Gene expression and in vitro development of inter-species nuclear transfer embryos

Production of wild buffalo (Bubalus arnee) embryos by interspecies somatic cell nuclear transfer using domestic buffalo (Bubalus bubalis) oocytes

The objective of this study was to explore the possibility of producing wild buffalo embryos by interspecies somatic cell nuclear transfer (iSCNT) through handmade cloning using wild buffalo somatic cells and domestic buffalo (Bubalus bubalis) oocytes. Somatic cells derived from the ear skin of wild buffalo were found to express vimentin but not keratin and cytokeratin-18, indicating that they were of fibroblast origin. The population doubling time of skin fibroblasts from wild buffalo was significantly (p < 0.05) higher, and the cell proliferation rate was significantly (p < 0.05) lower compared with that of skin fibroblasts from domestic buffalo. Neither the cleavage (92.6 ± 2.0% vs 92.8 ± 2.0%) nor the blastocyst rate (42.4 ± 2.4% vs 38.7 ± 2.8%) was significantly different between the intraspecies cloned embryos produced using skin fibroblasts from domestic buffalo and interspecies cloned embryos produced using skin fibroblasts from wild buffalo. However, the total cell number (TCN) was significantly (p < 0.05) lower (192.0 ± 25.6 vs 345.7 ± 42.2), and the apoptotic index was significantly (p < 0.05) higher (15.1 ± 3.1 vs 8.0 ± 1.4) for interspecies than that for intraspecies cloned embryos. Following vitrification in open-pulled straws (OPS) and warming, although the cryosurvival rate of both types of cloned embryos, as indicated by their re-expansion rate, was not significantly different (34.8 ± 1.5% vs 47.8 ± 7.8), the apoptotic index was significantly (p < 0.05) higher for vitrified-warmed interspecies than that for corresponding intraspecies cloned embryos (48.9 ± 7.2 vs 23.9 ± 2.8). The global level of H3K18ac was significantly (p < 0.05) lower in interspecies cloned embryos than that in intraspecies cloned embryos. The expression level of HDAC1, DNMT3a and CASPASE3 was significantly (p < 0.05) higher, that of P53 was significantly (p < 0.05) lower in interspecies than in intraspecies embryos, whereas that of DNMT1 was similar between the two types of embryos. In conclusion, these results demonstrate that wild buffalo embryos can be produced by iSCNT.

Interspecies somatic cell nuclear transfer: advancements and problems

Embryologists working with livestock species were the pioneers in the field of reprogramming by somatic cell nuclear transfer (SCNT). Without the "Dolly experiment," the field of cellular reprogramming would have been slow and induced plutipotent cells (iPSCs) would not have been conceived. The major drive of the work in mammalian cloning was the interest of the breeding industry to propagate superior genotypes. Soon it was realized that the properties of oocytes could be used also to clone endangered mammalian species or to reprogram the genomes of unrelated species through what is known as interspecies (i) SCNT, using easily available oocytes of livestock species. iSCNT for cloning animals works only for species that can interbreed, and experiments with taxonomically distant species have not been successful in obtaining live births or deriving embryonic stem cell (ESC) lines to be used for regenerative medicine. There are controversial reports in the literature, but in most cases these experiments have underlined some of the cellular and molecular mechanisms that are incomplete during cell nucleus reprogramming, including the failure to organize nucleoli, silence somatic cell genes, activate the embryonic genome, and resume mitochondrial replication and function, thus indicating nucleus-cytoplasmic incompatibility.

Transgenic chicken, mice, cattle, and pig embryos by somatic cell nuclear transfer into pig oocytes

This study explored the possibility of producing transgenic cloned embryos by interspecies somatic cell nuclear transfer (iSCNT) of cattle, mice, and chicken donor cells into enucleated pig oocytes. Enhanced green florescent protein (EGFP)-expressing donor cells were used for the nuclear transfer. Results showed that the occurrence of first cleavage did not differ significantly when pig, cattle, mice, or chicken cells were used as donor nuclei (p>0.05). However, the rate of blastocyst formation was significantly higher in pig (14.9±2.1%; p<0.05) SCNT embryos than in cattle (6.3±2.5%), mice (4.2±1.4%), or chicken (5.1±2.4%) iSCNT embryos. The iSCNT embryos also contained a significantly less number of cells per blastocyst than those of SCNT pig embryos (p<0.05). All (100%) iSCNT embryos expressed the EGFP gene, as evidenced by the green florescence under ultraviolet (UV) illumination. Microinjection of purified mitochondria from cattle somatic cells into pig oocytes did not have any adverse effect on their postfertilization in vitro development and embryo quality (p>0.05). Moreover, NCSU23 medium, which was designed for in vitro culture of pig embryos, was able to support the in vitro development of cattle, mice, and chicken iSCNT embryos up to the blastocyst stage. Taken together, these data suggest that enucleated pig oocytes may be used as a universal cytoplast for production of transgenic cattle, mice, and chicken embryos by iSCNT. Furthermore, xenogenic transfer of mitochondria to the recipient cytoplast may not be the cause for poor embryonic development of cattle-pig iSCNT embryos.

Histone deacetylase inhibitor improves the development and acetylation levels of cat-cow interspecies cloned embryos

Abnormal epigenetic reprogramming, such as histone acetylation, might cause low efficiency of interspecies somatic cell nuclear transfer (iSCNT). This study was conducted to evaluate the effects of trichostatin A (TSA) on the developmental competence and histone acetylation of iSCNT embryos reconstructed from cat somatic cells and bovine cytoplasm. The iSCNT cat and parthenogenetic bovine embryos were treated with various concentrations of TSA (0, 25, 50, or 100 nM) for 24 h, respectively, following fusion and activation. Treatment with 50 nM TSA produced significantly higher rates of cleavage and blastocyst formation (84.3% and 4.6%, respectively) of iSCNT embryos than the rates of non-TSA-treated iSCNT embryos (63.8% and 0%, respectively). Similarly, the treatment of 50 nM TSA increased the blastocyst formation rate of parthenogenetic bovine embryos. The acetylation levels of histone H3 lysine 9 (H3K9) in the iSCNT embryos with the treatment of 50 nM TSA were similar to those of in vitro-fertilized embryos and significantly higher (p<0.05) than those of non-TSA-treated iSCNT embryos (control), irrespective of the embryonic development stage (two-cell, four-cell, and eight-cell stages). These results indicated that the treatment of 50 nM TSA postfusion was beneficial for development to the blastocyst stage of iSCNT cat embryos and correlated with the increasing levels of acetylation at H3K9.

Heterochromatin reprogramming in rabbit embryos after fertilization, intra-, and inter-species SCNT correlates with preimplantation development

To investigate the embryonic genome organization upon fertilization and somatic cell nuclear transfer (SCNT), we tracked HP1β and CENP, two well-characterized protein markers of pericentric and centromeric compartments respectively, in four types of embryos produced by rabbit in vivo fertilization, rabbit parthenogenesis, rabbit-to-rabbit, and bovine-to-rabbit SCNT. In the interphase nuclei of rabbit cultured fibroblasts, centromeres and associated pericentric heterochromatin are usually isolated. Clustering into higher-order chromatin structures, such as the chromocenters seen in mouse and bovine somatic cells, could not be observed in rabbit fibroblasts. After fertilization, centromeres and associated pericentric heterochromatin are quite dispersed in rabbit embryos. The somatic-like organization is progressively established and completed only by the 8/16-cell stage, a stage that corresponds to major embryonic genome activation in this species. In SCNT embryos, pericentric heterochromatin distribution typical for rabbit and bovine somatic cells was incompletely reverted into the 1-cell embryonic form with remnants of heterochromatin clusters in 100% of bovine-to-rabbit embryos. Subsequently, the donor cell nuclear organization was rapidly re-established by the 4-cell stage. Remarkably, the incomplete remodeling of bovine-to-rabbit 1-cell embryos was associated with delayed transcriptional activation compared with rabbit-to-rabbit embryos. Together, the results confirm that pericentric heterochromatin spatio-temporal reorganization is an important step of embryonic genome reprogramming. It also appears that genome reorganization in SCNT embryos is mainly dependent on the nuclear characteristics of the donor cells, not on the recipient cytoplasm.

Early and delayed aspects of nuclear reprogramming during cloning

The successful production of viable progeny following adult somatic cell nuclear transfer (cloning) provides exciting new opportunities for basic research for investigating early embryogenesis, for the propagation of valuable or endangered animals, for the production of genetically engineered animals, and possibly for developing therapeutically valuable stem cells. Successful cloning requires efficient reprogramming of gene expression to silence donor cell gene expression and activate an embryonic pattern of gene expression. Recent observations indicate that reprogramming may be initiated by early events that occur soon after nuclear transfer, but then continues as development progresses through cleavage and probably to gastrulation. Because reprogramming is slow and progressive, cloned embryos have dramatically altered characteristics in comparison with fertilized embryos. Events that occur early following nuclear transfer may be essential prerequisites for the later events. Additionally, the later reprogramming events may be inhibited by sub-optimum culture environments that exist because of the altered characteristics of cloned embryos. By addressing the unique requirements of cloned embryos, the entire process of reprogramming may be accelerated, thus increasing cloning efficiency.

Development, embryonic genome activity and mitochondrial characteristics of bovine-pig inter-family nuclear transfer embryos

The best results of inter-species somatic cell nuclear transfer (iSCNT) in mammals were obtained using closely related species that can hybridise naturally. However, in the last years, many reports describing blastocyst development following iSCNT between species with distant taxonomical relations (inter-classes, inter-order and inter-family) have been published. This indicates that embryonic genome activation (EGA) in xeno-cytoplasm is possible, albeit very rarely. Using a bovine-pig (inter-family) iSCNT model, we studied the basic characteristics of EGA: expression and activity of RNA polymerase II (RNA Pol II), formation of nucleoli (as an indicator of RNA polymerase I (RNA Pol I) activity), expression of the key pluripotency gene NANOG and alteration of mitochondrial mass. In control embryos (obtained by IVF or iSCNT), EGA was characterised by RNA Pol II accumulation and massive production of poly-adenylated transcripts (detected with oligo dT probes) in blastomere nuclei, and formation of nucleoli as a result of RNA Pol I activity. Conversely, iSCNT embryos were characterised by the absence of accumulation and low activity of RNA Pol II and inability to form active mature nucleoli. Moreover, in iSCNT embryos, NANOG was not expressed, and mitochondria mass was significantly lower than in intra-species embryos. Finally, the complete developmental block at the 16-25-cell stage for pig-bovine iSCNT embryos and at the four-cell stage for bovine-pig iSCNT embryos strongly suggests that EGA is not taking place in iSCNT embryos. Thus, our experiments clearly demonstrate poor nucleus-cytoplasm compatibility between these animal species.

Nucleologenesis and embryonic genome activation are defective in interspecies cloned embryos between bovine ooplasm and rhesus monkey somatic cells

Background

Interspecies somatic cell nuclear transfer (iSCNT) has been proposed as a tool to address basic developmental questions and to improve the feasibility of cell therapy. However, the low efficiency of iSCNT embryonic development is a crucial problem when compared to in vitro fertilization (IVF) and intraspecies SCNT. Thus, we examined the effect of donor cell species on the early development of SCNT embryos after reconstruction with bovine ooplasm.

Results

No apparent difference in cleavage rate was found among IVF, monkey-bovine (MB)-iSCNT, and bovine-bovine (BB)-SCNT embryos. However, MB-iSCNT embryos failed to develop beyond the 8- or 16-cell stages and lacked expression of the genes involved in embryonic genome activation (EGA) at the 8-cell stage. From ultrastructural observations made during the peri-EGA period using transmission electron microscopy (TEM), we found that the nucleoli of MB-iSCNT embryos were morphologically abnormal or arrested at the primary stage of nucleologenesis. Consistent with the TEM analysis, nucleolar component proteins, such as upstream binding transcription factor, fibrillarin, nucleolin, and nucleophosmin, showed decreased expression and were structurally disorganized in MB-iSCNT embryos compared to IVF and BB-SCNT embryos, as revealed by real-time PCR and immunofluorescence confocal laser scanning microscopy, respectively.

Conclusion

The down-regulation of housekeeping and imprinting genes, abnormal nucleolar morphology, and aberrant patterns of nucleolar proteins during EGA resulted in developmental failure in MB-iSCNT embryos. These results provide insight into the unresolved problems of early embryonic development in iSCNT embryos.

Interspecies nuclear transfer: implications for embryonic stem cell biology

Accessibility of human oocytes for research poses a serious ethical challenge to society. This fact categorically holds true when pursuing some of the most promising areas of research, such as somatic cell nuclear transfer and embryonic stem cell studies. One approach to overcoming this limitation is to use an oocyte from one species and a somatic cell from another. Recently, several attempts to capture the promises of this approach have met with varying success, ranging from establishing human embryonic stem cells to obtaining live offspring in animals. This review focuses on the challenges and opportunities presented by the formidable task of overcoming biological differences among species.

Development of interspecies cloned monkey embryos reconstructed with bovine enucleated oocytes

This study was carried out to determine whether culture media reconstructed with bovine enucleated oocytes and the expression pattern of Oct-4 could support dedifferentiaton of monkey fibroblasts in interspecies cloned monkey embryos. In this study, monkey and bovine skin fibroblasts were used as donor cells for reconstruction with bovine enucleated oocytes. The reconstructed monkey interspecies somatic cell nuclear transfer (iSCNT) embryos were then cultured under six different culture conditions with modifications of the embryo culture media and normal bovine and monkey specifications. The Oct-4 expression patterns of the embryos were examined at the two-cell to blastocyst stages using immunocytochemistry. The monkey iSCNT embryos showed similar cleavage rates to those of bovine SCNT and bovine parthenogenetic activation (PA). However, the monkey iSCNT embryos were not able to develop beyond the 16-cell stage under any of the culture conditions. In monkey and bovine SCNT embryos, Oct-4 could be detected from the two-cell to blastocyst stage, and in bovine PA embryos, Oct-4 was detectable from the morula to blastocyst stage. These results suggested that bovine ooplasm could support dedifferentiation of monkey somatic cell nuclei but could not support embryo development to either the compact morula or blastocyst stage. In conclusion, we found that the culture conditions that tend to enhance monkey iSCNT embryo development and the expression pattern of Oct-4 in cloned embryos (monkey iSCNT and bovine SCNT) are different than in bovine PA embryos.

DNA Methylation Errors in Cloned Mice Disappear with Advancement of Aging

Cloned animals have various health problems. Aberrant DNA methylation is a possible cause of the problems. Restriction landmark genomic scanning (RLGS) that enabled us to analyze more than 1,000 CpG islands simultaneously demonstrated that all cloned newborns had aberrant DNA methylation. To study whether this aberration persists throughout the life of cloned individuals, we examined genome-wide DNA methylation status of newborn (19.5 dpc, n=2), adult (8-11 months old, n=3), and aged (23-27 months old, n=4) cloned mice using kidney cells as representatives. In the adult and aged groups, cloning was repeated using cumulus cells of the adult founder clone of each group as nucleus donor. Two newborn clones had three with aberrantly methylated loci, which is consistent with previous reports that all cloned newborns had DNA methylation aberrations. Interestingly, we could detect only one aberrantly methylated locus in two of the three adult clones in mid-age and none of four senescent clones, indicating that errors in DNA methylation disappear with advancement of animals' aging.

Expression of enhanced green fluorescent protein in porcine- and bovine-cloned embryos following interspecies somatic cell nuclear transfer of fibroblasts transfected by retrovirus vector

Interspecies somatic cell nuclear transfer (iSCNT) has emerged as an important tool for studying nucleo-cytoplasmic interactions and cloning of animals whose oocytes are difficult to obtain. This study was designed to explore the feasibility of employing transgenic fibroblasts as donor cells for iSCNT. The study examined the chromatin morphology, in vitro development, and expression of an enhanced green fluorescent protein (EGFP) gene in porcine- and bovine-cloned embryos produced by iSCNT of fetal fibroblast transfected with a pLNbeta-EGFP retroviral vector. Parthenogenetic and transfected or nontransfected intraspecies SCNT embryos were used as controls for comparison. Analysis of data revealed that xenogenic oocyte was able to reprogram somatic cells of different genus and supports their in vitro development to the blastocyst stage. However, the developmental rates of transgenic iSCNT embryos to the blastocyst stage were significantly lower than those of intraspecies SCNT embryos. The reduction in development rates was however, not due to integration of the transgene as the lower (P < 0.05) development rates of the intraspecies SCNT porcine or bovine embryos did not differ between transgenic and nontransgenic groups. Expression of EGFP was observed in 100% of blastocysts and mosaicism was not observed. Furthermore, after iSCNT of porcine or bovine donor nuclei into xenogenic ooplasm, patterns of nuclear remodeling in reconstructed embryos were similar. In conclusion, our data demonstrated the feasibility of producing transgenic iSCNT embryos. To our knowledge, this is the first report of transgenic cloned embryo production by iSCNT approach. In the future, this may provide a powerful research tool for studying developmental events in domestic animals and provide marked cell lines for other genetic manipulations.

Nuclear and microtubule remodeling and in vitro development of nuclear transferred cat oocytes with skin fibroblasts of the domestic cat (Felis silvestris catus) and leopard cat (Prionailurus bengalensis)

The leopard cat (Prionailurus bengalensis), a member of the felidae family, is a threatened animal in South Korea. In terms of protecting endangered felids, nuclear transfer (NT) is a potentially valuable technique for assuring the continuation of species with dwindling numbers. In the present experiment, nuclear and microtubule remodeling and the in vitro developmental potential of enucleated domestic cat oocytes reconstructed with nuclei of somatic cells from either domestic cat fibroblast (DCF) or leopard cat fibroblast (LCF) were evaluated. Microtubule aster is allocated to de-condensed chromatin following nuclear transfer (3h after activation) of fibroblast cells from both domestic and leopard cats, suggesting the introduction of a somatic cell centrosome. The transferred fibroblast nuclei formed a large, swollen, pronuclear-like structure in most reconstructed oocytes, in the cat or leopard cat. At 18h following nuclear transfer, mitosis occurred, and according to the photo (F) it appears that spindle microtubules and two asters were observed. The percentages of blastocyst formation from nuclear transfer embryos derived from domestic cat fibroblasts (4/46, 8.6%) were not significantly different than those for nuclear transfer embryos constructed with leopard cat fibroblasts (4/52, 7.6%). These results indicate that nuclear and microtubule remodeling processes and in vitro developmental ability are similar in reconstructed cat oocytes following transfer of nuclei from either domestic or leopard cats.

Nuclear transfer in cats and its application

Nuclear transfer (NT) technology is typically used for generating identical individuals, but it is also a powerful resource for understanding the cellular and molecular aspects of nuclear reprogramming. Most recently, the procedure has been used in humans for producing patient-specific embryonic stem cells. The successful application of NT in cats was demonstrated by the birth of domestic and non-domestic cloned kittens at a similar level of efficiency to that reported for other mammalian species. In cats, it has been demonstrated that either in vivo or in vitro matured oocytes can be used as donor cytoplasts. The length of in vitro oocyte maturation affects in vitro development of reconstructed embryos, and oocytes matured in vitro for shorter periods of time are the preferred source of donor cytoplasts. For NT, cat somatic cells can be synchronized into the G0/G1 phase of the cell cycle by using different methods of cell synchronization without affecting the frequency of in vitro development of cloned embryos. Also, embryo development to the blastocyst stage in vitro is not influenced by cell type, but the effect of cell type on the percentage of normal offspring produced requires evaluation. Inter-species NT has potential application for preserving endangered felids, as live offspring of male and female African wildcats (AWC, Felis silvestris lybica) have been born and pregnancies have been produced after transferring black-footed cat (Felis nigripes) cloned embryos into domestic cat (Felis silvestris catus) recipients. Also, successful in vitro embryo development to the blastocyst stage has been achieved after inter-generic NT of somatic cells of non-domestic felids into domestic cat oocytes, but no viable progeny have been obtained. Thus, while cat cytoplasm induces early nuclear remodeling of cell nuclei from a different genus, the high incidence of early embryo developmental arrest may be caused by abnormal nuclear reprogramming. Fetal resorption and abortions were frequently observed at various stages of pregnancy after transfer of AWC cloned embryos into domestic cat recipients. Abnormalities, such as abdominal organ exteriorization and respiratory failure and septicemia were the main causes of death in neonatal cloned kittens. Nonetheless, several live domestic and AWC cloned kittens have been born that are seemingly normal and healthy. It is important to continue evaluating these animals throughout their lives and to examine their capability for natural reproduction.

Mitochondria and the success of somatic cell nuclear transfer cloning: from nuclear-mitochondrial interactions to mitochondrial complementation and mitochondrial DNA recombination

The overall success of somatic cell nuclear transfer (SCNT) cloning is rather unsatisfactory, both in terms of efficacy and from an animal health and welfare point of view. Most research activities have concentrated on epigenetic reprogramming problems as one major cause of SCNT failure. The present review addresses the limited success of mammalian SCNT from yet another viewpoint, the mitochondrial perspective. Mitochondria have a broad range of critical functions in cellular energy supply, cell signalling and programmed cell death and, thus, affect embryonic and fetal development, suggesting that inadequate or perturbed mitochondrial functions may adversely affect SCNT success. A survey of perinatal clinical data from human subjects with deficient mitochondrial respiratory chain activity has revealed a plethora of phenotypes that have striking similarities with abnormalities commonly encountered in SCNT fetuses and offspring. We discuss the limited experimental data on nuclear-mitochondrial interaction effects in SCNT and explore the potential effects in the context of new findings about the biology of mitochondria. These include mitochondrial fusion/fission, mitochondrial complementation and mitochondrial DNA recombination, processes that are likely to be affected by and impact on SCNT cloning. Furthermore, we indicate pathways that could link epigenetic reprogramming and mitochondria effects in SCNT and address questions and perspectives for future research.

Reprogrammationet épigenèse

The fact that the nucleus of a differentiated somatic cell can be reprogrammed in order to sustain embryonic development is now well established. Experiments of somatic cell nuclear transfer (cloning) have proved that a foreign nucleus introduced into an enucleated oocyte can give rise to physiologically normal offsprings, with a normal lifespan. Such evidence of genome expression plasticity is also observed experimentally with heterokaryons, created by the fusion or the nuclear transfer between two somatic cells, where differentiated nuclei are able to express genes characteristic of the host cell. However, the epigenetic mechanisms that permit nuclear plasticity remain poorly understood. In this paper we present the main evidences showing important modifications of the large scale organisation of chromosomal domains and of the DNA methylation pattern upon nuclear transfer and during the first cleavages. These modifications of epigenetic marks, brought by an intimate contact between the chromatin and the recipient oocyte cytoplasmic factors, appear essential for further development. They are established over the first cell cycles of development. The onset of embryonic genome activation and the first cellular differentiation events that occur over the implantation period are two additional check-points of reprogramming that appear to be also highly dependent on epigenetic alterations. Beyond those stages, defective placental functions might be directly responsible for the fetal and postnatal physiopathologies frequently observed in cloned animals. No direct link between preimplantation reprogramming defaults, placental dysfunctions and low development to term has been established yet. The epigenetics studies which are now used to characterise loci specific and probably genotype dependent alterations in cloned animals of different species will provide invaluable help to define the role of epigenesis in the achievement of a developmental program.

Knockdown of the Dnmt1s transcript using small interfering RNA in primary murine and bovine fibroblast cells

RNA interference (RNAi) has rapidly developed into one of the most widely applied technologies in molecular and cellular research, and although young, is now an essential experimental tool. The versatility of RNAi, especially in mammalian species, lends to its potential applications in a wide array of fields. Without having to genetically manipulate the genome, the ability to selectively reduce the level of a specific transcript using small interfering RNA (siRNA) molecules has great appeal in studying reprogramming issues in somatic cell nuclear transfer (SCNT) embryos. In such embryos, the aberrant expression of the somatic isoform of Dnmt1 (Dnmt1s), the enzyme responsible for maintaining DNA methylation in all somatic cells, has been implicated as one factor in the improper reprogramming of the donor genome. In the present study, the ability to develop a method allowing for the knockdown, or reduction, of Dnmt1s in primary fibroblast cells, like those commonly used as karyoplast donors in SCNT studies, was investigated in primary murine and bovine fibroblast cells as well as in a compromised cell line (NIH/3T3). Two Dnmt1s-specific siRNA candidates were designed and tested. Using optimized conditions, these siRNAs were transiently transfected into the cells with total RNA and nuclear protein being collected. A 56.5% knockdown in Dnmt1s was achieved in the compromised and primary murine cells whereas Dnmt1s was reduced by 15.4% in the primary bovine cells. A reduction in Dnmt1s mRNA did not correspond to a reduction in protein as determined by immunodetection of Western blots. Overall, this study demonstrated the ability of siRNA to knockdown Dnmt1s mRNA in primary fibroblast donor cells. In order to substantially increase the efficiency while decreasing the anomalies seen in SCNT, novel techniques, like the one proposed, are needed to assist the oocyte's ability to reprogram a differentiated genome.

Cloning: questions answered and unsolved

Cloning by the transfer of adult somatic cell nuclei to oocytes has produced viable offspring in a variety of mammalian species. The technology is still in its initial stages of development. Studies to date have answered several basic questions related to such issues as genome potency, life expectancy of clones, mitochondrial fates, and feasibility of inter-species nuclear transfer. They have also raised new questions related to the control of nuclear reprogramming and function. These questions are reviewed here.