Donor cells for nuclear cloning: many are called, but few are chosen

Telomere length analysis in goat clones and their offspring

Incomplete epigenetic reprogramming of the donor genome is believed to be the cause behind the high rate of developmental mortality and post-natal anomalies observed in animal clones. It appears that overt phenotypic abnormalities are not transmitted to their progeny suggesting that epigenetic errors are corrected in the germline of clones. Here, we show variation in telomere lengths among Nigerian dwarf goat clones derived from different somatic cell types and that the offspring of two male clones have significantly shorter telomere lengths than age-matched noncloned animals. Telomere lengths were significantly shorter in skin biopsies of goat clones derived from adult granulosa cells compared to those measured for controls. Telomere lengths were highly variable in male goat clones reconstructed from fetal fibroblasts but their mean terminal repeat fragment (TRF) length was within normal range of normal goats. However, in the progeny of two male clones, mean TRF lengths were considerably shorter than age-matched controls for both skin and leukocyte samples. Evidence for possible inheritance of shortened telomeres was obtained by measuring telomere lengths in testicular biopsies obtained from the clones, which when compared with those from noncloned animals of a similar age were significantly shorter. The offspring exhibited telomere lengths intermediate to the TRF values obtained for their cloned fathers' and age-matched control testes. These results demonstrate that telomere length reprogramming in clones is dependent on the type of donor cell used and that the progeny of clones may inherit telomere length alterations acquired through the cloning procedure.

Cell confluency is as efficient as serum starvation for inducing arrest in the G0/G1 phase of the cell cycle in granulosa and fibroblast cells of cattle

The cell cycle stage of donor cells is an important factor influencing developmental ability of nuclear transfer embryos. In the present experiment, cumulus and fibroblast cells of cattle were subjected to flow cytometric cell cycle analysis before being used in somatic cloning experiments. The following experimental groups were analyzed for each cell type: (1) actively dividing cells, (2) cells confluent for 4 days, (3) cells starved for 1, 2, 3 or 5 days. Using the propidium iodide flow cytometric assay, there were no significant differences (P > or = 0.05) in the percentage of cells in G0/G1 regardless of origin and type of cell, after confluency or serum starvation. Differences with the growing cells were found (P < or = 0.01). To determine what subset of cells in G0/G1 were in the G0 subphase of the cell cycle, an immunofluorescence analysis was conducted using monoclonal anti-PCNA antibodies in a FACS assay. There were not statistically significant differences in the percentage of cells that enter G0, between confluent and any starved group for either type of cells. Bovine fibroblast cells, confluent or serum starved for 3 days, were used in nuclear transfer experiments. A slight trend for a more desirable fusion rate in starved cells was detected, and embryo cleavage was greater in starved cells, however, in vitro development to blastocysts was similar between groups. Data indicate that prolonged culture of cells in the absence of serum does not imply a shift in the percentage of cells that enter G0/G1 or G0 alone, and that confluency is sufficient to induce quiescence. This finding can be beneficial in nuclear transfer programs, because there are negative effects such as apoptosis, associated with serum starvation.

Isolation of Murine and Porcine Fetal Stem Cells from Somatic Tissue1

Adult stem cells have been previously isolated from a variety of somatic tissues, including bone marrow and the central nervous system; however, contribution of these cells to the germ line has not been shown. Here we demonstrate that fetal somatic explants contain a subpopulation of somatic stem cells (FSSCs), which can be induced to display features of lineage-uncommitted stem cells. After injection into blastocysts, these cells give rise to a variety of cell types in the resultant chimeric fetuses, including those of the mesodermal lineage; they even migrate into the genital ridge. In vitro, FSSCs exhibit characteristics of embryonic stem cells, including extended self-renewal; expression of stem cell marker genes, such as Pou5f1 (Oct4), Stat3, and Akp2 (Tnap) and growth as multicellular aggregates. We report that fetal tissue contains somatic stem cells with greater potency than previously thought, which might form a new source of stem cells useful in somatic nuclear transfer and cell therapy.

Duration of In Vitro Maturation of Recipient Oocytes Affects Blastocyst Development of Cloned Porcine Embryos

This study investigated the effects of different incubation periods for oocyte maturation and contact inhibition of donor cells as well as different osmolarities for storage of recipient oocytes on fusion rates, cleavage rates, and blastocyst yields of porcine somatic nuclear transfer (SCNT) derived embryos. In addition, the in vivo developmental potential of cloned embryos derived from the most promising SCNT protocol was tested by transfer to recipient gilts. Storage of in vitro-matured oocytes for 7.5 h in calcium-free TL-HEPES medium at 295 or 320 mOsmol prior to activation yielded significantly (p < 0.05) higher parthenogenetic blastocyst rates compared to storage in TL-HEPES with an osmolarity of 270 mOsmol (24.4 +/- 3.0% and 26.2 +/- 4.3% vs. 18.3 +/- 6.4%, respectively, mean +/- SD) and improved the visibility of the polar body. Electrical fusion of fibroblasts to enucleated oocytes matured for 38, 40, or 42 h resulted in similar fusion and cleavage rates (74.8-84.4%). However, nuclear transfer with oocytes matured for 40 h in vitro yielded significantly higher (p < 0.05) development to the blastocyst stage after 7 days of culture (14.7 +/- 1.7%) than with oocytes matured for 38 h (9.5 +/- 2.1%) or 42 h (5.1 +/- 2.1%). Contact inhibition for 24, 48, or 72 h significantly (p < 0.05) increased the proportion of cells at G0/G1 compared with cycling fibroblasts. However, duration of contact inhibition of the donor cells for either 24, 48, or 72 h had no effect on blastocyst rates of SCNT embryos. Four gilts received an average of 150 SCNT embryos (range 138-161) reconstructed with oocytes matured for 40 h; two of these became pregnant; one of them went to term and farrowed four piglets on day 115 of pregnancy. Microsatellite analysis confirmed that the clones were genetically identical with the donor cells. These results show that changes of the in vitro maturation protocol may affect in vitro development of reconstructed porcine embryos, while duration of the contact inhibition period plays a minor role for the success of porcine SCNT. The effects on in vivo development are yet to be determined.

Isolation, culture, and characterization of embryonic cell lines from vitrified sheep blastocysts

This study was conducted to isolate, to culture, and to characterize embryonic cell lines from in vitro produced vitrified sheep blastocysts. Embryos were produced and vitrified at the expanded blastocyst stage. Ten inner cell masses arising from day 6-7 blastocysts were isolated by immunosurgery, disaggregated, and cultured onto mitomocin-C-inactivated mouse STO fibroblasts (MIF). After 5 or 6 days of culture the primary cell colonies were disaggregated, seeded in a new MIF, and cultured for 3 or 4 days to form new colonies called Passage 1. These cells were then disaggregated and cultured for other two passages. The primary cell colonies and Passage 2 colonies expressed stage specific embryonic markers SSEA-1, SSEA-3, and SSEA-4, and were alkaline phosphatase positive. In the absence of feeder layer and human leukemia inhibitory factor (LIF), these cells differentiated into variety of cell types and formed embryoid bodies. When cultured for an extended period of time, embryoid bodies differentiated into derivatives of three embryonic germ (EG) layers. These were characterized by detection of specific markers for differentiation such early mesoderm (FE-C6), embryonic myosin (F1-652), neural precursor (FORSE-1), and endoderm (anti-cytokeratin 18). To our knowledge, this is the first time that embryonic cell lines from in vitro produced and vitrified ovine blastocysts have been isolated and examined for detection of SSEA markers, and embryoid bodies have been cultured and examined for specific cell surface markers for differentiation.

From fibroblasts and stem cells: implications for cell therapies and somatic cloning

Pluripotent embryonic stem cells (ESCs) from the inner cell mass of early murine and human embryos exhibit extensive self-renewal in culture and maintain their ability to differentiate into all cell lineages. These features make ESCs a suitable candidate for cell-replacement therapy. However, the use of early embryos has provoked considerable public debate based on ethical considerations. From this standpoint, stem cells derived from adult tissues are a more easily accepted alternative. Recent results suggest that adult stem cells have a broader range of potency than imagined initially. Although some claims have been called into question by the discovery that fusion between the stem cells and differentiated cells can occur spontaneously, in other cases somatic stem cells have been induced to commit to various lineages by the extra- or intracellular environment. Recent data from our laboratory suggest that changes in culture conditions can expand a subpopulation of cells with a pluripotent phenotype from primary fibroblast cultures. The present paper critically reviews recent data on the potency of somatic stem cells, methods to modify the potency of somatic cells and implications for cell-based therapies.

Comparison of neurosphere cells with cumulus cells after fusion with embryonic stem cells: reprogramming potential

Embryonic stem cells (ESCs) are the pluripotent cells that also have the capacity to induce the genomic reprogramming of differentiated somatic cells. The progressively restricted genomic potential of somatic cells observed during embryonic development can be reverted to a pluripotent state by the formation of cell hybrids with ESCs. To assess the reprogramming potential of ESCs, we investigated the reprogramming of one of two different somatic cell populations, neurosphere cells (NSCs) and cumulus cells (CCs), after fusion with ESCs. Specifically, hybrid cells were produced by cell fusion of E14 ESCs with either NSCs or CCs containing the neo/lacZ and Oct4-GFP transgenes. The first reprogramming event, observed by the presence of Oct4-GFP in the hybrid cells, could be identified on Day 2, at approximately 45 h after fusion in both ESC-NSC and ESC-CC hybrids. In addition, the two ESC–somatic cell hybrids exhibit a similar reprogramming rate and share characteristics with the E14 ESC line: (1) expression of pluripotent markers (Oct4, Rex-1 and nanog); (2) inactivation of differentiated tissue-specific gene expression; and (3) the capacity to differentiate into all three germ layers. Taken together, our results suggest that the ESC–somatic cell hybrids have fully acquired ESC characteristics and that somatic cells of different tissue origin have the same potential to be reprogrammed after fusion with ESCs.

Attempts towards derivation and establishment of bovine embryonic stem cell-like cultures

Current knowledge on the biology of mammalian embryonic stem cells (ESC) is stunningly sparse in light of their potential value in studies of development, functional genomics, generation of transgenic animals and human medicine. Despite many efforts to derive ESC from other mammalian species, ESC that retain their capacity for germ line transmission have only been verified in the mouse. However, the criterion of germ line transmission may not need to be fulfilled for exploitation of other abilities of these cells. Promising results with human ESC-like cells and adult stem cells have nourished great expectations for their potential use in regenerative medicine. However, such an application is far from reality and substantial research is required to elucidate aspects of the basic biology of pluripotent cells, as well as safety issues associated with the use of such cells in therapy. In this context, methods for the derivation, propagation and differentiation of ESC-like cultures from domestic animals would be highly desirable as biologically relevant models. Here, we review previously published efforts to establish bovine ESC-like cells and describe a procedure used in attempts to derive similar cells from bovine Day 12 embryos.

Comparison of two approaches to nuclear transfer in the bovine: hand-made cloning with modifications and the conventional nuclear transfer technique

The aim of the present study was to compare the in vitro and in vivo developmental competence of hand-made cloning (HMC) embryos with the conventional nuclear transfer (NT) method using five somatic cell lines and in vitro-fertilised (IVF; control) embryos. Modifications to the HMC procedure included fusion efficiency optimisation, effect of cytoplasmic volume and cloned embryo aggregation. The developmental competence of blastocysts from each of the treatment groups and cell lines used was assessed following transfer to 345 recipients. Vitrification was also used to enable management of recipient resources and to assess the susceptibility of membranes to cryopreservation following zona removal. Increasing cytoplasmic volume to 150% or aggregating two embryos improved the blastocyst development rate and increased the total cell number. Although HMC embryo transfers established a significantly higher pregnancy rate on Day 30 than fresh IVF or NT embryo transfers, the overall outcome in terms of cloned live births derived from either fresh or vitrified/thawed HMC or NT embryo transfers across the five cell lines did not differ. The birth and continued survival of clones produced with HMC technology with equivalent efficiency to NT shows that it can be used as an alternative method for the generation of cloned offspring in the bovine.

Expression profiling of genes crucial for placental and preimplantation development in bovine in vivo, in vitro, and nuclear transfer blastocysts

Placental abnormalities and failed implantation are characterized phenotypes that occur in many species as a result of somatic cell cloning. This study examines a number of genes, critical for early placental development and reports aberrant expression patterns in a number of cloned bovine blastocysts, thus implicating a role of these genes in failed implantation. Messenger RNA (mRNA) expression of eight genes critical for early placental and preimplantation development including Acrogranin, Cdx2, Eomes, ErbB3, ERR2, Hand1, MRJ, and Rex1 were analyzed in single, in vivo, in vitro, and cloned bovine blastocysts (produced by hand-made cloning (HMC) and serial hand-made cloning (SHMC)) following complementary DNA (cDNA) amplification with a SMART cDNA synthesis kit. Aberrant expression of Acrogranin, Cdx2, and ERR2 was detected in a number of blastocysts produced by SHMC. Other genes, Eomes and Hand1, were not detectable in, in vivo bovine blastocysts, suggesting a differential expression pattern between bovine and murine embryos. A number of control marker genes including Oct4, IFN-tau, and PolyA were expressed in all single blastocysts analyzed. This is the first study to report that failure of implantation may be due to aberrant expression of genes in the preimplantation cloned embryo, which are crucial for the early regulation and differentiation of the placenta.

Embryos Derived from Porcine Skin-Derived Stem Cells Exhibit Enhanced Preimplantation Development1

Ongoing research to identify the most suitable type of donor cell for nuclear transfer (NT) has suggested that less differentiated stem cells may be better donors than other somatic cell types. Recently, we have reported the isolation and characterization of porcine skin-originated sphere (PSOS) stem cells from fetal skin, making it possible to test this hypothesis in a nonrodent animal model. In the present study, we have investigated and compared the feasibility and preimplantation developmental efficiency of using fetal PSOS cells and fibroblasts as nuclear-transfer donors. The majority of fetal PSOS cells are in the G1/ G0 stage of the cell cycle, which is desirable for NT. During long-term in vitro culture, fetal PSOS cells had greater genome stability, with a lower frequency of abnormal karyotypes than fetal fibroblast cells. Embryos cloned from PSOS cells showed enhanced preimplantation development compared with fibroblast cloned embryos, which is indicated by an increased rate of blastocyst development and a higher total cell number in Day 7 blastocysts. The gene expression profile of genes critical for early development from eight-cell-stage PSOS NT embryos more closely resembled the pattern observed from in vivo-produced embryos compared with that of fibroblast-cloned embryos. Cumulatively, our data suggest that fetal PSOS cells may be better donor cells for NT in the pig.

Cloning cattle

Over the past six years, hundreds of apparently normal calves have been cloned worldwide from bovine somatic donor cells. However, these surviving animals represent less than 5% of all cloned embryos transferred into recipient cows. Most of the remaining 95% die at various stages of development from a predictable pattern of placental and fetal abnormalities, collectively referred to as the "cloning-syndrome." The low efficiency seriously limits commercial applicability and ethical acceptance of somatic cloning and enforces the development of improved cloning methods. In this paper, we describe our current standard operating procedure (SOP) for cattle cloning using zona-free nuclear transfer. Following this SOP, the output of viable and healthy calves at weaning is about 9% of embryos transferred. Better standardization of cloning protocols across and within research groups is needed to separate technical from biological factors underlying low cloning efficiency.

Interpretation of reprogramming to predict the success of somatic cell cloning

In the context of mammalian somatic cell cloning, the term reprogramming refers to the processes that enable a somatic cell nucleus to adopt the role of a zygotic nucleus. Gene re-expression is one measure of reprogramming if correlated with subsequent developmental potential. This paper describes several experiments utilizing pre-implantation gene expression to evaluate reprogramming and clone viability. We have established a direct correlation between Oct4 expression in mouse clones at the blastocyst stage and their potential to maintain pluripotent embryonic cells essential for post-implantation development. Furthermore, the quality of gene expression in clones dramatically improves when genetically identical clones are combined in clone-clone aggregate chimeras. Clone--clone aggregates exhibit a higher developmental potential than single clones both in vitro and in vivo. This could be mediated by complementation between blastomeres from epigenetically different clones within the aggregate rather than by the increase in cell number resulting from aggregation. We also discuss the use of tetraploid embryos as a model to evaluate reprogramming using gene expression and demonstrate that somatic cell nuclei can be reprogrammed by blastomeres to re-express embryonic specific genes but not to contribute to post-implantation development.

Stem cells with multilineage potential derived from porcine skin

Stem cells from farm animals are valuable cell models for the study of development, differentiation, and are potential efficient donors for nuclear transfer. Here we report the isolation and characterization of stem cells from porcine skin. These porcine skin-originated sphere (PSOS) cells expressed the neural progenitor marker, nestin, as well as genes that are critical for pluripotency such as Oct4 and Stat3. The PSOS cells proliferated actively in vitro and retained normal karyotype after long-term culture. When cultured in defined medium, they generated cells with characteristics of neurons and astrocytes. A subpopulation of cells differentiated into adipocyte-like cells when cultured in 10% fetal bovine serum. Clonal study demonstrated that PSOS exhibited clonal-generating capability. Clonal populations from individual stem cells could form neuron-, astrocyte-, and adipocyte-like cells upon inducted differentiation. Our findings represent the first report of skin-originated stem cells isolated from non-rodent animals.

Review of Enucleation Methods and Procedures Used in Animal Cloning: State of the Art

Enucleation of a recipient oocyte is a crucially important process for nuclear transfer efficiency. Several procedures have been developed and used in the production of nuclear transfer embryos. Although the use of excitable fluorochromes and ultraviolet (UV) light are commonly used for complete enucleation, they also pose the risk of damaging the maternal cytoplast. Telophase and chemically assisted enucleation have also been used for cloning, but the quality and quantity of the recipient cytoplasm varies with the procedure used. This paper reviews various methods used for enucleation, and discusses their benefits and limitations with respect to cloning efficiency.

Nuclear reprogramming in mammalian somatic cell nuclear cloning

Nuclear cloning is still a developing technique used to create genetically identical animals by somatic cell nuclear transfer into unfertilized eggs. Despite an intensive effort in a number of laboratories, the success rate of obtaining viable offspring from this technique remains less than 5%. In the past few years many investigators reported the reprogramming of specific nuclear activities in cloned animals, such as genome-wide gene expression patterns, DNA methylation, genetic imprinting, histone modifications and telomere length regulation. The results highlight the tremendous difficulty the clones face to reprogram the original differentiation status of the donor nuclei. Nevertheless, nuclei prepared from terminally differentiated lymphocytes can overcome this barrier and produce apparently normal mice. Study of this striking nuclear reprogramming activity should significantly contribute to our understanding of cell differentiation in more physiological settings.

Nuclear-cytoplasmic interactions affect in utero developmental capacity, phenotype, and cellular metabolism of bovine nuclear transfer fetuses

We generated a clone of bovine somatic cell nuclear transfer embryos using oocyte pools from defined maternal sources to study nuclear-cytoplasmic interactions. Nucleocytoplasmic hybrids were reconstructed with Bos taurus (Brown Swiss) granulosa cells and oocytes that contained B. taurus A (Simmental), B. taurus B (Simmental), or Bos indicus (Dwarf Zebu) cytoplasm. Another set of embryos was reconstructed with randomly selected Brown Swiss (B. taurus R) oocytes. Embryo transfer resulted in nine (12.5%), nine (13.8%), three (50%), and 11 (16.7%) Day 80 fetuses, of which eight (11.1%), three (4.6%), three (50%), and 10 (15.2%) were viable, respectively. The proportion of viable fetuses was affected by cytoplasm (likelihood ratio test, P < 0.02) and was higher for embryos with B. indicus cytoplasm than for the B. taurus A (P < 0.05) and B (P < 0.01) groups. Furthermore, the proportion of surviving Day 80 fetuses was reduced for B. taurus B as compared with B. taurus A and B. taurus R cytoplasm (P < 0.05 and P < 0.02). Body weight of nucleocytoplasmic hybrid fetuses was not significantly different from Brown Swiss control fetuses produced by artificial insemination (AI), but fetuses reconstructed with random cytoplasts of the same breed as the nuclear donor exhibited overgrowth (P < 0.01) and a higher coefficient of variation in weight. Furthermore, body weight, crown rump length, thorax circumference (P < 0.05), and femur length (P < 0.01) of fetuses with B. taurus A cytoplasm differed from fetuses with B. taurus R cytoplasms. Fetal skin, heart, and liver cells with B. indicus cytoplasm showed a greater increase in number per time period (P < 0.001) and oxygen consumption rate per cell (skin and liver, P < 0.001; heart, P < 0.08) in comparison with their counterparts with B. taurus A cytoplasm. These data point to complex oocyte cytoplasm-dependent epigenetic modifications and/or nuclear DNA-mitochondrial DNA interactions with relevance to nuclear transfer and other reproductive technologies such as ooplasmic transfer in human assisted reproduction.

Introduction to Cloning by Nuclear Transplantation

Despite its long history, the cloning of animals by nuclear transplantation is going through a "renaissance" after the birth of Dolly. The amount of work and achievements obtained in the last seven years are probably greater than those obtained in half a century of research. However, the principal obstacles outlined years ago with the work on somatic cell cloning in amphybia, are all still there in mammals. The importance of somatic cell nuclear transfer is, without any doubt, beyond the scope of replicating superior animal genotypes. It is an invaluable experimental tool to address fundamental scientific issues such as nuclear potency, cell de-differentiation, chromatin structure and function, epigenetics, and genome manipulation. For these reasons the importance of cloning is not for what it can achieve but for the technical support it can provide to biomedical research and in particular to the study of epigenetics, cancer and stem cell biology, cell therapy and regenerative medicine. In this introductory paper we will summarize the intellectual and technical framework of cloning animals by nuclear transfer that still remains the only absolute way of judging the success of the procedure. Together with the achievements of the recent past we will mention the very last developments and the many questions that still remain open. Current research efforts are expected to provide some answers and certainly new questions.

Architectural defects in pronuclei of mouse nuclear transplant embryos

Reprogramming somatic nuclear function by transplantation of nuclei into recipient oocytes is associated with a morphological remodeling of the somatic nucleus. Successful cloning of animals by nuclear transplantation (NT) demonstrates that reprogramming somatic cell function is possible. However, low pregnancy rates and high frequencies of lethal abnormalities in animals born suggest that reprogramming is rarely complete. To address this issue, we tested the hypothesis that nuclear transplantation leads to nuclear remodeling deficiencies. We report the identification of several markers of morphological remodeling, or lack thereof, of mouse cumulus cell nuclei after transplantation into oocytes. Notably, nuclear transplant mouse embryos exhibit nuclear assembly of the differentiated cell-specific A-type lamins at the one-cell stage, as a result of misregulation of lamin A gene expression. The transplanted nuclei also display enhanced concentration of the nuclear matrix-associated protein NuMA as a result of translation from maternal mRNA and de novo transcription. The A-kinase anchoring protein 95 (AKAP95), a marker of the nuclear envelope-chromatin interface, is of somatic origin. Furthermore, greater resistance of AKAP95 and DNA to in situ extractions of one-cell stage NT embryos with non-ionic detergent, DNase, RNase and NaCl reflects an enhanced proportion of heterochromatin in these embryos. Passage through first embryonic mitosis does not rescue the defects detected in one-cell stage embryos. We propose that somatic nuclear reprogramming deficiencies by NT might emanate from, at least in part, failure to remodel the somatic nucleus morphologically into a functional embryonic nucleus.

Nuclear transfer of synchronized african wild cat somatic cells into enucleated domestic cat oocytes

The African wild cat is one of the smallest wild cats and its future is threatened by hybridization with domestic cats. Nuclear transfer, a valuable tool for retaining genetic variability, offers the possibility of species continuation rather than extinction. The aim of this study was to investigate the ability of somatic cell nuclei of the African wild cat (AWC) to dedifferentiate within domestic cat (DSH) cytoplasts and to support early development after nuclear transplantation. In experiment 1, distributions of AWC and DSH fibroblasts in each cell-cycle phase were assessed by flow cytometry using cells cultured to confluency and disaggregated with pronase, trypsin, or mechanical separation. Trypsin (89.0%) and pronase (93.0%) yielded higher proportions of AWC nuclei in the G0/G1 phase than mechanical separation (82.0%). In contrast, mechanical separation yielded higher percentages of DSH nuclei in the G0/G1 phase (86.6%) than pronase (79.7%) or trypsin (74.2%) treatments. In both species, pronase induced less DNA damage than trypsin. In experiment 2, the effects of serum starvation, culture to confluency, and exposure to roscovitine on the distribution of AWC and DSH fibroblasts in various phases of the cell cycle were determined. Flow cytometry analyses revealed that the dynamics of the cell cycle varied as culture conditions were modified. Specifically, a higher percentage of AWC and DSH nuclei were in the G0/G1 phase after cells were serum starved (83% vs. 96%) than were present in cycling cells (50% vs. 64%), after contact inhibition (61% vs. 88%), or after roscovitine (56% vs. 84%) treatment, respectively. In experiment 3, we evaluated the effects of cell synchronization and oocyte maturation (in vivo vs. in vitro) on the reconstruction and development of AWC-DSH- and DSH-DSH-cloned embryos. The method of cell synchronization did not affect the fusion and cleavage rate because only a slightly higher percentage of fused couplets cleaved when donor nuclei were synchronized by serum starvation (83.0%) than after roscovitine (80.0%) or contact-inhibition (80.0%). The fusion efficiency of in vivo and in vitro matured oocytes used as recipient cytoplasts of AWC donor nuclei (86.6% vs. 85.2%) was similar to the rates obtained with DSH donor nuclei, 83.7% vs. 73.0%, respectively. The only significant effect of source of donor nucleus (AWC vs. DSH) was on the rate of blastocyst formation in vitro. A higher percentage of the embryos derived from AWC nuclei developed to the blastocyst stage than did embryos produced from DSH nuclei, 24.2% vs. 3.3%, respectively (P < 0.05). In experiment 4, the effect of calcium in the fusion medium on induction of oocyte activation and development of AWC-DSH-cloned embryos was determined. The presence of calcium in the fusion medium induced a high incidence of cleavage of DSH oocytes (54.3%), while oocyte cleavage frequency was much lower in the absence of calcium (16.6%). The presence or absence of calcium in the fusion medium did not affect the fusion, cleavage, and blastocyst development of AWC-DSH-cloned embryos. In experiment 5, AWC-DSH-cloned embryos were transferred to the uteri of 11 synchronized domestic cat recipients on Day 6 or 7 after oocyte aspiration. Recipients were assessed by ultrasonography on Day 21 postovulation, but no pregnancies were observed. In the present study, after NT, AWC donor nuclei were able to dedifferentiate in DSH cytoplasts and support high rates of blastocyst development in vitro. Incomplete reprogramming of the differentiated nucleus may be a major constraint to the in vivo developmental potential of the embryos.

Nuclear cloning, stem cells, and genomic reprogramming

The generation of adult animals by nuclear cloning from adult donor cells is extremely inefficient, with most clones dying soon after implantation. In contrast, cloning from embryonic stem cell donor nuclei is significanty more efficient than from adult donor cells. However, regardless of donor cell type, all clones that survive to birth and beyond suffer serious phenotypic and gene expression abnormalities. All available evidence is consistent with the notion that the anomalous phenotypes of cloned animals are caused by faulty epigenetic reprogramming of the donor nucleus. Faulty reprogramming appears to be caused by the cloning process itself as well as by the epigenetic state of the donor nucleus. In contrast to reproductive cloning, faulty reprogramming of the donor nucleus does not tend to interfere with the application of nuclear transfer technology for therapeutic purposes (therapeutic cloning).

Practical aspects of donor cell selection for nuclear cloning

Choosing the right nuclear donor is the most critical decision in cloning by nuclear transfer (NT), or nuclear cloning, because the cloned animal will be a genetic copy of the donor cell genome used for NT. Both donor cell type and cell cycle stage are important methodological parameters and influence nuclear cloning efficiency. Cloning, however, is a multi-step procedure and the exact contribution of the nuclear donor to overall cloning success must be determined in comparative studies. This requires strict standardization of isolation, purification, and culture protocols, and application of stringent identification criteria in order to obtain a homogenous donor cell population. In all these respects, the standards in the cloning field are currently poor. The aim of this review is to provide a brief guideline for the major practical aspects of donor cell selection, cell cycle synchronization and preparation for NT.

Coordination between donor cell type and cell cycle stage improves nuclear cloning efficiency in cattle

Several studies have shown that both quiescent and proliferating somatic donor cells can be fully reprogrammed after nuclear transfer (NT) and result in viable offspring. So far, however, no comparative study has conclusively demonstrated the relative importance of donor cell cycle stage on nuclear cloning efficiency. Here, we compare two different types of bovine fetal fibroblasts (BFFs) that were synchronized in G(0), G(1), and different phases within G(1). We show that for non-transgenic (non-TG) fibroblasts, serum starvation into G(0) results in a significantly higher percentage of viable calves at term than synchronization in early G(1) or late G(1). For transgenic fibroblasts, however, cells selected in G(1) show significantly higher development to calves at term and higher post-natal survival to weaning than cells in G(0). This suggests that it may be necessary to coordinate donor cell type and cell cycle stage to maximize overall cloning efficiency.