A novel approach to sperm cryopreservation

Human spermatozoa have unusual cryobiological behaviour and improvements in their survival have not been achieved by the standard approaches of cryobiology. Conventional approaches to cryopreservation impose a linear change of temperature with time; however, the stresses that cells encounter during cryopreservation are all non-linear with time. In this paper it is shown that improved methods of cryopreservation may be developed by specifically manipulating the manner in which cells experience physical changes instead of imposing a linear temperature reduction. Several treatments were compared: control of solidification to achieve constant ice formation with time was more damaging than the standard linear reduction in temperature. However, treatments which followed a chosen non-linear concentration profile, referred to as 'controlled concentration' allowed recovery of almost all the cells which were motile before freezing. The biophysical basis of these different responses was examined using the cryostage of a scanning electron microscope and freeze substitution and it was found that, surprisingly, all samples of spermatozoa in the frozen state were neither osmotically dehydrated nor had any visible intracellular ice. Viability on thawing did not appear to correlate with conventional theories of cellular freezing injury, which suggests that for human spermatozoa other factors determine viability following freezing and thawing.     

Membrane fluidity predicts the outcome of cryopreservation of human spermatozoa

Semen cryopreservation is an important procedure in the treatment of human infertility. However, the ability of spermatozoa to survive freeze/thaw processes varies between patients. Cryopreservation-induced stress may result in membrane injury with consequent loss of sperm motility and viability. We investigated the relationship between the physico-chemical state of the human sperm membranes and their tolerance to cryopreservation. Conventional characteristics of 20 semen samples were analysed before and after cryopreservation as well as their membrane fluidity assessed by measuring the fluorescence polarization anisotropy, which is inversely proportional to the fluidity. Correlation between fluidity and post-thaw recoveries of motile and viable spermatozoa were examined. Results showed that membrane anisotropy markedly varies between patients. In cryopreserved spermatozoa, anisotropy values were significantly higher than in fresh spermatozoa. Furthermore, recovery of motile and viable spermatozoa after freeze/thaw was strongly correlated with anisotropy of fresh spermatozoa (P < 0.05). The higher the membrane fluidity was before freezing, the better was the response of spermatozoa to cryopreservation. The results indicate that the freeze/thaw process results in a rigidifying effect on the sperm membrane and suggest that sperm adaptability to freeze/thaw-induced stress could be dependent on their initial membrane fluidity. The latter finding has practical implications for predicting the response of spermatozoa following freezing and thawing and for improving the recovery of viable spermatozoa.     

Cryopreservation of single human spermatozoa

A procedure is described that allows cryopreservation and efficient post-thaw recovery of either a single or a small group of human spermatozoa. This is achieved by injecting them into cell-free human, mouse or hamster zonae pellucidae before the addition of cryoprotectant. The method involves a combination of physical micromanipulation procedures and glycerol-mediated cryoprotection. Zonae were tracked by positioning them in straws between two small air bubbles prior to freezing. Spermatozoa from poor specimens were cryopreserved and their fertilizing ability after thawing was compared with that of fresh spermatozoa from fertile men. Human eggs used for fertilization testing were either 1 day old or in-vitro matured. Only 2% of the frozen zonae were lost and >75% of spermatozoa cryopreserved in this manner were recovered and prepared for intracytoplasmic sperm injection. The feasibility of cryopreserving a single spermatozoon was assessed. Fifteen motile spermatozoa were frozen in 15 zonae, of which 14 were recovered after thawing. Ten were injected into spare eggs, of which eight became fertilized. Spermatozoa recovered mechanically from human zonae fertilized the same proportion of oocytes as fresh fertile control spermatozoa. The recovery and fertilization rates with spermatozoa frozen in animal zonae were 87 and 78% respectively. The fertilization rate was marginally higher (P < 0.05) than that for spermatozoa frozen in human zonae, perhaps because the latter may have acrosome reacted more frequently. The zona pellucida appears to be an ideally suited sterile vehicle for storage of single spermatozoa.      

Changes in carnitine and acetylcarnitine in human semen during cryopreservation.

L-Carnitine and acetylcarnitine concentrations were determined in spermatozoa and seminal plasma from 15 men, in both fresh ejaculate and frozen-thawed semen with cryoprotective medium. Sperm motility was also evaluated. In fresh samples, the levels of carnitine and acetylcarnitine in seminal plasma were comparable whereas in spermatozoa, acetylcarnitine predominated. Cryopreservation did not change the carnitine and acetylcarnitine levels in seminal plasma nor the carnitine concentration in spermatozoa; by contrast, the acetylcarnitine level in spermatozoa was decreased in 14 cases (110 +/- 8 versus 210 +/- 20 nmol/10(8) cells). This decrease in acetylcarnitine content was greater during semen dilution in cryoprotectant than after the freezing/thawing process. Motility was also decreased in all cases after the freezing/thawing process. These results suggest that acetylcarnitine recovery in spermatozoa is further evidence of the deleterious effect of the cryoprotective medium in the cryopreservation of semen.     

Improvement in motion characteristics and acrosome status in cryopreserved human spermatozoa by swim-up processing before freezing

The purpose of this study was to examine if selecting a sperm population with improved motion characteristics before freezing reduces the deleterious effects of cryopreservation. Semen specimens from 15 normal donors were divided into two equal aliquots. The first aliquot received no treatment (control), and the second was processed by swim-up from a washed sperm preparation to select a sperm population with better motility and motion characteristics (swim-up). Both aliquots were cryopreserved by the liquid nitrogen vapour method. Percentage motility and motion characteristics were evaluated by computer-assisted semen analysis. Acrosome integrity as well as spontaneous and calcium ionophore-induced acrosome reactions before freezing and after thawing were assessed by fluorescein isothiocyanate conjugated peanut agglutinin combined with a supra vital dye (Hoechst-33258). Swim-up processing enabled selection of a sperm population with better motion characteristics, percentage motility and viability before freezing (P < 0.001), but with no difference in percentage of acrosome-intact spermatozoa (P = 0.63). After thawing, the swim-up specimens exhibited faster velocity and progression than untreated specimens (P < 0.001). They also had higher percentages of spermatozoa with intact acrosomes and spermatozoa able to undergo acrosome reaction in response to calcium ionophore (P < 0.05). Selecting a highly motile sperm population before freezing enhances overall post-thaw spermatozoa quality.     

An efficient method for the cryopreservation of fetal human liver hematopoeitic progenitor cells

The use of human hematopoietic progenitor cells (HPC) for transplantation requires efficient recovery methods and cryopreservation procedures. The purpose of this study was to determine cryopreservation techniques for fetal human liver (FHL) CD34(+) cells. We assessed FHL HPC recovery efficiency after freezing and thawing by viability testing, fluorescence-activated cell sorting analysis, and colony-forming ability under different conditions. We also determined optimal cell freezing concentrations and the effect of rate-controlled freezing on cell recovery. Lastly, cell recovery after varying freezing time periods was examined. Our results indicated that optimal cell recovery occurs when: A) cryopreservation medium consists of either 5% dimethylsulphoxide (DMSO) or 10% DMSO in combination with either 20% fetal bovine serum (FBS) or 70% FBS and when Iscove's modified Dulbecco's medium consists of not more than 10% DMSO; B) a rate-controlled freezing device container is used; C) CD34(+) cells are frozen at a concentration of 1 x 10(6)/ml, and D) a thawing temperature of 37 degrees C is used. These observations indicate that cryopreservation of FHL HPC is possible for up to 18 months in optimal conditions without losing hematopoietic activity.     

Andrology: Effect of freezing method, thawing temperature and post-thaw dilution/washing on motility (CASA) and morphology characteristics of high-quality human sperm

Sixteen semen samples, 12 donor and four patient samples ofhigh initial quality, were processed to compare the effect oftwo freezing methods, two thawing temperatures and the effectof dilution and washing on sperm motility and morphology characteristics.Sperm samples were divided in two equal parts and frozen eitherby fast vapour freezing or by slow computer-controlled freezing.For each freezing method, half of the straws were thawed atroom temperature (22°C), the other half were thawed at 37°C.From each freeze—thawing treatment, one straw was evaluatedimmediately post-thawing; another straw was washed to removethe cryoprotectant solution. In this way, each semen samplewas subjected to eight freeze—thawing treatments. No effectof the freezing method and thawing temperature was observedon motility characteristics evaluated by computer-assisted semenanalysis, nor on light-microscopical morphology parameters.Post-thaw dilution and washing, however, exerted a deleteriouseffect on sperm motility, by reducing percentage motility by50% compared to unwashed thawed specimens. Linearity and percentageof morphologically normal spermatozoa were obviously impaired,while percentage of abnormal tails and beat cross frequencyincreased significantly. In general, freeze-thawing was mostsuccessful when rapid vapour freezing was followed by 37°Cthawing, and when slower computer-controlled freezing was combinedwith 22°C thawing, causing significant interactions betweenthe freezing method and the thawing temperature. For semen samplesof high initial quality, vapour and computer-controlled freezingwere equally effective in terms of recovery of morphologicallynormal, motile spermatozoa.     

Assessment of DNA integrity and morphology of ejaculated spermatozoa from fertile and infertile men before and after cryopreservation

Cryopreservation of human spermatozoa is extensively used in artificial insemination and IVF programmes. Despite various advances in cryopreservation methodology, the recovery rate of functional post-thaw spermatozoa remains mediocre, with sperm motility being significantly decreased after freezing. This aim of this study was to investigate the effects of cryopreservation on both DNA integrity and morphology of spermatozoa from fertile and infertile men. Semen samples were obtained from 17 fertile and 40 infertile men. All samples were prepared by discontinuous Percoll density centrifugation (95.0:47.5). Samples were divided into aliquots to allow direct comparison of fresh and frozen spermatozoa from the same ejaculate. Aliquots for cryopreservation were mixed with a commercial cryoprotectant and frozen by static phase vapour cooling before plunging into liquid nitrogen. Thawing was carried out slowly at room temperature. Sperm DNA integrity was determined using a modified alkaline single cell gel electrophoresis (comet) assay and sperm morphology analysed using the Tygerberg criteria. DNA of semen and prepared spermatozoa from fertile men was found to be unaffected by cryopreservation. In marked contrast, spermatozoa from infertile men were significantly damaged by freeze-thawing. Cryopreservation had a detrimental effect on morphology of semen and prepared samples from fertile and infertile men.     

Cryopreservation of human spermatozoa with pentoxifylline improves the post-thaw agonist-induced acrosome reaction rate

Cryopreservation causes extensive damage to spermatozoa, thereby impairing their fertilizing ability. The purpose of this study was to determine if the direct addition of pentoxifylline to the seminal plasma before cryopreservation improved sperm motility and acrosome reaction. Semen specimens from 15 healthy volunteers were divided into two aliquots. One aliquot was treated by adding 5 mM pentoxifylline directly to the seminal plasma (treatment group) and the other aliquot received no treatment (control group). Both aliquots were then cryopreserved by using the liquid nitrogen freezing method. The percentage of motile spermatozoa and various motion characteristics were then evaluated by performing computer-assisted semen analysis. The sperm viability was determined with a supra-vital dye, Hoechst-33258, and the acrosome reaction (spontaneous and calcium ionophore-induced) was monitored using fluorescein isothiocyanate-conjugated peanut lectin (FITC-PNA) binding assays. Pentoxifylline treatment significantly increased the sperm motility, the amplitude of lateral head displacement, the hyperactivation status, and the frequency of spontaneous acrosome reactions before freezing (P < 0.05). After post- thaw, no difference in motion characteristics (except percentage motility) between treated and control groups were observed. Acrosome loss due to the freeze-thaw process was less in the pentoxifylline- treated group (P = 0.0003). In addition, the percentage of cryopreserved acrosome-intact spermatozoa that underwent further acrosome reactions in response to calcium-ionophore challenge was significantly higher in the treated group (P = 0.03). Pentoxifylline treatment before freezing improved the acrosome reaction to ionophore challenge in cryopreserved spermatozoa. Treatment with pentoxifylline appears to minimize sperm damage during the freeze-thaw process and may improve fertilization rates with assisted reproductive procedures such as intrauterine insemination or in-vitro fertilization.      

Light and electron microscopic analysis of human testicular spermatozoa and spermatids from frozen and thawed testicular biopsies.

The morphological changes caused by freezing and thawing human testicular spermatozoa have been assessed here. Retrieval of testicular biopsies was carried out on six patients with obstructive azoospermia preparatory to intracytoplasmic sperm injection (ICSI). Light microscope analysis was carried out on testicular cells and ultrastructural analysis was carried out on spermatozoa and different spermatid stages before and after the freezing procedure. Upon examination under light microscopy, all germ cells presented increased vacuolization in their cytoplasm and shrinkage or swelling of the nuclei and cytoplasmic membranes. These altered structures were accentuated in the spermatocyte I cell which often presented disrupted membranes. The ultrastructural findings under transmission electron microscopy demonstrated that after freezing and thawing the major types of cryoinjury were the swelling and rupture of inner and outer acrosomal and plasma membranes. The acrosome material often appeared as dispersed material or as condensed spots or was even lost. Such damage was observed mainly at the spermatozoa and late spermatid stages. We conclude that the freezing and thawing of testicular biopsies causes similar morphological damage to testicular spermatozoa and frozen-thawed ejaculated spermatozoa. It is still unclear whether these changes in testicular spermatozoa after freezing and thawing may compromise its use in the ICSI procedure.     

The hypoosmotic swelling test and cryosurvival of human spermatozoa

The hypoosmotic swelling test is a simple laboratory test to evaluate the functional integrity of the membrane of human spermatozoa. This test was performed on 83 samples of human semen before cryopreservation to determine whether it has any predictive value for the cryosurvival of human spermatozoa. Stepwise regression analysis demonstrated that conventional sperm characteristics, including the concentration, motility, normal morphology and viability, of pre-freeze semen samples were of limited value in predicting the cryosurvival of human spermatozoa. Further, the hypoosmotic swelling test results from pre-freeze semen samples did not correlate with the post-thaw motility or the survival rate of spermatozoa after cryopreservation.     

The effect of butylated hydroxytoluene (BHT) on bull spermatozoa frozen in two different extenders

This study evaluated the protective effect of butylated hydroxytoluene (BHT), a lipid-soluble antioxidant against cryopreservation damage on bull spermatozoa. Four BHT concentrations (0.5, 1, 2 and 4?mM) were evaluated. Sperm characteristics were evaluated when BHT was added to a post-thaw–freezing extender by measuring the degree of sperm lipid peroxidation (using malondialdehyde, MDA) and by measuring parameters such as motility and viability of spermatozoa. Production of MDA as an indicator of lipid peroxidation was obtained when BHT ranged from 0.5 to 1?mM in both extenders (P?<?0.0001). Sperm motility and viability evaluated immediately after thawing was higher in BHT-treated spermatozoa, this was significant (P?<?0.001) when the freezing egg yolk–citrate extender was supplemented with 0.5 and 1?mM BHT and when egg yolk–Tris extender was supplemented with 0.5?mM BHT. The addition of 0.5 and 1?mM BHT to semen egg yolk–citrate extender increased the viability of frozen semen after thawing (P?<?0.007). The addition of 0.5 and 1?mM BHT to both extenders resulted in a significant (P?<?0.0001) decrease in MDA production. In conclusion, the addition of BHT to freezing egg yolk–citrate extender improved the overall efficiency of thawed bull spermatozoa, but the addition of BHT to the freezing egg yolk–Tris extender did not.     

Observational clinical follow-up of oocyte cryopreservation using a slow-freezing method with 1,2-propanediol plus sucrose followed by ICSI

BACKGROUND: The value of oocyte cryopreservation remains controversial. Two major problems exist: poor survival and injury to the oocyte meiotic spindle after freezing and thawing. METHODS: For slow oocyte cryopreservation, we used 1.5 mol/l 1,2-propanediol and 0.3 mol/l sucrose. We waited 3 h after thawing for possible recovery of the meiotic spindles before performing ICSI. RESULTS: Forty-three women undergoing IVF or ICSI cycles cryopreserved some or all of their harvested oocytes; of these, 20 thawed their cryopreserved oocytes for personal use and one for donation. The survival rate of oocytes after thawing was 75%, with 67% of oocytes fertilizing normally after ICSI. All 21 cycles (100%) resulted in fertilization and embryo transfers. Seven pregnancies (33%) resulted. Four women delivered five babies with normal karyotypes. Three conceptions are ongoing. Compared to 38 cycles of frozen-thawed embryos at the pronuclear stage in the same period, the percentages of survival, pregnancy and implantation were similar. Additionally, four unmarried women with white blood cell diseases underwent oocyte freezing before preconditioning treatment for haematopoietic stem cell transplantation. CONCLUSIONS: This protocol achieved reproducible success of survival, fertilization and pregnancy for freezing and thawing of human oocytes. The 3 h post-thaw incubation could permit restoration of the meiotic spindles, thus facilitating normal fertilization.     

Effects of cryopreservation on human sperm acrosomes.

Total acrosin activity and acrosomal status were determined before and after cryopreserving human spermatozoa. Three different cryopreservation protocols were used. Both acrosin activity and the incidence of intact acrosomes decreased during cryopreservation. The magnitudes of the decreases were weakly but significantly correlated (r = 0.29, P less than 0.05), suggesting that acrosomal loss contributed to the decrease in acrosin activity. The effects of the three cryopreservation protocols were not significantly different. Motility decreased more (average 43%) than did the percentage of spermatozoa with intact acrosomes (27%) and the total acrosin activity (24%). These measurements suggested that acrosomal damage may have been secondary to cell death. This hypothesis was tested by determining the acrosomal status of spermatozoa that survived cryopreservation. Spermatozoa that were motile after thawing averaged 96% acrosome-intact; their acrosin activity, however, was significantly less than that of motile, unfrozen spermatozoa. These observations support the idea that the acrosomal loss due to cryopreservation is associated with cell death but also demonstrate decreased total acrosin activity of the acrosome-intact spermatozoa that survive cryopreservation.     

DNA integrity and motility of human spermatozoa after standard slow freezing versus cryoprotectant-free vitrification

BACKGROUND: In contrast to the technique of conventional freezing, the vitrification of spermatozoa requires high cooling rates (720 000 degrees K/min), which could be damaging for spermatozoa. The aim of our study was to compare slowly frozen and vitrified spermatozoa in terms of their post-thaw DNA integrity and motility. METHODS: Semen samples were prepared according to the routine swim-up technique and divided into aliquots for comparison of fresh, conventionally frozen and vitrified spermatozoa from the same ejaculate in the presence or absence of cryoprotectants. Spermatozoa motility and DNA integrity were determined. RESULTS: The motility of spermatozoa conventionally (slowly) frozen with a cryoprotectant was similar to that recorded for spermatozoa vitrified in the absence of cryoprotectant (47 versus 52%). The DNA integrity was unaffected by the cryopreservation method or presence of cryoprotectants. CONCLUSION: The vitrification of human spermatozoa in the absence of conventional cryoprotectants is indeed feasible. The DNA integrity of vitrified sperm is comparable with that shown by standard slow-frozen/thawed spermatozoa, yet the method is quick and simple and does not require special cryobiological equipment.     

种属和发育期及不同实验条件对卵裂期胚胎冷冻复苏结局的影响

背景：人卵裂期胚胎冷冻复苏的研究中,不同的实验条件及实验动物模型是否与人类胚胎具有相同的敏感性从而反映出实验方案的优劣值得探讨。 目的：观察胚胎种属和发育期及不同冷冻条件对卵裂期胚胎冷冻复苏结局的影响。 方法：将人卵裂期胚胎作为对照组,将KM小鼠胚分为2细胞,4细胞,8细胞组。各组胚胎随机采用以下实验方案：①冷冻操作环境温度18~20 ℃、24~26 ℃和37 ℃。②慢速程序化方案、自制straw叶片玻璃化方案和CPS玻璃化方案。③与玻璃化液接触时间< 40 s、40~60 s和60~90 s。各组胚胎比较不同实验条件下胚胎复苏率和培养24 h发育率。 结果与结论：①对照组24~26 ℃冷冻操作环境温度复苏率高于37 ℃冷冻操作环境(P < 0.05)。②对照组和4细胞鼠胚组采用自制straw叶片玻璃化方案复苏率高于慢速程序化方案(P < 0.05),培养24 h胚胎发育率差异无显著性意义(P > 0.05)。③各组胚胎与冷冻保护剂接触不同时间胚胎复苏率差异无显著性意义(P > 0.05)。表明卵裂期胚胎玻璃化冷冻复苏效果优于慢速程序化,24~26 ℃操作环境、减少冷冻保护剂剂量和缩短接触时间可改善玻璃化冷冻复苏结局;相同冷冻条件下,胚胎种属和发育阶段对冷冻复苏结局有影响,4细胞鼠胚更适合作为研究人类卵裂期胚胎冷冻复苏的实验模型。     

圆头精子的冷冻保存

目的:探讨圆头精子的冷冻保存效果及其精子功能改变。方法:6例圆头精子症患者各提供1-3份精液标本,应用不含卵黄冷冻保护剂和二步降温方法冷冻保存圆头精子症标本,检测冷冻精子活动率、头-尾膜完整率、存活时间和顶体蛋白酶活性。结果:圆头精子标本冷冻保存后,精子活动率显著减少,膜完整率显著下降,头膜损伤-尾膜完整的精子率显著升高(n=13,P＜0.01)。体外存活时间缩短。精子冷冻前后无顶体蛋白酶活性。不含卵黄冷冻保护剂与含卵黄冷冻保护剂,以及两步冷冻与多步冷冻的效果比较未见显著差异(n=7,P＞0.05)。结论:圆头精子可经历冷冻保存后存活,但冷冻复苏率低。冷冻-解冻过程显著降低精子功能。不含卵黄冷冻保护剂和两步降温可应用于冷冻保存圆头精子症标本。     

Triploid pregnancy after ICSI of frozen testicular spermatozoa into cryopreserved human oocytes: case report

Although freezing oocytes is ethically more acceptable than cryopreservation of embryos, variable oocyte survival, fertilization rate and possible risk of increased ploidy after cryopreservation have precluded the widespread clinical application of oocyte cryopreservation in assisted reproduction techniques. We report a triploid pregnancy from intracytoplasmic sperm injection of recombinant FSH-stimulated frozen/thawed testicular spermatozoa into cryopreserved oocytes in a hormone replacement cycle. To our knowledge, this is the first report of a pregnancy where both gametes have been frozen. It illustrates the need for further research when applying new techniques in assisted reproduction.     

Cryopreservation of human ovarian tissue using dimethylsulphoxide and propanediol-sucrose as cryoprotectants

Pieces of ovarian cortical tissue (03–2 mm in diameter)were obtained during gynaecological operations by biopsy oras a result of oophorectomy from 19 women aged 19–44 years.The tissue was frozen in a programmable freezer using one oftwo different cryoprotectants, either 1.5 M dimethylsulphoxide(DMSO), or a combination of 1, 2-propanediol (1.5 M) and sucrose(0.1 M). After cryopreservation lasting from 24 h to 5 weeks,the ovarian pieces were thawed and studied histologically. Specimenstaken before and after cryopreservation with either pro-tectantshowed no signs of tissue necrosis. Follicles at similar developmentalstages were found before and after freezing. The proportionsof follicles showing signs of atresia, 27% in the non-frozentissue and 19% in the frozen-thawed tissue, were not significantlydifferent. Oocytes, too, had the same appearance after freezingand thawing with both cryoprotectants as was seen in the specimenstaken before freezing. These results suggest that cryopreservationof human ovarian tissue is feasible. However, the normalityof the oocytes taken from tissue which has been frozen stillneeds to be established. Cryopreservation of ovarian tissuewould be potentially an excellent method for storage of humanoocytes once methods for their maturation in vitro have beendeveloped.     

Polscope analysis of meiotic spindle changes in living metaphase II human oocytes during the freezing and thawing procedures

BACKGROUND: The clinical efficacy of the current methods used for cryopreservation of metaphase II human oocytes is low. Meiotic spindle disorders are thought to be largely responsible for this situation. METHODS: Supernumerary fresh metaphase II human oocytes were cryopreserved in 1,2-propanediol with 0.1 M sucrose using a slow freezing/rapid thawing programme. Meiotic spindles were analysed in these living metaphase II oocytes at sequential steps of the freezing and thawing procedures with the use of a computer-assisted polarization microscopy system (Polscope). RESULTS: The meiotic spindle was detected in all 56 oocytes (from 16 patients) before freezing and remained visible in all these oocytes throughout the preparation for freezing up to the time that they were loaded into cryopreservation straws. Immediately after thawing, the spindle was visible in 35.7% of oocytes, but it disappeared in all of the thawed oocytes during the subsequent washing steps. However, the spindle reappeared in all surviving thawed oocytes after washing (57.4%), by 3 h of incubation at 37 degrees C in culture medium. CONCLUSIONS: The current techniques of oocyte freezing and thawing inevitably cause meiotic spindle destruction. All spindles observed in thawed oocytes result from post-thaw reconstruction.