Genetic control of survival of frozen mouse embryos

Embryo vitrification in rabbits: Consequences for progeny growth

The objective of this research is to examine if there are any effects of the rederivation procedures on rabbit growth pattern and on weight of different organ in adults. For this purpose, three experiments were conducted on two different groups of animals (control group and vitrified-transferred group) to evaluate the possible effect of embryo manipulation (vitrification and transfer procedures) on future growth traits. The first experiment studies body weight from 1 to 9 weeks of age from the two groups. The second experiment describes the growth curve of progeny from experimental groups and analyzes their Gompertz curve parameters, including the estimation of adult body weight. The third experiment has been developed to study if there are any differences in different organ weight in adult males from the two experimental groups. In general, the results indicate that rederivation procedures had effect on the phenotypic expression of growth traits. The results showed that rabbit produced by vitrification and embryo transfer had higher body weight in the first four weeks of age than control progeny. Results from body weight (a parameter) and b parameter estimated by fitting the Gompertz growth curve did not show any difference between experimental groups. However, differences related with growth velocity (k parameter of the Gompertz curve) were observed among them, showing that the control group had higher growth velocity than the vitrified-transferred group. In addition, we found that liver weight at 40th week of age exhibits significant differences between the experimental groups. The liver weight was higher in the control males than in the VF males. Although the present results indicate that vitrification and transfer procedures might affect some traits related with growth in rabbits, further research is needed to assess the mechanisms involved in the appearance of these phenotypes and if these phenotypes could be transferred to the future progeny.

In vitro fertility rate of 129 strain is improved by buserelin (gonadotropin-releasing hormone) administration prior to superovulation

The 129 mice are well recognized for their low fertility and it is speculated that this lack of fertility may be due to the oocyte condition. In this study we investigated superovulation regimens for the 129S1/SvImJ mouse strain to improve the oocyte quality and fertility rate of in vitro fertilization (IVF). Female mice were divided into four groups based on hormone and timing of injection. Group 1 received pregnant mare serum gonadotropin (PMSG) and 48 h later human chorionic gonadotropin (hCG); using the same dose, group 2 received hCG 52 h post-PMSG and group 3, 55 h post-PMSG. Group 4 received buserelin (gonadotropin-releasing hormone agonist [GnRH]) followed 24 h later by PMSG and then hCG 55 h post-PMSG. IVF was performed using 129S1/SvImJ oocytes and sperm; C57BL/6J sperm with 129S1/SvImJ oocytes was used as fertility control. The IVF fertility rate was 1% (Groups 1 and 2), 17% (Group 3) and 55% (Group 4) for 129 oocytes fertilized with 129 sperm. For 129 oocytes fertilized with C57BL/6J sperm, the fertility rate was 5% (Group 1), 10% (Group 2), 40% (Group 3) and 59% (Group 4). These results suggest that extending the interval time between PMSG and hCG and giving GnRH in addition to the standard PMSG and hCG treatments can improve IVF fertility rate of 129S1/SvImJ mouse strains significantly.

Fertility comparison between wild type and transgenic mice by in vitro fertilization

Transgenic mice are increasingly used as animal models for studies of gene function and regulation of mammalian genes. Although there has been continuous and remarkable progress in the development of transgenic technology over several decades, many aspects of the resulting transgenic model's phenotype cannot be completely predicted. For example, it is well known that as a consequence of the random insertion of the injected DNA construct, several founder mice of the new line need to be analyzed for possible differences in phenotype secondary to different insertion sites. The Knock out technique for transgenic production disrupts a specific gene by insertion or homologous recombination creating a null expression or replacement of the gene with a marker to localize it expression. This modification could result in pleiotropic phenotype if the gene is also expressed in tissues other than the target organs. Although the future breeding performance of the newly created model is critical to many studies, it is rarely anticipated that the new integrations could modify the reproductive profile of the new transgenic line. To date, few studies have demonstrated the difference between the parent strain's reproductive performance and the newly developed transgenic model. This study was designed to determine whether a genetic modification, knock out (KO) or transgenics, not anticipated to affect reproductive performance could affect the resulting reproductive profile of the newly developed transgenic mouse. More specifically, this study is designed to study the impact of the genetic modification on the ability of gametes to be fertilized in vitro. We analyzed the reproductive performance of mice with different background strains: FVB/N, C57BL/6 (129Sv/J x C57Bl/6)F1 and outbred CD1((R)) and compared them to mice of the same strain carrying a transgene or KO which was not anticipated to affect fertility. In vitro Fertilization was used to analyze the fertility of the mice. Oocytes from superovulated females were inseminated with sperm of same background. Fertility rate was considered as the percentage of two cell embryos scored 24 h after insemination. The data collected from this study shows that the fertilization rate is affected (reduced to half fold) in some of the transgenic mice compared to the respective Wild Type (WT) mice. For the WT the average fertility rate ranged from 80% (C57BL/6), 90% (FVB/N), 45% (129Sv/J x C57Bl/6)F1 and 43% (CD1). For transgenic mice it was 52% (C57BL/6), 65% (FVB/N), 22% (129Sv/J x C57Bl/6)F1 and 25% (CD1).

Osmotic tolerance of mouse spermatozoa from various genetic backgrounds: acrosome integrity, membrane integrity, and maintenance of motility

All cells have an intrinsic biophysical property related to their ability to undergo osmotically driven volume changes. This project is of fundamental importance to our understanding of the basic cryobiology of mouse spermatozoa. The objectives of this study were to determine the osmotic tolerance limits for (1) motility, (2) acrosome integrity, and (3) membrane integrity of mouse spermatozoa from multiple genetic backgrounds including: C57BL/6, BALB/c, FVB, C3H, 129/SVS2 hsd B6C3F1, CB6F1, and ICR. The maintenance of acrosomal and plasma membrane integrity was not affected by genetic background (p=0.13), however, there was an interaction between genetic background and osmolality. In addition, acrosome and plasma membrane integrity was highly correlated within each strain (p<0.01). In contrast to acrosome and plasma membrane integrity, the motility of spermatozoa from different genetic backgrounds fell sharply on both sides of isosmolality, both with and without return to isosmotic conditions. Exposure to hyposmotic conditions caused morphological changes in the spermatozoa, which inhibited motility. However, this morphological change was not reversible in all cases when returned to isosmotic conditions. The ability to maintain motility in an anisosmotic media was affected by genetic background, osmolality as well as the interaction between genetic background and osmolality (p<0.05). In conclusion, mice with different genetic backgrounds appear to have similar tolerance to osmotic changes in terms of sperm acrosome and plasma membrane integrity; however, the ability to maintain motility differs between genetic backgrounds.

Successful cryopreservation of mouse ovaries by vitrification

We developed a new method of cryopreservation of whole ovaries by vitrification using DAP213 (2 M dimethyl sulfoxide, 1 M acetamide, and M propylene glycol) as a cryoprotectant. Four-week-old C57BL/6 mice that underwent partial ovariectomy were orthotopically transplanted with cryopreserved or fresh ovaries (experimental or control group) isolated from 10-day-old green fluorescent protein (GFP)-transgenic mice (+/+). GFP-positive pups were similarly obtained from both groups by natural mating or in vitro fertilization (IVF) followed by embryo transfer, indicating that the cryopreserved ovaries by vitrification retain their fecundity. However, a statistically significant difference (P < 0.05) was found between both groups with respect to the following parameters: the number of GFP-positive pups born by natural mating/grafted ovary (0.8 +/- 0.3 for the experimental group versus 2.0 +/- 0.7 for the control group, mean +/- SEM), the number of collected oocytes by superovulation per mouse (7.0 +/- 1.7 for the experimental group versus 22.7 +/- 3.2 for the control group), the percentage of two-cell embryos obtained from GFP-positive oocytes by IVF (38.5% for the experimental group versus 90.0% for the control group). Histologically, normal development of follicles and formation of corpora lutea were observed in frozen-thawed grafts. However, estimated number of follicles decreased in frozen-thawed ovaries compared with fresh ovaries. Taken together, cryopreservation of the ovary by vitrification seems a promising method to preserve ovarian function, but further studies are required to overcome the possible inhibitory effects of this method on the growth of the ovarian graft.

Use of cryopreserved pronuclear embryos for the production of transgenic mice

A series of experiments was conducted to test the hypothesis that an improved cryopreservation protocol for pronuclear stage mouse embryos will produce transgenic (Tg) mice by pronuclear gene injection at a rate not significantly different from noncryopreserved embryos. In the first experiment, three cryoprotective agents (CPAs) (dimethyl sulfoxide [DMSO], propylene glycol [PG], ethylene glycol [EG]) and two cryopreservation protocols, currently used for pronuclear embryos, were compared in regard to their ability to maintain post-thaw morphological integrity and in vitro developmental competence. In the second and third experiments, the optimal cryopreservation protocol determined from the first experiment was used to evaluate in vitro developmental competence of pronuclear embryos following green fluorescence protein gene injection and in vivo developmental competence as well as the gene integration rates. Survival (morphological integrity and development to two cells) of embryos cryopreserved in the presence of DMSO was higher (P < 0.05) than those cryopreserved with either PG or EG. Postinjection developmental competence (development to two cells) of cryopreserved CBA, C57B6/JxCBA-F1 and noncryopreserved (control) embryos was not different (P > 0.05). Postinjection blastocyst formation rate of cryopreserved and noncryopreserved C57B6/JxCBA-F1 embryos was similar (P > 0.05); however, noncryopreserved CBA embryos resulted in a higher blastocyst formation than controls (P < 0.05). While there was no difference in the percentage of transgenic fetuses between cryopreserved and control CBA embryos (P > 0.05), cryopreserved C57B6/JxCBA-F1 embryos resulted in lower transgenic fetuses than control (P < 0.05). These results indicate that the use of cryopreserved mouse pronuclear embryos can be a useful and efficient approach to the production of Tg mice.

Cryopreservation of murine spermatozoa

Cryopreservation of mouse sperm provides an economic option for preserving the large number of mouse strains now being generated by transgenic and targeted mutation methodologies. The ability of a spermatozoan cell to survive cryobiological preservation depends on general biophysical constraints that apply to all cells, such as the avoidance or minimization of the formation of intracellular ice during cooling. This action is typically achieved by use of cryoprotectant substances and by controlled, slow rates of cooling. Superimposed on those general constraints may be special characteristics of mouse spermatozoa, such as more narrow, osmotically driven volume tolerance limits and the fact that relatively successful freezing can be obtained without the use of a permeating cryoprotective agent. The lack of important information regarding sperm cells fundamental cryobiological properties, including their osmotic and membrane permeability characteristics, has hindered progress in developing anything but empirically derived methods. Genetic differences between inbred mouse strains are reflected in motility and fertility characteristics of mouse sperm and contribute to the difficulty of developing successful cryopreservation methods. Recovery of live young from frozen sperm has been much more successful with sperm from hybrid mice than from most inbred strains. There have been no published reports of successful cryopreservation of rat sperm. Nevertheless, in mice, success in deriving live young from intracytoplasmic sperm injection using sperm frozen under suboptimal conditions raises the possibility of using this technique for the ultimate rescue of sperm regardless of the success of cryopreservation. This technique, however, requires additional development and verification of its efficacy before it will be suitable for general laboratory use. Although cryopreservation of mouse sperm is not yet universally successful, it can be used reliably to supplement cryopreservation of embryos and other germline cells or tissues for preserving biomedically important strains of mice for research.

Factors affecting the efficiency of embryo cryopreservation and rederivation of rat and mouse models

The efficiency of embryo banking for rat and mouse models of human disease and normal biological processes depends on the ease of obtaining embryos. Authors report on the effect of genotype on embryo production and rederivation. In an effort to establish banks of cryopreserved embryos, they provide two databases for comparing banking efficiency: one that contains the embryo collection results from approximately 11,000 rat embryo donors (111 models) and another that contains the embryo collection results from 4,023 mouse embryo donors (57 induced mutant models). The genotype of donor females affected the efficiency of embryo collection in two ways. First, the proportion of females yielding embryos varied markedly among genotypes (rats: 16-100 %, mean =71 %; mice: 24-95 %, mean =65 %). Second, the mean number of embryos recovered from females yielding embryos varied considerably (rats: 4-10.6, mean =7.8; mice 5.3-32.2, mean =13.7). Genotype also affected the efficiency of rederivation of banked rat and mouse embryos models by embryo transfer. For rats, thawed embryos (n =684) from 33 genotypes were transferred into 66 recipient females (pregnancy rate, 78 %). The average rate of developing live newborns for individual rat genotypes was 30 % with a range of 10 to 58 %. For mice, thawed embryos (n =2,064) from 59 genotypes were transferred into 119 pseudopregnant females (pregnancy rate: 76 %). The average rate of development of individual mouse genotypes was 33 % with a range of 11 to 53 %. This analysis demonstrates that genotype is an important consideration when planning embryo banking programs.

In vitro fertilization with cryopreserved inbred mouse sperm

Sperm from C57BL/6J, DBA/2J, BALB/cJ, 129S3/SvImJ, and FVB/NJ inbred mice were cryopreserved in 3% skim milk/18% raffinose cryoprotectant solution. The post-thaw sperm from all strains were evaluated for their viability and fertility by comparing them against B6D2F1 sperm used as a control. The protocol used for freezing mouse sperm was effective in different strains, because the motility was decreased by 50% after cryopreservation similar to other mammalian sperm. However, the progressive motility and the fertility of each inbred strain were affected differently. The C57BL/6J, BALB/cJ, and 129S3/SvImJ strains were the most affected; their fertility (two-cell cleavage) decreased from 70%, 34%, and 84% when using freshly collected sperm to 6%, 12%, and 6% when using frozen/thawed sperm, respectively. Live newborns derived from frozen/thawed sperm were obtained from all strains in the study. These results corroborate the genetic variation among strains with regard to fertility and susceptibility to cryopreservation.

Estimates of heterosis for in vitro embryo production using reciprocal crosses in cattle

In vitro embryo production and exploitation of heterosis are two methods of increasing productivity and accelerating genetic progress in many cattle production systems. However, it is not known if heterosis exists in bovine embryos produced in vitro. Tests for heterosis in in vitro embryo production were conducted in two experiments using reciprocal crosses. In the first, gametes from Bos taurus and Bos indicus were used; in the second, gametes from dairy and beef breeds of Bos taurus were used. In each experiment, both parental groups were used as sperm and oocyte donors, producing crossbred and purebred embryos. Oocytes obtained from abattoir-derived ovaries underwent in vitro maturation and in vitro fertilization with frozen semen. Embryos were cultured to blastocyst stage and observed. In the first experiment, higher (P < 0.05) rates of blastocyst formation were found for Bos taurus both as sires and as dams. Approximately 36% of the purebred Bos taurus oocytes and 21% of the purebred Bos indicus oocytes developed to blastocyst. Crosses averaged 16% resulting in a heterosis estimate of 45%. Ovaries from Bos indicus cows had more harvestable oocytes than did those from Bos taurus cows (P < 0.05). No evidence for heterosis was found for crosses within Bos taurus. Oocytes from beef cows had a higher rate of blastocyst formation than did those from dairy cows (30 vs. 24%, P < 0.05). These seemingly disparate results concerning heterosis were discussed in light of the period of genetic isolation of the parental populations in the two experiments.

Cryobiology of rat embryos I: determination of zygote membrane permeability coefficients for water and cryoprotectants, their activation energies, and the development of improved cryopreservation methods

New rat models are being developed at an exponential rate, making improved methods to cryopreserve rat embryos extremely important. However, cryopreservation of rat embryos has proven to be difficult and expensive. In this study, a series of experiments was performed to characterize the fundamental cryobiology of rat fertilized 1-cell embryos (zygotes) and to investigate the effects of different cryoprotective agents (CPAs) and two different plunging temperatures (T(p)) on post-thaw survival of embryos from three genetic backgrounds. In the initial experiments, information on the fundamental cryobiology of rat zygotes was determined, including 1) the hydraulic conductivity in the presence of CPAs (L(p)), 2) the cryoprotectant permeability (P(CPA)), 3) the reflection coefficient (sigma), and 4) the activation energies for these parameters. P(CPA) values were determined for the CPAs, ethylene glycol (EG), dimethyl sulfoxide (DMSO), and propylene glycol (PG). Using this information, a cryopreservation method was developed and the cryosurvival and fetal development of Sprague-Dawley zygotes cryopreserved in either EG, DMSO, or PG and plunged at either -30 or -80 degrees C, were assessed. The highest fetal developmental rates were obtained using a T(p) of -30 degrees C and EG (61.2% +/- 2.4%), which was not different (P > 0.05) from nonfrozen control zygotes (54.6% +/- 3.0%).

Effect of genotype on the efficiency of mouse embryo cryopreservation by vitrification or slow freezing methods

We examined possible genotype effects on the survival of 8- to 16-cell mouse embryos isolated from four inbred strains (C57BL/6N, BALB/cAnN, DBA/2N, and C3H/HeN), a outbred stock (ICR), and various crosses after cryopreservation by vitrification or conventional slow freezing using glycerol solutions. The rates of in vitro development of C57BL/6N, BALB/cAnN, C3H/HeN, and ICR embryos to expanded blastocysts ranged from 86% to 94% after slow freezing and 85% to 97% after vitrification. The cryopreservation method did not significantly influence in vitro embryo survival after thawing (P > 0.05). Although genotype significantly influenced the in vitro survival of embryos (P = 0.008), this presumably resulted from an increased difficulty in assessing the quality grade of C3H/HeN embryos prior to cryopreservation. The rates in vivo development of C57BL/6N, BALB/cAnN, C3H/HeN, DBA/2N, and ICR embryos to normal day 18-19 fetuses ranged from 19% to 64% after slow freezing and from 18% to 63% after vitrification. The in vivo development of cryopreserved embryos was significantly influenced by cryopreservation method and genotype (P = 0.01 and P = 0.001, respectively). Vitrification yielded significantly higher rates of in vivo development than that after slow freezing (P > 0.05). In vivo development rates of DBA/2N and ICR female X B6D2F1 male embryos after cryopreservation were significantly higher than that of embryos from BALB/cAnN and C3H/HeN mice (P < 0.05). These results indicate that parental genotype exerts little or no effect on the ability of embryos to develop in vitro after vitrification or slow freezing. Differences in the ability of cryopreserved embryos to develop normally in vivo may reflect inherent genotype related differences in their post-implantation developmental potential and not their sensitivity to cryoinjury.

Effect of cryoprotectant and genetic selection for body fat content on embryonic cryosurvival in mice

Lines of mice selected for high (HF) or low (LF) 12-week epididymal fat pad weight as a percentage of body weight were used to investigate the effects of genotype, two cryoprotectants [glycerol (GLY) and propylene glycol (PG)] and genotype x cryoprotectant interaction on cryosurvival of four and eight-cell embryos. Embryos were collected from selection lines and reciprocal crosses of selection lines (HFLF and LFHF) and frozen by established slow-cool methods. Embryos were thawed for 40s at room temperature and then placed in a 37° C waterbath for 1 min. Cryoprotectant was diluted from embryos with either 0.5 M sucrose (GLY-treated) or 1.0 M sucrose (PG-treated). Post-thaw survival was measured as the percentage of embryos developing to 36 h (PTS36), 48 h (PTS48) and hatched blastocyst (PTSHB), respectively. Non-frozen controls were cultured concurrently with frozen embryos. No significant genotype or genotype x cryoprotectant interaction effects were found. Results of the embryo freezing study indicated that selection for high or low fat content did not affect the ability of embryos to survive cryopreservation. There was no indication of embryo heterosis for post-thaw survial. Embryos frozen with GLY survived the freeze-thaw stress significantly better than those frozen in PG (P < 0.05). In vitro development of non-frozen controls at 36 and 48 h did not vary significantly among lines, but in vitro development was significantly different among lines at the hatched blastocyst stage (P < 0.05). Linear contrasts showed that the embryonic genome was responsible for differential in vitro development at the hatched blastocyst stage between these selected lines (HF > LF; P < 0.05); asymmetric response also occurred in that both HF and LF exceeded the unselected control line (P < 0.05).