings have important implications for the use of DMSO as a solvent in studies that involve glutamatergic neurotransmission. Our data also identify a mechanism that might explain clinical effects of DMSO on both peripheral and CNS neurons and suggest a potential use for DMSO in the treatment of excitotoxic neurodegenerative conditions.

A theoretical model of intracellular devitrification

Devitrification of the intracellular solution can cause significant damage during warming of cells cryopreserved by freezing or vitrification. Whereas previous theoretical investigations of devitrification have not considered the effect of cell dehydration on intracellular ice formation, a new model which couples membrane-limited water transport equations, classical nucleation theory, and diffusion-limited crystal growth theory is presented. The model was used to explore the role of cell dehydration in devitrification of human keratinocytes frozen in the presence of glycerol. Numerical simulations demonstrated that water transport during cooling affects subsequent intracellular ice formation during warming, correctly predicting observations that critical warming rate increases with increasing cooling rate. However, for cells with a membrane transport activation energy less than approximately 50 kJ/mol, devitrification was also affected by cell dehydration during warming, leading to a reversal of the relationship between cooling rate and critical warming rate. Thus, for low warming rates (less than 10 degrees C/min for keratinocytes), the size and total volume fraction of intracellular ice crystals forming during warming decreased with decreasing warming rate, and the critical warming rate decreased with increasing cooling rate. The effects of water transport on the kinetics of intracellular nucleation and crystal growth were elucidated by comparison of simulations of cell warming with simulations of devitrification in H(2)O-NaCl-glycerol droplets of constant size and composition. These studies showed that the rate of intracellular nucleation was less sensitive to cell dehydration than was the crystal growth rate. The theoretical methods presented may be of use for the design and optimization of freeze-thaw protocols.

Effects of chilling to 0 degrees C on the morphology of meiotic spindles in human metaphase II oocytes

OBJECTIVE

To determine the effects of chilling to 0 degrees C on the meiotic spindle of human metaphase II oocytes, as observed by optical sectioning microscopy.

DESIGN

Laboratory study.

SETTING

Academic research laboratory in a medical school.

PATIENT(S)

Seventy-two women undergoing infertility treatment donated a total of 108 oocytes.

INTERVENTION(S)

Metaphase II oocytes were stripped of their cumulus cells, cooled directly to 0 degrees C, and held for periods of 1 to 10 minutes. They were then fixed at 37 degrees C, stained for immunofluorescence, and examined microscopically.

MAIN OUTCOME MEASURE(S)

Morphology of the meiotic spindle in chilled and control oocytes.

RESULT(S)

Microscopic evaluations of 46 chilled oocytes revealed various time-dependent changes in microtubules compared to 9 control oocytes. After 1 minute at 0 degrees C, spindle damage was negligible, but in oocytes cooled for 2 or 3 minutes, there was obvious shortening of the spindle and loss of polarity. Cooling to 0 degrees C for 4 to 9 minutes resulted in increasingly more drastic changes; by 10 minutes the spindles had totally disappeared. Despite depolymerization of microtubular tubulin at 0 degrees C, the chromosomes did not become dispersed, but remained anchored even in the absence of spindles.

CONCLUSION(S)

Even brief exposure of human oocytes to temperatures near 0 degrees C causes profound alterations of the meiotic spindle.

Human oocyte cryopreservation: new perspectives regarding oocyte survival

The success of human oocyte cryopreservation depends on morphological and biophysical factors that could influence oocyte survival after thawing. Various attempts to cryopreserve human oocytes have been performed with contrasting results. Therefore the effect of some factors, such as the presence or absence of the cumulus oophorus, the sucrose concentration in the freezing solution and the exposure time to cryoprotectants, on human oocyte survival after thawing were investigated. The oocytes were cryopreserved in 1,2-propanediol added with sucrose, using a slow-freezing-rapid-thawing programme. After thawing, the oocytes were inseminated by intracytoplasmic sperm injection (ICSI) and the outcomes of insemination and subsequent embryo development were also recorded. The post-thaw cryosurvival rate was not different for the oocytes cryopreserved with their cumuli partially removed mechanically (56%) when compared with those cryopreserved with their cumuli totally removed enzymatically (53%). On the contrary, a significantly higher survival rate was obtained when the oocytes were cryopreserved in the presence of a doubled sucrose concentration (0.2 mol/l) in the freezing solution and the survival rate was even higher when the sucrose concentration was tripled (0.3 mol/l) (60 versus 82% P < 0.001). Furthermore, a longer exposure time (from 10.5 to 15 min) to cryoprotectants, before lowering the temperature, significantly increased the oocyte survival rate (P < 0.005). Intracytoplasmic sperm injection produced a good fertilization rate (57%) of thawed oocytes and a high embryo cleavage rate (91%) and a satisfactory embryo morphology was observed (14 and 34% for grade I and grade II embryos respectively).

Technical aspects of oocyte cryopreservation

Since the successful development in the mouse, the oocyte cryopreservation has been applied with varying success to a number of different species including the human. The recently reported successes in terms of pregnancies obtained by human oocyte cryopreservation are encouraging. Several studies typically reported different rates of survival (20-80%), fertilization (30-60%) and cleavage (32-100%). This variability of results throws some doubts on the usefulness of oocyte cryopreservation in IVF treatment cycles. It remains to be determined whether the relatively different success rates reported in literature, mainly in terms of survival rate, are due to methodological differences. We tried to investigate the effect of some factors on the oocyte survival rate after thawing: the presence or absence of cumulus oophorus and the exposure time of the oocytes to cryoprotectant. We suggest that a combination of several factors including both morphological and biophisical ones can affect the oocyte survival rate.

Cryopreservation of rhesus macaque (Macaca mulatta) spermatozoa and their functional assessment by in vitro fertilization

Although spermatozoa from several species of nonhuman primates have been cryopreserved, there has been no report of success with rhesus macaque spermatozoa as judged by functional assays. Two Tris--egg yolk freezing media, TEST and TTE, which have been successfully used for cynomolgus macaque (Macaca fascicularis) spermatozoa, were compared for cryopreservation of spermatozoa from four rhesus macaques (Macaca mulatta). The postthaw motility (percentage and duration) of spermatozoa cryopreserved in TTE was much higher than that for spermatozoa cryopreserved in TEST. The function of sperm cryopreserved in TTE was evaluated by in vitro fertilization of oocytes collected from gonadotropin-stimulated prepubertal rhesus macaques. Of the inseminated oocytes, 82 +/- 13% were fertilized and 63 +/- 22 and 39 +/- 21% of the resulting zygotes developed into morulae and blastocysts, respectively. These results indicate that rhesus macaque spermatozoa can be effectively cryopreserved in TTE medium. This finding will facilitate the application of in vivo and in vitro assisted reproductive technologies in this species.

Fundamental cryobiology of mammalian oocytes and ovarian tissue

Embryo cryopreservation is a widely used and relatively well-established procedure. By contrast, ovarian tissue and unfertilized oocytes are only rarely cryopreserved, even though for germ line storage these often would be preferable to embryo cryopreservation. There are many reasons for this discrepancy. Unfertilized mature (MII) stage oocytes are more difficult to cryopreserve than cleavage stage embryos of the same species. Many factors contribute to this including the oocyte's surface to volume ratio, single membrane, temperature-sensitive metaphase spindle and zona, and its susceptibility to parthenogenetic activation and chill-injury. A completely different set of problems applies to primordial follicles. Oocytes in primordial follicles are very small and tolerate cryopreservation by slow cooling very well. The problem lies in the difficulty in producing mature oocytes from these primordial follicles. Better and/or more convenient cryopreservation procedures for both oocytes and ovarian tissue are being developed. This paper describes some of the advances in this area and outlines the relative merits and limitations of several currently available egg and ovarian tissue cryopreservation procedures.

Comparison of pregnancy outcome of pronuclear- and multicellular-stage frozen-thawed embryo transfers

PURPOSE

Our purpose was to determine if supernumerary embryos generated by in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) should be frozen (using 1,2-propanediol) at the pronuclear or multicellular stage.

METHODS

The study was a retrospective analysis conducted at the Dubai Gynaecology & Fertility Centre of the Department of Health & Medical Services, Dubai, U.A.E. One hundred forty-one women undergoing frozen-thawed embryo replacement cycles with IVF generated embryos and 84 women undergoing the same with ICSI generated embryos.

RESULTS

Supernumerary, IVF-generated embryos frozen at the multicellular stage had a significantly higher rate of survival on thawing (73.9%) than embryos frozen at the pronuclear stage (64.4%). The morphological grades of the embryos in the two groups were similar, but a significantly higher pregnancy rate was obtained with embryos frozen at the multicellular stage (22.8%) than with pronuclear-stage embryos (14.8%). Similarly, with ICSI-generated embryos, significantly higher survival was seen with multicellular-stage frozen embryos (74.8%) than pronuclear-stage embryos (64.4%). The morphological grades of the embryos and pregnancy outcomes of the two groups were similar.

CONCLUSIONS

Supernumerary embryos generated by IVF and ICSI should be frozen at the multicellular stage so as to allow selection of the best embryos for transfer and embryo freezing of only robust embryos.

Effects of cryopreservation procedures on the cytology and fertilization rate of in vitro-matured bovine oocytes

The survival and developmental capacity of bovine oocytes after cryopreservation are greatly impaired, possibly due to organelle damage caused by freezing procedures. Distributions of chromosomes, microtubules, and microfilaments in bovine oocytes matured in vitro were examined after cooling, ethylene glycol (EG) exposure, or freezing. Oocytes were incubated after treatment for 20 min or 1 or 3 h, fixed, and evaluated using specific fluorescent probes. Abnormal cytological features increased over control levels after cooling or EG exposure and rewarming. Changes observed in oocytes during prefreezing manipulations included chromosome dispersal and clumping, microtubule depolymerization and alteration of spindle structure, and formation of craters and discontinuity in cytoskeletal actin staining. Freezing also led to an increase in the occurrence of cytological abnormalities. Less than 31% of frozen-thawed oocytes contained a normal chromosome arrangement 3 h postthaw (versus 90% of controls). Only 7-14% of frozen-thawed oocytes had normal spindles (versus 59-71% of controls). Normal distribution of filamentous actin was observed in less than 30% of oocytes postthaw (versus 62-89% of controls). These results indicate that the steps in a conventional freezing procedure cause irreversible alterations in multiple cytological components of bovine oocytes, demonstrating the need for improved strategies for preventing cellular damage during cryopreservation procedures.

Freezing of oocytes

Storing oocytes is one of the challenges of assisted reproduction which may provide an alternative to embryo cryopreservation. Despite early disappointing results regarding survival, fertilization and cleavage rates, which led to only sporadic pregnancies in more than ten years, the recent introduction of technical modification greatly improved the clinical efficiency, with the birth of several healthy children.

Ovarian injury and modern options to preserve fertility in female cancer patients treated with high dose radio-chemotherapy for hemato-oncological neoplasias and other cancers

High dose chemotherapy and radiotherapy have radically increased long term survival of young cancer patients. Among the side effects of chemotherapy treatments are ovarian failure and infertility, which are of particularly great concern to young cancer patients. Recently, cryopreservation procedures such as in vitro fertilization and embryo storage, or ovarian tissue cryopreservation have been used to preserve fertility in patients subjected to cancer treatments. Knowledge of the risks and probabilities of ovarian failure as well as the risks of the cryopreservation procedures is crucial for patients and physicians in order to make informed choices that will best serve the patients interests. This article presents data of a prospective study that determines the risk of ovarian failure following exposure to chemotherapy as well as a review of related publications. Progressive, dose-related depletion of primordial follicles is noted on histology, explaining the risk of undergoing premature ovarian failure years after exposure to chemotherapy. The safety of ovarian tissue cryopreservation procedures with a new round biopter was evaluated, as well as the risk of malignant cell transmission. It has been shown that laproscopic ovarian biopsy performed with the round biopter is a safe and efficient method for collection of ovarian tissue in cancer patients. In Hodgkin's disease patients' ovarian cortical tissue obtained for cryopreservation does not contain malignant cells. However the risk of cryopreserving and transferring malignant cells should be tested separately for each disease according to the risk of ovarian metastasis and the ability to detect single malignant cells.

Comparison of glycerol, other polyols, trehalose, and raffinose to provide a defined cryoprotectant medium for mouse sperm cryopreservation

Most procedures for mouse sperm cryopreservation have utilized raffinose to provide hypertonicity for cell desiccation prior to freezing and glycerol to block intracellular ice formation. Trehalose has been shown in other cell systems to provide positive protection to the plasma membrane and so was examined as a replacement for raffinose. Comparison of 3 and 6% glycerol and 7.5 and 20% sugar showed that 6% glycerol and 7.5% sugar gave maximal protection consistently and so were adopted as standard. Comparison of raffinose and trehalose at this concentration showed trehalose to give significantly better recovery of intact cells: 48 +/- 6% for trehalose, 36 +/- 9% for raffinose (+/- SE, n = 5; arc sine transformed data; P < 0.01). Less hydrophilic polyols should prove more permeant to the membrane than glycerol, enter the cell rapidly, and so possibly inhibit lethal intracellular ice formation effectively. We hypothesized that one of these polyols plus glycerol would be a more effective cryoprotectant than glycerol alone. The polyols tested as supplements to 6% glycerol were propane-1,2-diol, propane-1,3-diol, 1,1,1-tris-(hydroxymethyl)ethane (THME), and 2-ethyl-2-(hydroxymethyl)-propane-1,3-diol (EHMP). With 6% glycerol and 7.5% raffinose or trehalose, the two diols and THME gave less cryoprotection than with glycerol alone, and EHMP reduced postthaw membrane integrity to nil, thus invalidating the hypothesis. Comparison of bicarbonate-containing medium MJB to bicarbonate-free medium NTP, both with 6% glycerol/7.5% trehalose, showed no difference in recovery of membrane-intact cells. For ease of pH maintenance, NTP was chosen for studies of addition prefreeze and removal postthaw of 6% glycerol/7.5% trehalose cryoprotectant with in vitro fertilization as endpoint. Three protocols for cryoprotectant handling were tested: serial addition/dilution; dialysis addition and removal; and dialysis addition and direct insemination without cryoprotectant removal. The last proved significantly superior (P < 0.01), giving 62% fertilized eggs, normalized to controls, compared to 21% for dialysis addition and removal and 32% for serial addition and dilution. The glycerol/trehalose combination thus provides a defined cryoprotectant which, when used with addition by dialysis prefreeze and direct insemination postthaw, yields a satisfactory yield of fertilized eggs in an in vitro fertilization system.

Detrimental effects of sodium during mouse oocyte cryopreservation

Cryopreservation is an established way of storing embryos, but effective methods are not available for freezing eggs. Most freezing damage is caused by high solute concentration (solution effects) and intracellular ice. Sodium salts are the major components of cryopreservation media, and the main contributor to the solution effects. The present experiments examine the effect of substituting choline for sodium as the major extracellular cation in the cryopreservation of mouse eggs. The effects of serum and various cryoprotectants were also examined. Survival, fertilization, and development were inversely related to the concentration of sodium in the freezing medium. Oocytes frozen in a choline-based medium had the highest (p < 0.001) survival and development rates. The absence of serum during thawing inhibited fertilization, whereas exposure to serum or opening the zona allowed fertilization to reach the control level. Dimethyl sulfoxide was as effective as 1,2 propanediol for obtaining high survival and fertilization rates. These results support the hypothesis that the high concentration of sodium in conventional freezing media is detrimental to cells and show that choline is a promising replacement for sodium. Reducing or eliminating sodium may allow oocytes and other cells to be frozen more efficiently.

Cryopreservation of kunming mouse oocytes using slow cooling, ultrarapid cooling and vitrification protocols

The cryopreservation of oocytes has been only marginally successful with any of the current protocols, including slow cooling, rapid cooling and vitrification. We wished to test the hypothesis that oocytes from a single mouse strain would freeze successfully by 1 of the 3 mentioned protocols. Unfertilized Kunming mouse oocytes obtained 14 h after PMSG/hCG administration were randomly assigned to be cryopreserved after slow cooling, ultra rapid cooling and vitrification. Oocytes were thawed by straws being placed into 37 degrees C water, and their morphological appearance and in vitro fertilization capability were compared with that of oocytes that had not undergone cryopreservation. Survival of oocytes was indicated by the absence of darkened ooplasm or by broken membranes or zona pellucida. Functional integrity was evaluated by the formation of a 2-cell embryo after IVF. Survival rate of slow cooled oocytes did not differ from that seen in vitrified oocytes (55.1 vs 65.9%) but was significantly lower in the rapidly cooled oocytes (24.2%; P < 0.01). The results of IVF of slow cooled and vitrified oocytes were similar to those of the control group (72 and 73 vs 77%; P > 0.05). It appears that Kunming mouse oocytes can be successfully cryopreserved using the slow cooling method with 1,2-propanediol and vitrification, which contains both permeating and nonpermeating cryoprotectants.

Temperature dependence of mature mouse oocyte membrane permeabilities in the presence of cryoprotectant

Knowledge of cell membrane permeability characteristics facilitates the design of cryopreservation protocols which minimize damage from osmotic stress and reduce the incidence of intracellular freezing. Such permeability characteristics can be determined for oocytes from volume measurements taken during exposure to cryoprotectant. Individual mouse oocytes were held using negative pressure applied to the zona pellucida by means of a micropipet. Each oocyte was perfused with 1 ml 1.5 mol liter-1 dimethyl sulfoxide (Me2SO) or propane-1,2-diol at 30, 23, or 10 degrees C. The osmotic response of each oocyte before, during, and after perfusion was recorded by videomicroscopy until equilibrium was reached. Mean cell diameter across three axes was used to calculate oocyte volume, assuming sphericity, and, using mathematical modeling, values for hydraulic conductivity (Lp) were found to be 0.64, 0.41, and 0.20 micron min-1 atm-1 in the presence of Me2SO and 0.53, 0.36 and 0.15 in the presence of propane-1,2-diol at 30, 23, and 10 degrees C, respectively. Cryoprotectant permeability (omega) was 0.37, 0.16, and 0.035 for Me2SO and 0.43, 0.24, and 0.04 for propane-1,2-diol, while the reflection coefficient was 0.98, 0.94, and 0.99 (Me2SO) and 0.76, 0.99, and 0.95 (propane-1,2-diol) all at 30, 23, and 10, respectively. The corresponding activation energies (Ea) were 11.65 and 12.23 kCal mol-1 for Lp and 23.52 and 22.48 kCal mol-1 for omega, in the presence of Me2SO and propane-1,2-diol, respectively. Values generated for Lp and associated Ea were similar to those found for mouse oocytes in the absence of cryoprotectant, while omega and its Ea were similar to those found for oocytes of other species.

Deleterious effect of equilibration temperature on the toxicity of propanediol during cryopreservation of mouse zygotes

PURPOSE

Our aim was to determine the effect of temperature and incubation time on the toxicity of propanediol on mouse zygotes.

METHOD

Zygotes were pooled and randomly allocated to one of the treatment groups. Zygotes were incubated in PBS or PBS containing 1.5M propanediol at 37 degrees C or room temperature (RT) for up to 30 min, washed, and cultured. Similarly, zygotes were incubated at 4 degrees C or RT for 30 min and either washed and cultured or frozen-thawed, washed, and cultured. Zygotes were examined at 24, 48, and 96 hr of culture and embryo quality was determined.

RESULTS

Exposure of zygotes to 1.5 M propanediol at 37 degrees C significantly impaired first cleavage and blastocyst formation compared to exposure at RT (P < 0.0007). Incubation of zygotes for more than 5 min at 37 degrees C significantly reduced embryo development (P < 0.00001). Exposure to propanediol at 4 degrees C results in similar embryo development before and after freeze-thawing compared to exposure at RT.

CONCLUSIONS

Propanediol is toxic to mouse zygotes in a temperature- and time-dependent fashion. Cryopreservation of zygotes after exposure at 4 degrees C appears to be no better than after exposure at RT.

Effect of chilling bovine oocytes on their developmental competence

Bovine oocytes are damaged when chilled to temperatures near 0 degree C. We have determined the temperatures at which this injury occurs, as well as its kinetics and the functional consequences for oocytes both at the germinal vesicle-stage (GV) and after in vitro maturation (IVM). Cooling GV oocytes had no effect on their nuclear maturation or fertilization. Compared to control oocytes held at 30 degrees C, the development of GV oocytes into blastocysts following maturation and fertilization was unaffected by cooling them to 20 degrees C for 30 min (blastocyst formation: 25% vs 26%, respectively), but development decreased after cooling them to 10 degrees C and 0 degree C (blastocyst: 6% and 1%, respectively). Cooling oocytes after maturation gave similar results, with no difference between controls and oocytes cooled to 20 degrees C (blastocyst: 25% and 26%, respectively). However, cooling them to 10 degrees C and 0 degree C did reduce development (blastocyst: 8% and 3%, respectively). Chilling oocytes to 0 degree C for 30 sec reduced their cleavage and blastocyst formation by > 40%; there was a high negative correlation between the length of exposure and subsequent survival, both for GV-stage and for IVM oocytes. The extreme sensitivity of both GV and IVM oocytes to chilling can explain the limited success obtained for cryopreservation of bovine oocytes by conventional slow-cooling procedures.

Effects of alcohols on murine preimplantation development: relationship to relative membrane disordering potency

During in vitro culture of murine preimplantation embryos, we have observed that exposure to 0.1% ethanol induces an immediate increase in intracellular calcium levels and subsequently accelerates embryogenesis. If the observed effects of ethanol on developing embryos is mediated by its membrane disordering potency, we hypothesized that the relative membrane disordering potencies of related alcohols would correspondingly effect embryonic intracellular calcium levels and developmental rates. Two-cell embryos were exposed to 0.1% ethanol or 0.05 to 1.0% (w/v) n-butanol, n-propanol, isopropanol, 1,2-propanediol, glycerol, or methanol for 24 hr at 37 degrees C, and development to the blastocyst stage was monitored after 5 days. n-Butanol, n-propanol, isopropanol, and methanol treatment caused a dose-dependent inhibition (p < 0.01) of development to the blastocyst stage, whereas 1,2-propanediol or glycerol neither accelerated nor inhibited development. In a second experiment, 8-cell morulae were treated with 1,2-propanediol or glycerol, and cavitation rates were examined. There was no significant difference from control embryos in the onset of cavitation or the blastocoel expansion rate of 1,2-propanediol- or glycerol-exposed embryos, whereas exposure to 0.1% ethanol accelerate cavitation (p > 0.05). In a third experiment, morulae were exposed to 0.1% or 1.0% of each alcohol and were monitored for changes in intracellular calcium levels using the fluorescent indicator, fluo-3-acetoxymethyl ester. There was an immediate increase in intracellular calcium levels when morulae were treated with 1.0% ethanol or n-butanol, but only ethanol induced an increase (p < 0.05) in the level of intracellular calcium at 0.1%. These data suggest that ethanol is unique in its ability to accelerate embryogenesis and that the membrane disordering potency of ethanol does not directly underlie its effects on intracellular calcium release and the acceleration of preimplantation development.

Clinical factors for successful cryopreserved-thawed embryo transfer

PURPOSE

To study how clinical factors such as embryo quality, cell stage of embryo at cryopreservation, and synchronization of developmental stages between embryo and endometrium at thawing affect the implantation rate after cryopreserved-thawed embryo transfer (ET), these factors were examined in 106 cryopreserved-thawed ETs including 204 embryos.

RESULTS

In 86 embryos graded as high quality before transfer by morphological evaluation, 31 implantations were successful, while the other, low-quality embryos did not implant at all. High-quality embryos received less cryoinjury during cryopreservation compared to the injuries sustained by embryos in moderate and poor quality. When cyopreservation was performed at the 1-, 2-, and 3-day cell stages, pregnancies were similiarly achieved among most of the embryos at all cell stages. At thawed ETs in natural ovulation cycles, there were some differences among the developmental stages between thawed embryo and endometrium that received the thawed transferred embryos. Although the transfer timelag ranged a day or more, asynchronism of endometrium growth to the cell stage did not reduce pregnancy rates.

CONCLUSIONS

These results indicated that embryo quality evaluated morphologically was the most important clinical factor for successful implantation of cryopreserved-thawed ET.

Developmental potential of frozen-thawed human blastocysts

PURPOSE

To examine the possibility of freezing human embryos at late cleaved stages (morula or blastocyst stage), we cryopreserved human embryos 5 days (day 5) or 6 days (day 6) after insemination and investigated their developmental potential after thawing.

MATERIALS AND METHODS

One hundred nineteen morphologically good-quality human embryos from 43 women undergoing in vitro fertilization treatment between 1991 and 1992 were frozen using dimethylsulfoxide as a cryoprotectant. The embryos were cryopreserved for 5 to 30 months. After thawing they were then cultured in vitro for 24 hr to investigate their developmental potential. Survival rates and developmental rates were morphologically assessed after 24 hr of in vitro culture.

RESULTS

Developmental rates were significantly (P < 0.01 or P < 0.05) lower than survival rates at every developmental stage. There was no difference in total survival rates between embryos frozen 5 days after insemination (78.2%; 54/69) and embryos frozen 6 days after insemination (70.0%; 35/54). However, the developmental rates after 24 hr of culture was significantly (P < 0.05) lower for embryos frozen 6 days after insemination (6.0%; 3/50) than for embryos frozen 5 days after insemination (18.8%; 13/69). Only two embryos developed into fetuses after transfer into the uterus (1.7%; 2/119).

CONCLUSIONS

From the results, the developmental potential of frozen-thawed human blastocysts was found to be significantly reduced, even though the blastocysts were of morphologically good quality. Longer in vitro exposure of embryos appears to reduce their developmental potential.

The protective effects of polymers in the cryopreservation of human and mouse zonae pellucidae and embryos

OBJECTIVES

To evaluate the occurrence of injury due to physical factors in embryo cryopreservation and the effect of the polymers dextran, polyvinylpyrrolidone (PVP), and Ficoll on this mechanical damage.

DESIGN

Damage to the zona pellucida (ZP) observed after cryopreservation was taken as indication of cryoinjury caused exclusively by physical factors. Human and mouse ZPs from oocytes remaining unfertilized after previous IVF attempts and mouse two-cell embryos were frozen in the presence of different polymers. After thawing, they were checked carefully for signs of physical damage (cracks). A possible toxicity of the use of the polymers in cryoprotection was evaluated by development to the blastocyst stage of mouse two-cell embryos that survived the freezing and thawing process.

RESULTS

Incidences of damaged ZPs in groups of human and mouse ZPs and two-cell embryos frozen without polymers were found to vary between 20% and 29%. The use of any of the tested polymers resulted in significantly lower incidences of damaged ZPs (0% to 15%). Damage to the ZP after freezing and thawing in mouse embryos was accompanied by low survival rates of the embryo itself. Of mouse embryos that survived the cryopreservation process, blastocyst formation was not significantly different in groups frozen without polymer (80%) or in the presence of either dextran (90%) or Ficoll (82%); however, embryos frozen in the presence of PVP showed low blastocyst formation (12%).

CONCLUSIONS

Polymers can protect embryos against cryoinjury by avoiding mechanical strain occurring during cryopreservation. Polyvinylpyrrolidine is toxic to mouse two-cell embryos when present during freezing and thawing.

The use of a liquid-phase controlled rate freezing system for the successful cryopreservation of mouse pre-embryos

OBJECTIVES

To investigate the ability of a liquid-phase, controlled-rate freezing machine to generate reproducible freezing gradients with a constant rate of change; temperature fluctuations and heat dissipation during seeding; to assess the viability of mouse pre-embryos exposed to the silicone liquid cooling phase and the rates of survival and viability of mouse pre-embryos cryopreserved using this system.

DESIGN

Freezing gradients were generated from the bath or a sample and compared for reproducibility and slope. Temperature fluctuations and gradients around the freezing chamber and the temperature rises and dissipation during seeding were measured. The toxicity of the silicone polymer freezing-phase was tested with mouse pronuclear pre-embryos. Different developmental stages of mouse pre-embryos were frozen and thawed and survival recorded in vitro and in vivo.

SETTING

Research Laboratories, Sinai Hospital of Baltimore, Baltimore, Maryland.

MAIN OUTCOME MEASURES

The reproducibility of freezing gradients; temperature fluctuations during seeding; and the in vitro and in vivo viability of mouse pre-embryos exposed to the silicone polymer or frozen and thawed.

RESULTS

The freezing gradients generated were constant and reproducible with a constant rate of change; no temperature differences were recorded around the freezing chamber; temperature changes at seeding are minimal and rapidly dispersed. The silicone polymer was nontoxic to mouse pre-embryos and mouse pre-embryos frozen with the system and subsequently thawed are viable both in vitro and in vivo.

CONCLUSIONS

This liquid-phase cryopreservation system is an attractive option for assisted reproductive technologies laboratories because liquid nitrogen is not required for operation, it is reliable, there are small temperature fluctuations during seeding, and it can be kept constantly running.

Nonequilibrium freezing of one-cell mouse embryos. Membrane integrity and developmental potential

A thermodynamic model was used to evaluate and optimize a rapid three-step nonequilibrium freezing protocol for one-cell mouse embryos in the absence of cryoprotectants (CPAs) that avoided lethal intracellular ice formation (IIF). Biophysical parameters of one-cell mouse embryos were determined at subzero temperatures using cryomicroscopic investigations (i.e., the water permeability of the plasma membrane, its temperature dependence, and the parameters for heterogeneous IIF). The parameters were then incorporated into the thermodynamic model, which predicted the likelihood of IIF. Model predictions showed that IIF could be prevented at a cooling rate of 120 degrees C/min when a 5-min holding period was inserted at -10 degrees C to assure cellular dehydration. This predicted freezing protocol, which avoided IIF in the absence of CPAs, was two orders of magnitude faster than conventional embryo cryopreservation cooling rates of between 0.5 and 1 degree C/min. At slow cooling rates, embryos predominantly follow the equilibrium phase diagram and do not undergo IIF, but mechanisms other than IIF (e.g., high electrolyte concentrations, mechanical effects, and others) cause cellular damage. We tested the predictions of our thermodynamic model using a programmable freezer and confirmed the theoretical predictions. The membrane integrity of one-cell mouse embryos, as assessed by fluorescein diacetate retention, was approximately 80% after freezing down to -45 degrees C by the rapid nonequilibrium protocol derived from our model. The fact that embryos could be rapidly frozen in the absence of CPAs without damage to the plasma membrane as assessed by fluorescein diacetate retention is a new and exciting finding. Further refinements of this protocol is necessary to retain the developmental competence of the embryos.

A self-programmable in vitro fertilization/gamete intrafallopian transfer patient database management system for MacIntosh computers

PURPOSE

Our purpose was to develop a data processing system for a large in Vitro Fertilization/Gamete Intrafallopian Transfer (IVF/GIFT) practice which would (1) require minimal data entry time, (2) be easy to operate, (3) be simple to construct (no knowledge of procedural language or programming necessary), and (4) quickly collate and reduce data.

RESULTS

A database management system was successfully constructed on an Apple MacIntosh computer which met the above criteria. The key elements of this database were its user-friendly features (MacIntosh-based system), adaptability (user was constantly able to update and revise the program as informational needs changed), and ability to perform complex searches and data analyses imposed by the individual operators.

CONCLUSIONS

The software and hardware described in this report were found to be highly effective in meeting the ever-changing administrative and clinical needs of our IVF/GIFT program.

The effects of cryopreservation on early development and chromosome constitution in Chinese hamster embryos

The effects of cryopreservation on early embryonic development were investigated in Chinese hamster embryos. Embryos were randomly divided into 3 groups, as follows: the control group, embryos which were simply cultured; the DMSO group, embryos which were exposed to dimethyl sulfoxide (DMSO) and then cultured; the cryopreservation group, embryos which were cryopreserved and then cultured. The percentages of embryos which developed into blastocysts after 40 hours of cultivation were high in all groups. However, there were significant differences in the mean number of blastomeres with lower values after an exposure to DMSO and cryopreservation (73.2 in the control group, 62.0 in the DMSO group, and 40.2 in the cryopreservation group). No significant differences in chromosome abnormality rate were evident and there was no distinct tendency for variation in karyotype among the 3 groups. These results indicate that DMSO adversely affects the division of blastomeres, and that cryopreservation with DMSO as a cryoprotectant might aggravate these adverse effects.

Vitrification of mouse oocytes using short cryoprotectant exposure: effects of varying exposure times on survival

The effects on oocyte viability of varying the duration of exposure to cryoprotectants before rapid cooling to -196 degrees C were examined, using the vitrification protocol of Nakagata. A very short exposure (15 sec) was found to be optimal, resulting in an overall rate of development from vitrified oocytes to hatching blastocysts of 31.8%. Very high rates of survival (77-89%) of oocytes exposed to the cryoprotectant media, but without the vitrification, together with extreme variability in results between straws in the vitrified groups, suggest that losses in viability during vitrification may result from ice damage during devitrification of the medium.

Cryopreservation of human embryos

This chapter describes general cryobiological principles and the different methods for cryopreservation of supernumerary human embryos obtained after several procedures for medically assisted procreation. Different factors that influence the performance of freezing and thawing of human embryos are reviewed: the data from international and national surveys of results, the cryopreservation procedures, the stages of embryonic development, the morphological appearance of the cryopreserved embryos, the numbers of embryos transferred, the storage times of embryos in liquid nitrogen, the ovarian stimulation protocols in the IVF cycle and the replacement cycles of cryopreserved embryos. Cryopreservation of embryos circumvents the difficult problem of synchrony between the ovarian cycles of donor and acceptor patients in an oocyte donation programme.

Unfixed tissue for electron immunocytochemistry: a simple preparation method for colloidal gold localization of sensitive epitopes using ethanediol dehydration

A quick, simple protocol is described for the preparation of tissue for electron immunocytochemistry without the use of fixatives or deleterious solvents. Fresh, normal human colon was rapidly dehydrated in ethanediol (ethylene glycol) then embedded directly in low-acid glycol methacrylate. Using both mono- and polyclonal antibodies, in conjunction with colloidal gold probes, a range of intra- and extracellular epitopes were localized; these epitopes included lysozyme, chromogranin, desmin and collagen IV. Overall, the tissue compared well with material fixed in glutaraldehyde, partially dehydrated and embedded in LR White acrylic resin. Ultrastructural detail was good and was further enhanced, without affecting probe density and epitope localization, by the addition of 1% tannic acid or 1% uranyl acetate to the dehydrant. The technique is applicable to a wide range of tissues, allowing excellent antigen retention which might prove useful for the immunolocalization of sensitive epitopes.

Plasma membrane water permeabilities of human oocytes: the temperature dependence of water movement in individual cells

Membrane water permeability values were measured in individual fresh human pre-ovulatory oocytes using real time microscopy in a microscope diffusion chamber. The cells were exposed to anisosmotic conditions, their volume responses measured, and from these data the Lp values were computed employing the Kedem-Katchalsky analyses of irreversible thermodynamics. Lp values were measured at four temperatures for each oocyte between 37 degrees C and 10 degrees C, and the temperature-related Arrhenius activation energy (Ea) calculated. It was apparent that individual oocytes exhibited a wide range of Lp values; at 37 degrees C Lp values ranged between 0.33 and 1.80 microns/atm/min. However, each oocyte exhibited the expected inverse linear correlation between Lp and temperature, with high linear correlations (R2 values between 0.73 and 0.96). A mean value for Ea of 8.61 +/- 5.11 Kcal/mol was computed. It is apparent that pre-ovulatory human oocytes express a range of biological diversity in terms of membrane water transport, and this fact needs to be considered when attempting to formulate cryopreservation protocols for storage of these oocytes.

Vitrification of mouse oocytes: improved rates of survival, fertilization, and development to blastocysts

Rall and Fahy's (1985) vitrification procedure for the cryopreservation of 8-cell embryos was applied to unfertilized mouse oocytes. Unchanged, this method resulted in a mean of 24.1% of vitrified oocytes fertilizing and developing to blastocysts in vitro. Exposure of oocytes to the cryoprotectant media, but without the vitrification, resulted in 30.8% developing to blastocysts. Modifications to the durations of and media used in the dilution and equilibration steps of the procedure produced a final protocol giving a mean of 55.4% of vitrified oocytes and 72.4% of nonvitrified VS1-exposed oocytes developing to blastocysts; 85.7% of control oocytes develop to blastocysts. Osmotically induced damage was found to be the most important cause of loss of viability in these methods. Cooling of oocytes to 5-8 degrees C during the procedure had no significant effect on their viability. No parthenogenetic activation of oocytes occurred as a result of exposure to the procedure.

Cryopreservation of postnatal rat retinal ganglion cells: persistence of voltage- and ligand-gated ionic currents

Established methods for cryopreservation of living cells were modified for freeze-storage of postnatal retinal ganglion cells from rat. Retinal cell suspensions containing fluorescently labeled ganglion cells were frozen after addition of 8% dimethyl sulfoxide and stored at -80 degrees C for up to 66 days. Viability of identified retinal ganglion cells was assessed by their ability to take up and cleave fluorescein diacetate to fluorescein. No significant difference was found in the number of living retinal ganglion cells when cells obtained from the same dissociation were counted before and after freezing (6.65 +/- 2.37 x 10(4) vs 7.05 +/- 3.67 x 10(4) retinal ganglion cells per ml, respectively; mean +/- S.D., n = 4). In culture following cryopreservation, the cells appeared morphologically normal, and developed neurites and growth cones similar to their freshly dissociated counterparts. Since very little is known about the electrophysiology and membrane properties of neurons after cryopreservation, we used the whole-cell configuration of the patch-clamp technique to study voltage- and ligand-gated conductances in cryopreserved retinal ganglion cells. The cryopreserved retinal ganglion cells studied under current-clamp maintained resting potentials of -60.9 +/- 6.6 mV (n = 10) and upon depolarization fired action potentials. During voltage-clamp in the whole-cell mode, depolarizing voltage steps activated Na(+)-(INa), Ca(2+)-(ICa), and K(+)-currents in all cells tested (n = 122). INa could be reversibly blocked by 1 microM tetrodotoxin added to the external solution. ICa was blocked by external 250 microM Cd2+ or 3 mM Co2+. In some cells, ICa consisted of both a transient and prolonged component. The outward K(+)-current consisted of Ca(2+)-dependent and -independent components. The Ca(2+)-insensitive portion of the K+ outward current was separated into four distinct components based upon pharmacological sensitivity and biophysical properties. In many cells, a rapidly inactivating current similar to the A-type K(+)-current (IA) observed in freshly cultured retinal ganglion cells was isolated by its greater sensitivity to 4-aminopyridine (5 mM) than to tetraethylammonium (20 mM). A tetraethylammonium-sensitive current with a more prolonged time course reminiscent of IK, the delayed rectifier, was also found. When the 4-aminopyridine- and tetraethylammonium-insensitive portions of the outward current were further analysed with voltage protocols, an additional slowly decaying potassium current became apparent. The inhibitory amino acids, GABA (20 microM) and glycine (100 microM), activated chloride-selective currents that were selectively blocked by bicuculline methiodide (10 microM) and strychnine (5 microM), respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Fresh versus cryopreserved cultured allografts for the treatment of chronic skin ulcers

Both fresh and cryopreserved cultured epithelial allografts were used to treat 20 patients with a total of 30 chronic ulcers. A profound stimulation of host epithelialization from the wound edges and epidermal appendages was observed. Our data suggest that allografts, prepared using a simple cryopreservation technique, retain their morphological and functional characteristics, and are as effective as fresh allografts in healing chronic ulceration.

Screening of conditions for rapid freezing of human oocytes: preliminary study toward their cryopreservation

One hundred and twenty-one freshly-collected human oocytes and 839 unfertilized human oocytes after insemination were cryopreserved by vitrification. The cryoprotectants used were dimethylsulfoxide (DMSO) and sucrose. Vital staining and morphological criteria were used to assess injuries to cells. Variation of the time exposure to DMSO and sucrose, and cryoprotectants concentrations, followed by extraction-dilution in sucrose without freezing made it possible to study chemical toxicities. Variation of cryoprotectant concentrations followed by immersion in liquid nitrogen, thawing, extraction, and dilution made it possible to choose optimal conditions for vitrification. The sucrose concentration upon extraction after freezing and thawing which was lower than that during soaking enhanced the oocyte survival rate as did the choice of duration and temperature of soaking. No parthenogenetical activation of these unfertilized ovum was observed. This study indicates that with a certain combination of DMSO and sucrose concentrations up to 80% of morphologically intact human oocytes can be recovered after rapid freezing and thawing.

Parthenogenetic activation of unfertilized mouse oocytes by exposure to 1,2-propanediol is influenced by temperature, oocyte age, and cumulus removal

Cumulus-intact and -denuded unfertilized oocytes from two mouse strains were exposed to 1.5 M ethanol (EtOH) or two cryoprotectant solutions, 1.5 M propanediol (PROH) or 1.5 M dimethylsulfoxide (DMSO), for 4.5 min at 27 degrees C, and the proportion of activating or degenerating oocytes studied. Exposure to DMSO did not significantly increase activation above that of oocytes not exposed to DMSO. Treatment of oocytes in PROH resulted in the activation of up to 87% of viable oocytes. This was significantly higher (P less than .01) than in control oocytes and comparable to the rate of activation after treatment with EtOH (59-96% activation). In solutions at 1 degree C, 47% of control oocytes were activated, which was not significantly different from the rate of activation in EtOH (36%) or PROH (50%) at 1 degree C. Following treatment with PROH, up to 87% of oocytes degenerated within a period of 6 h in vitro. The age of the oocytes (h post hCG) and the time of cumulus removal with the enzyme hyaluronidase, relative to the time of exposure to the chemicals, influenced the level of degeneration in most groups. Significantly fewer oocytes degenerated when cumulus cells were removed before treatment (0-31%) than when the cumulus was left intact throughout the treatment and 6 h culture period (10-87%). Exposure to PROH at 1 degree C reduced oocyte degeneration to 5%. We conclude that PROH causes significantly greater losses of oocytes as a result of parthenogenetic activation and degeneration than of exposure to DMSO.

Hamster oocyte penetration tests with oocytes frozen in propanediol: comparison with non-frozen oocytes

Hamster oocytes were frozen using a 1,2-propanediol-sucrose procedure, which resulted in over 90% survival. After thawing and zona removal the oocytes were compared with non-frozen oocytes in a zona-free hamster egg test employing spermatozoa from human semen donors and suspected infertility patients. Similar data were obtained, indicating that propanediol-sucrose frozen hamster eggs may be used in place of fresh eggs for convenience and to avoid scheduling problems.

The effect on fertilization of exposure of mouse oocytes to dimethyl sulfoxide: an optimal protocol

Mouse oocytes, with or without an intact cumulus mass, were exposed to various concentrations of dimethyl sulfoxide (DMSO) at different temperatures for different periods of time and using different protocols of DMSO addition and removal. The effect of these procedures on the chymotrypsin sensitivity of the zona pellucida and the fertilizability of the oocytes was then assessed. Some procedures were found to affect adversely both the zona pellucida and the cumulus mass, resulting in reductions in the fertilization rate. As a result of both these and previously reported experiments (1-3), an optimal schedule is proposed for the handling of mouse oocytes during cryopreservation, namely, to equilibrate cumulus-intact oocytes in 1.5 M DMSO precooled to 4 degrees C prior to freezing, to remove DMSO at 4 degrees C after thawing prior to restoring the oocytes to 37 degrees C, to loosen or remove the cumulus cells, and then to hold oocytes at 37 degrees C for at least 1 hr to allow recovery of the spindle prior to insemination.