Generation of transgenic rabbits by the novel technique of chimeric somatic cell cloning

The effect of L-carnitine supplementation during in vitro maturation on oocyte maturation and somatic cloned embryo development

The quality of the recipient cytoplasm was reported as a crucial factor in maintaining the vitality of SCNT embryos and SCNT efficiency for dairy cows. Compared with oocytes matured in vivo, oocytes matured in vitro showed abnormal accumulation and metabolism of cytoplasmic lipids. L-carnitine treatment was found to control fatty acid transport into the mitochondrial β-oxidation pathway, which improved the process of lipid metabolism. The results of this study show that 0.5 mg/ml L-carnitine significantly reduced the cytoplasmic lipid content relative to control. No significant difference was observed in the rate of oocyte nuclear maturation, but the in vitro developmental competence of SCNT embryos was improved in terms of increased blastocyst production and lower apoptotic index in the L-carnitine treatment group. In addition, the pregnancy rate with SCNT embryos in the treatment group was significantly higher than in the control group. In conclusion, the present study demonstrated that adding L-carnitine to the maturation culture medium could improve the developmental competence of SCNT embryos both in vitro and in vivo by reducing the lipid content of the recipient cytoplasm.

Improving the developmental competences of porcine parthenogenetic embryos by Notoginsenoside R1-induced enhancement of mitochondrial activity and alleviation of proapoptotic events

Notoginsenoside R1 (NGR1), derived from the Panax notoginseng root and rhizome, exhibits diverse pharmacological influences on the brain, neurons, and osteoblasts, such as antioxidant effects, mitochondrial function protection, energy metabolism regulation, and inhibition of oxygen radicals, apoptosis, and cellular autophagy. However, its effect on early porcine embryonic development remains unclear. Therefore, we investigated NGR1's effects on blastocyst quality, reactive oxygen species (ROS) levels, glutathione (GSH) levels, mitochondrial function, and embryonic development-related gene expression in porcine embryos by introducing NGR1 during the in vitro culture (IVC) of early porcine embryos. Our results indicate that an addition of 1 μM NGR1 significantly increased glutathione (GSH) levels, blastocyst formation rate, and total cell number and proliferation capacity; decreased ROS levels and apoptosis rates in orphan-activated porcine embryos; and improved intracellular mitochondrial distribution, enhanced membrane potential, and reduced autophagy. In addition, pluripotency-related factor levels were elevated (NANOG and octamer-binding transcription factor 4 [OCT4]), antioxidant-related genes were upregulated (nuclear factor-erythroid 2-related factor 2 [NRF2]), and apoptosis- (caspase 3 [CAS3]) and autophagy-related genes (light chain 3 [LC3B]) were downregulated. These results indicate that NGR1 can enhance early porcine embryonic development by protecting mitochondrial function.

Perspectives in Genome-Editing Techniques for Livestock

Genome editing of farm animals has undeniable practical applications. It helps to improve production traits, enhances the economic value of livestock, and increases disease resistance. Gene-modified animals are also used for biomedical research and drug production and demonstrate the potential to be used as xenograft donors for humans. The recent discovery of site-specific nucleases that allow precision genome editing of a single-cell embryo (or embryonic stem cells) and the development of new embryological delivery manipulations have revolutionized the transgenesis field. These relatively new approaches have already proven to be efficient and reliable for genome engineering and have wide potential for use in agriculture. A number of advanced methodologies have been tested in laboratory models and might be considered for application in livestock animals. At the same time, these methods must meet the requirements of safety, efficiency and availability of their application for a wide range of farm animals. This review aims at covering a brief history of livestock animal genome engineering and outlines possible future directions to design optimal and cost-effective tools for transgenesis in farm species.

The Uterine Melatonergic Systems of AANAT and Melatonin Membrane Receptor 2 (MT2) Are Essential for Endometrial Receptivity and Early Implantation in Mice

In the current study, using Aanat and Mt2 KO mice, we observed that the preservation of the melatonergic system is essential for successful early pregnancy in mice. We identified that aralkylamine N-acetyltransferase (AANAT), melatonin receptor 1A (MT1), and melatonin receptor 1B (MT2) were all expressed in the uterus. Due to the relatively weak expression of MT1 compared to AANAT and MT2, this study focused on AANAT and MT2. Aanat and Mt2 KO significantly reduced the early implantation sites and the abnormal morphology of the endometrium of the uterus. Mechanistical analysis indicated that the melatonergic system is the key player in the induction of the normal nidatory estrogen (E2) response for endometrial receptivity and functions by activating the STAT signaling pathway. Its deficiency impaired the interactions between the endometrium, the placenta, and the embryo. The reduction in melatonin production caused by Aanat KO and the impairment of signal transduction caused by Mt2 KO reduced the uterine MMP-2 and MMP-9 activity, resulting in a hyperproliferative endometrial epithelium. In addition, melatonergic system deficiency also increased the local immunoinflammatory reaction with elevated local proinflammatory cytokines leading to early abortion in the Mt2 KO mice compared to the WT mice. We believe that the novel data obtained from the mice might apply to other animals including humans. Further investigation into the interaction between the melatonergic system and reproductive effects in different species would be worthwhile.

Occurrence of late-apoptotic symptoms in porcine preimplantation embryos upon exposure of oocytes to perfluoroalkyl substances (PFASs) under in vitro meiotic maturation

The objectives of this study were to evaluate the effect of perfluoroalkyl substances on early embryonic development and apoptosis in blastocysts using a porcine in vitro model. Porcine oocytes (N = 855) collected from abattoir ovaries were subjected to perfluorooctane sulfonic acid (PFOS) (0.1 μg/ml) and perfluorohexane sulfonic acid (PFHxS) (40 μg/ml) during in vitro maturation (IVM) for 45 h. The gametes were then fertilized and cultured in vitro, and developmental parameters were recorded. After 6 days of culture, resulting blastocysts (N = 146) were stained using a terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay and imaged as stacks using confocal laser scanning microscopy. Proportion of apoptotic cells as well as total numbers of nuclei in each blastocyst were analyzed using objective image analysis. The experiment was run in 9 replicates, always with a control present. Effects on developmental parameters were analyzed using logistic regression, and effects on apoptosis and total numbers of nuclei were analyzed using linear regression. Higher cell count was associated with lower proportion of apoptotic cells, i.e., larger blastocysts contained less apoptotic cells. Upon PFAS exposure during IVM, PFHxS tended to result in higher blastocyst rates on day 5 post fertilization (p = 0.07) and on day 6 post fertilization (p = 0.05) as well as in higher apoptosis rates in blastocysts (p = 0.06). PFHxS resulted in higher total cell counts in blastocysts (p = 0.002). No effects attributable to the concentration of PFOS used here was seen. These findings add to the evidence that some perfluoroalkyl substances may affect female reproduction. More studies are needed to better understand potential implications for continued development as well as for human health.

Profiling the transcriptomic signatures and identifying the patterns of zygotic genome activation - a comparative analysis between early porcine embryos and their counterparts in other three mammalian species

BACKGROUND

The transcriptional changes around zygotic genome activation (ZGA) in preimplantation embryos are critical for studying mechanisms of embryonic developmental arrest and searching for key transcription factors. However, studies on the transcription profile of porcine ZGA are limited.

RESULTS

In this study, we performed RNA sequencing in porcine in vivo developed (IVV) and somatic cell nuclear transfer (SCNT) embryo at different stages and compared the transcriptional activity of porcine embryos with mouse, bovine and human embryos. The results showed that the transcriptome map of the early porcine embryos was significantly changed at the 4-cell stage, and 5821 differentially expressed genes (DEGs) in SCNT embryos failed to be reprogrammed or activated during ZGA, which mainly enrichment to metabolic pathways. c-MYC was identified as the highest expressed transcription factor during ZGA. By treating with 10,058-F4, an inhibitor of c-MYC, the cleavage rate (38.33 ± 3.4%) and blastocyst rate (23.33 ± 4.3%) of porcine embryos were significantly lower than those of the control group (50.82 ± 2.7% and 34.43 ± 1.9%). Cross-species analysis of transcriptome during ZGA showed that pigs and bovines had the highest similarity coefficient in biological processes. KEGG pathway analysis indicated that there were 10 co-shared pathways in the four species.

CONCLUSIONS

Our results reveal that embryos with impaired developmental competence may be arrested at an early stage of development. c-MYC helps promote ZGA and preimplantation embryonic development in pigs. Pigs and bovines have the highest coefficient of similarity in biological processes during ZGA. This study provides an important reference for further studying the reprogramming regulatory mechanism of porcine embryos during ZGA.

Genetically modified rabbit models for cardiovascular medicine

Genetically modified (GM) rabbits are outstanding animal models for studying human genetic and acquired diseases. As such, GM rabbits that express human genes have been extensively used as models of cardiovascular disease. Rabbits are genetically modified via prokaryotic microinjection. Through this process, genes are randomly integrated into the rabbit genome. Moreover, gene targeting in embryonic stem (ES) cells is a powerful tool for understanding gene function. However, rabbits lack stable ES cell lines. Therefore, ES-dependent gene targeting is not possible in rabbits. Nevertheless, the RNA interference technique is rapidly becoming a useful experimental tool that enables researchers to knock down specific gene expression, which leads to the genetic modification of rabbits. Recently, with the emergence of new genetic technology, such as zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), clustered regularly interspaced short palindromic repeats (CRISPR), and CRISPR-associated protein 9 (CRISPR/Cas9), major breakthroughs have been made in rabbit gene targeting. Using these novel genetic techniques, researchers have successfully modified knockout (KO) rabbit models. In this paper, we aimed to review the recent advances in GM technology in rabbits and highlight their application as models for cardiovascular medicine.

Technical, Biological and Molecular Aspects of Somatic Cell Nuclear Transfer – A Review

Since the announcement of the birth of the first cloned mammal in 1997, Dolly the sheep, 24 animal species including laboratory, farm, and wild animals have been cloned. The technique for somatic cloning involves transfer of the donor nucleus of a somatic cell into an enucleated oocyte at the metaphase II (MII) stage for the generation of a new individual, genetically identical to the somatic cell donor. There is increasing interest in animal cloning for different purposes such as rescue of endangered animals, replication of superior farm animals, production of genetically engineered animals, creation of biomedical models, and basic research. However, the efficiency of cloning remains relatively low. High abortion, embryonic, and fetal mortality rates are frequently observed. Moreover, aberrant developmental patterns during or after birth are reported. Researchers attribute these abnormal phenotypes mainly to incomplete nuclear remodeling, resulting in incomplete reprogramming. Nevertheless, multiple factors influence the success of each step of the somatic cloning process. Various strategies have been used to improve the efficiency of nuclear transfer and most of the phenotypically normal born clones can survive, grow, and reproduce. This paper will present some technical, biological, and molecular aspects of somatic cloning, along with remarkable achievements and current improvements.

Extranuclear Inheritance of Mitochondrial Genome and Epigenetic Reprogrammability of Chromosomal Telomeres in Somatic Cell Cloning of Mammals

The effectiveness of somatic cell nuclear transfer (SCNT) in mammals seems to be still characterized by the disappointingly low rates of cloned embryos, fetuses, and progeny generated. These rates are measured in relation to the numbers of nuclear-transferred oocytes and can vary depending on the technique applied to the reconstruction of enucleated oocytes. The SCNT efficiency is also largely affected by the capability of donor nuclei to be epigenetically reprogrammed in a cytoplasm of reconstructed oocytes. The epigenetic reprogrammability of donor nuclei in SCNT-derived embryos appears to be biased, to a great extent, by the extranuclear (cytoplasmic) inheritance of mitochondrial DNA (mtDNA) fractions originating from donor cells. A high frequency of mtDNA heteroplasmy occurrence can lead to disturbances in the intergenomic crosstalk between mitochondrial and nuclear compartments during the early embryogenesis of SCNT-derived embryos. These disturbances can give rise to incorrect and incomplete epigenetic reprogramming of donor nuclei in mammalian cloned embryos. The dwindling reprogrammability of donor nuclei in the blastomeres of SCNT-derived embryos can also be impacted by impaired epigenetic rearrangements within terminal ends of donor cell-descended chromosomes (i.e., telomeres). Therefore, dysfunctions in epigenetic reprogramming of donor nuclei can contribute to the enhanced attrition of telomeres. This accelerates the processes of epigenomic aging and replicative senescence in the cells forming various tissues and organs of cloned fetuses and progeny. For all the above-mentioned reasons, the current paper aims to overview the state of the art in not only molecular mechanisms underlying intergenomic communication between nuclear and mtDNA molecules in cloned embryos but also intrinsic determinants affecting unfaithful epigenetic reprogrammability of telomeres. The latter is related to their abrasion within somatic cell-inherited chromosomes.

Genetically Modified Rabbits for Cardiovascular Research

Rabbits are one of the most used experimental animals for investigating the mechanisms of human cardiovascular disease and lipid metabolism because they are phylogenetically closer to human than rodents (mice and rats). Cholesterol-fed wild-type rabbits were first used to study human atherosclerosis more than 100 years ago and are still playing an important role in cardiovascular research. Furthermore, transgenic rabbits generated by pronuclear microinjection provided another means to investigate many gene functions associated with human disease. Because of the lack of both rabbit embryonic stem cells and the genome information, for a long time, it has been a dream for scientists to obtain knockout rabbits generated by homologous recombination-based genomic manipulation as in mice. This obstacle has greatly hampered using genetically modified rabbits to disclose the molecular mechanisms of many human diseases. The advent of genome editing technologies has dramatically extended the applications of experimental animals including rabbits. In this review, we will update genetically modified rabbits, including transgenic, knock-out, and knock-in rabbits during the past decades regarding their use in cardiovascular research and point out the perspectives in future.

Transgenic Rabbit Models: Now and the Future

Transgenic rabbits have contributed to the progress of biomedical science as human disease models because of their unique features, such as the lipid metabolism system similar to humans and medium body size that facilitates handling and experimental manipulation. In fact, many useful transgenic rabbits have been generated and used in research fields such as lipid metabolism and atherosclerosis, cardiac failure, immunology, and oncogenesis. However, there have been long-term problems, namely that the transgenic efficiency when using pronuclear microinjection is low compared with transgenic mice and production of knockout rabbits is impossible owing to the lack of embryonic stem cells for gene targeting in rabbits. Despite these limitations, the emergence of novel genome editing technology has changed the production of genetically modified animals including the rabbit. We are finally able to produce both transgenic and knockout rabbit models to analyze gain- and loss-of-functions of specific genes. It is expected that the use of genetically modified rabbits will extend to various research fields. In this review, we describe the unique features of rabbits as laboratory animals, the current status of their development and use, and future perspectives of transgenic rabbit models for human diseases.

Production of cloned cats using additional complimentary cytoplasm

Somatic cell nuclear transfer (SCNT) is an important technique for producing cloned animals. It, however, is inefficient when there is use of SCNT for cloned animal production. Cytoplasm injection cloning technology (CICT) was developed to overcome the inefficiencies of SCNT use of this purpose. The use of CICT involves additional cytoplasm fusing with enucleated oocytes to restore the cytoplasmic volume, thus improving the in vitro developmental competence and quality of cloned embryos. In this study, there was application of CICT in cats to improve the in vitro developmental competence of cloned embryos, as well as the production of the offspring. The results of this study were that fusion rate of the cloned embryos with use of the CICT method was greater than that with SCNT (80.0 ± 4.8% compared with 67.8 ± 11.3%, respectively), and more blastocysts developed with use of CICT than SCNT (20.0 ± 2.0% compared with 13.5 ± 5.0%, respectively). The 62 cloned embryos that were produced with use of CICT were transferred into five estrous synchronized recipients, and 151 cloned embryos produced using SCNT were transferred to 13 estrous-synchronized recipients. After the embryo transfer, there was birth from surrogate mothers of one live-born kitten that resulted using SCNT compared with three live-born kittens using CICT. The number of CICT-cloned embryos born was greater than that of SCNT-cloned embryos (4.8 ± 2.3% compared with 0.7 ± 1.3%, P <  0.05). These results indicate that the CICT technique can be used to produce cloned kittens, including endangered feline species.

Identifying Biomarkers of Autophagy and Apoptosis in Transfected Nuclear Donor Cells and Transgenic Cloned Pig Embryos

In this study, we first investigated the effects of 3-methyladenine (3-MA), an autophagy inhibitor, and the inducer – rapamycin (RAPA) on the incidence of programmed cell death (PCD) symptoms during in vitro development of porcine somatic cell nuclear transfer (SCNT)-derived embryos. The expression of autophagy inhibitor mTOR protein was decreased in porcine SCNT blastocysts treated with 3MA. The abundance of the autophagy marker LC3 increased in blastocysts following RAPA treatment. Exposure of porcine SCNT-derived embryos to 3-MA suppressed their developmental abilities to reach the blastocyst stage. No significant difference in the expression pattern of PCD-related proteins was found between non-transfected dermal cell and transfected dermal cell groups. Additionally, the pattern of PCD in SCNT-derived blastocysts generated using SC and TSC was not significantly different, and in terms of porcine SCNT-derived embryo development rates and total blastocyst cell numbers, there was no significant difference between non-transfected cells and transfected cells. In conclusion, regulation of autophagy affected the development of porcine SCNT embryos. Regardless of the type of nuclear donor cells (transfected or non-transfected dermal cells) used for SCNT, there was no difference in the developmental potential and quantitative profiles of autophagy/apoptosis biomarkers between porcine transgenic and non-transgenic cloned embryos. These results led us to conclude that PCD is important for controlling porcine SCNT-derived embryo development, and that transfected dermal cells can be utilized as a source of nuclear donors for the production of transgenic cloned progeny in pigs.

Chimerism in piglets developed from aggregated cloned embryos

Porcine chimeras are valuable in the study of pluripotency, embryogenesis and development. It would be meaningful to generate chimeric piglets from somatic cell nuclear transfer embryos. In this study, two cell lines expressing the fluorescent markers enhanced green fluorescent protein (EGFP) and tdTomato were used as donor cells to produce reconstructed embryos. Chimeric embryos were generated by aggregating two EGFP‐cell derived embryos with two tdTomato‐cell derived embryos at the 4‐cell stage, and embryo transfer was performed when the aggregated embryos developed into blastocysts. Live porcine chimeras were successfully born and chimerism was observed by their skin color, gene integration, microsatellite loci composition and fluorescent protein expression. The chimeric piglets were largely composed of EGFP‐expressing cells, and this phenomenon was possibly due to the hyper‐methylation of the promoter of the tdTomato gene. In addition, the expression levels of tumorigenicity‐related genes were altered after tdTomato transfection in bladder cancer cells. The results show that chimeric pigs can be produced by aggregating cloned embryos and that the developmental capability of the cloned embryo in the subsequent chimeric development could be affected by the growth characteristics of its donor cell.

Postimplantation Development of Cloned Rabbit Embryos Reconstructed with Foetal and Adult Skin-Derived Fibroblast Cell Nuclei

The aim of the study was to determine the postimplantation developmental potential of nuclear transfer (NT) derived rabbit embryos, which were reconstructed with foetal fibroblast (FF) or adult skin fibroblast (AF) cell nuclei. A total of 97 embryos reconstructed with FF cell nuclei (Group I) were transferred into the oviducts of 6 pseudopregnant recipients and 101 embryos reconstructed with AF cell nuclei (Group II) were transferred to 6 foster mothers. The presence of fetuses (with the symptoms of early resorption of amniotic sacs) was confirmed in the 4/6 (66.7%) and 1/6 (16.7%) recipient-females in Group I and Group II, respectively. The implantation rate was significantly higher for cloned embryos originating from the oocytes receiving foetal fibroblasts than for those derived from adult skin fibroblasts (P<0.1). Nonetheless, all pregnancies were lost and no progeny were obtained.

Transgenic mammalian species, generated by somatic cell cloning, in biomedicine, biopharmaceutical industry and human nutrition/dietetics--recent achievements

Somatic cell cloning technology in mammals promotes the multiplication of productively-valuable genetically engineered individuals, and consequently allows also for standardization of transgenic farm animal-derived products, which, in the context of market requirements, will have growing significance. Gene farming is one of the most promising areas in modern biotechnology. The use of live bioreactors for the expression of human genes in the lactating mammary gland of transgenic animals seems to be the most cost-effective method for the production/processing of valuable recombinant therapeutic proteins. Among the transgenic farm livestock species used so far, cattle, goats, sheep, pigs and rabbits are useful candidates for the expression of tens to hundreds of grams of genetically-engineered proteins or xenogeneic biopreparations in the milk. At the beginning of the new millennium, a revolution in the treatment of disease is taking shape due to the emergence of new therapies based on recombinant human proteins. The ever-growing demand for such pharmaceutical or nutriceutical proteins is an important driving force for the development of safe and large-scale production platforms. The aim of this paper is to present an overall survey of the state of the art in investigations which provide the current knowledge for deciphering the possibilities of practical application of the transgenic mammalian species generated by somatic cell cloning in biomedicine, the biopharmaceutical industry, human nutrition/dietetics and agriculture.

Live birth of somatic cell-cloned rabbits following trichostatin A treatment and cotransfer of parthenogenetic embryos

Somatic cell nuclear transfer (SCNT) efficiency is still low in rabbit. Previous studies indicated that trichostatin A (TSA) treatment could improve cloning efficiency and term development in the mouse, and cotransfer of parthenogenetic (PA) embryos benefited the pregnancy of cloned embryos in porcine and the mouse. In this study we investigated the effect of TSA treatment on the term development of the SCNT rabbit embryos, and the possibility of the pregnancy maintenance of clones by cotransfer of PA embryos. The SCNT embryos were produced by fusing cumulus cells with enucleated cytoplasts before activation by electrical stimulation, and Dimethylaminopurine (6-DMAP) and Cyclohexamide (CHX) treatments. They were cultured in EBSS-complete medium regardless of their treatment with or without TSA. In vitro developmental data showed no differences in the cleavage and the blastocyst rates, and the blastocyst cell number between the TSA-treated and the untreated SCNT embryos. Two of the six recipients became pregnant after the embryo transfer (ET) in the TSA-treated group, and one pregnant female delivered seven live and three stillborn pups. The death of all live pups occurred within an hour to 19 days. Four of the seven recipients became pregnant in the TSA-untreated group. Three of them gave birth to six live and eight stillborn pups. Four pups of the TSA-untreated group have grown into adulthood, and three of them produced progeny. Cotransfer of three to four PA embryos with 26-32 SCNT embryos to the same recipient resulted in pregnancy and birth rates statistically no different compared to the control SCNT ET group. In conclusion, our results indicate that TSA treatment has a limited effect on the in vitro development of the SCNT embryos; furthermore, both the TSA-treated and the untreated clones can develop to term in rabbits, but none of the offspring from TSA-treated embryos survived to adulthood in our experiment.

Transgene expression of enhanced green fluorescent protein in cloned rabbits generated from in vitro-transfected adult fibroblasts

Live rabbits have previously been generated through nuclear transfer using adult cells as nuclear donors. We demonstrated in this study that transfected adult rabbit fibroblasts are also capable of supporting full-term development. The fibroblasts were transfected with a pEGFP-C1 plasmid using lipofectamine() 2000, and the transgenic cells were derived from conditioned medium. The transgenic fibroblasts were cultured until confluent and then serum-starved prior to be used as nuclear donors. After nuclear transfer and activation, 22% (12/55) of the transgenic cloned embryos developed to the blastocyst stage. A total of 114 embryos at the 4- to 8-cell stage were transferred to the oviducts of 8 pseudo-pregnant mothers; 5 of these animals became pregnant, and 3 of the 5 mother rabbits carried the pregnancy to term. Caesarean section was performed on the 3 pregnant mothers, yielding 4 kits, one of which has survived for more than 9 months. Green fluorescence could be detected in the toenails of the living cloned rabbit and the offspring from the living cloned rabbit under ultraviolet light. DNA analyses confirmed that all 4 cloned rabbits were genetically identical to the transgenic donor cells, and that they all carried the EGFP gene. The present study demonstrated that transgenic rabbits can be generated through nuclear transfer. These results may facilitate future developments in the genetic engineering of rabbits.

Mouse zygotes as recipients in embryo cloning

Zygotes have not been recognized as nuclear recipients since enucleated zygotes receiving nuclei from beyond two-cell stage embryos are not able to form blastocysts. In the present study, a new technique of zygote enucleation is presented, which consists in selectively removing the nuclear membrane with genetic material of pronuclei, but leaving other pronuclear components in the cytoplasm. With selective enucleation it is possible - after transfer of eight-cell stage nuclei - to obtain 70.5 and 7.8% of preimplantation and full-term development respectively. Origin of cloned mice from introduced nuclei was confirmed by the coat colour and glucose phosphate isomerase (GPI) isozyme of the donor. We suggest that some pronuclear factors - taken away from the zygotes in the karyoplasts upon classical enucleation - are needed to reprogram the introduced nuclei.