Influence of cell cycle stage at nuclear transplantation on the development in vitro of mouse embryos

From germ cell preservation to regenerative medicine: an exciting research career in biotechnology

Collection, manipulation, assessment, and storage of mammalian gametes, embryos, and stem cells are providing important opportunities in agriculture, research, and medicine. Semen and embryo freezing in livestock are used in breeding schemes, especially in cattle and for international trade, with no risk of spreading disease. In human medicine, they are used in treatment of infertility. Usually, knowledge gained in one species is applicable in the others. In one exception, some ruminant embryos cultured according to protocols used in human in vitro fertilization become unusually large offspring. This is due to disturbances in expression of imprinted genes. The nuclear transfer procedure developed at the Roslin Institute is being used to make genetic modifications in livestock to either direct production of biomedical proteins, create animal models of human disease, or enhance animal health and productivity. Human pluripotent cells are being used in Edinburgh to identify drugs to treat degenerative diseases.

Sheep: the first large animal model in nuclear transfer research

The scope of this article is not to provide an exhaustive review of nuclear transfer research, because many authoritative reviews exist on the biological issues related to somatic and embryonic cell nuclear transfer. We shall instead provide an overview on the work done specifically on sheep and the value of this work on the greater nuclear transfer landscape.

Assessment of cytoplasmic effects on the development of mouse embryonic nuclei transferred to enucleated zygotes

In this study, cytoplasmic effects on the development of nuclear transplant embryos were examined. In addition, the production of offspring from nuclear transplant embryos was attempted. Nuclei from cleavage-stage embryos were transplanted to enucleated zygotes at different cell cycle stages and with different cytoplasmic volumes. A greater developmental rate to the blastocyst stage was observed in reconstituted late stage zygotes that received nuclei from late 2-cell stage embryos than in early stage zygotes (46.3% vs. 16.9%). A further increase in developmental rate to the blastocyst stage (85.5%) and in cell number was obtained in reconstituted late stage zygotes with reduced cytoplasmic volume. However, developmental potential of nuclei from 4- and 8-cell stage embryos was very limited, although they were transferred to enucleated late stage zygotes with reduced cytoplasm. After the transfer of blastocysts derived from nuclear transplant embryos to recipient females, live young were obtained from reconstituted embryos that received nuclei from late 2-cell stage embryos (28.6%). These results confirm that the development of nuclear transplant embryos can be affected by recipient cell cycle stage and cytoplasmic volume. Furthermore, the nuclei from late 2-cell stage embryos in which activation of the embryonic genome had occurred can be reprogrammed to a certain extent when transplanted into enucleated zygotes, especially late stage zygotes with reduced cytoplasmic content.

Improvement of the developmental ability of nuclear transfer embryos by using blastomeres from in vitro fertilized embryos selected according to the early developmental stage and cell division status as donor cells in cattle

This study was conducted to improve the developmental ability of nuclear transfer (NT) embryos by using blastomeres from in vitro fertilized (IVF) embryos with high quality as donor cells. The IVF embryos selected at the 2-cell stage at 24-h postinsemination (hpi) and again at the ≥8-cell stage at 48 hpi (Selected-IVF-embryos) showed the highest blastocyst formation rate among embryos. When blastomeres from the Selected-IVF-embryos (Selected-NT group) or Nonselected-IVF-embryos (Non-selected-NT group) were used as donor cells for NT, the blastocyst formation rate in the Selected-NT group (25.6%) was significantly higher than that in the Non-selected-NT group (13.5%). When blastomeres from the Selected-IVF-embryos at 108 (contained many cells before cell division) and 126 hpi (contained many cells immediately after cell division) were used as donor cells for NT (108- and 126-NT groups, respectively), the 126-NT group showed a significantly higher blastocyst formation rate (32.1%) than the 108-NT group (16.8%). Embryo transfer of blastocysts in the 126-NT group showed that 11 of 23 recipients became pregnant; nine calves were obtained. For the NT embryos reconstructed using in vivo derived embryos, 9 of 20 recipients became pregnant; seven calves were obtained. These results indicate that the blastocyst formation rate of NT embryos can be improved by using blastomeres from IVF embryos selected at the early developmental stage, especially immediately after cell division, and that the resultant NT embryos have a high developmental ability to progress to term that is comparable to NT embryos reconstructed using in vivo derived embryos.

Enucleated GV oocytes as recipients of embryonic nuclei in the G1, S, or G2 stages of the cell cycle

Universal recipients in the G2 phase of mitotic cell cycle (preactivated oocytes, zygotes, blastomeres) accept embryonic nuclei in all the stages of their cell cycle. To test if recipients in the G2 of meiotic cycle (immature oocytes) are universal recipients, mouse germinal vesicle (GV) oocytes were enucleated and reconstructed with blastomere nuclei in the G1, S, or G2 stages. Analysis of their maturation has shown that about 30% of the G1 nuclei and 60% of G2 nuclei allow for normal metaphase II (MII), both in the oocytes with and without the first polar body (1st PB). Among oocytes reconstructed with the S phase nuclei, only 8% or less have normal MII, although 75% of them extrude 1st PB. No phase of donor cell cycle prevented the abnormal acceleration of 1st PB extrusion, found in reconstructed GV oocytes. In conclusion, enucleated GV oocytes are not universal recipients of embryonic nuclei, because they do not accept the S donors. However, both the G1 and G2 donor nuclei can be reprogrammed in the GV oocyte cytoplasm.

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.

Telophase-stage host ooplasts support complete reprogramming of roscovitine-treated somatic cell nuclei in cattle

Nuclear-cytoplasmic incompatibilities are known to play a significant role in the developmental outcome of embryos produced by nuclear transfer, particularly when metaphase arrested oocytes are used as hosts for interphase donor nuclei. To further our understanding of how cell cycle coordination affects somatic cell cloning, somatic cells at different stages of the cell cycle were fused to host oocytes either before (metaphase II, M-II) or after (telophase II, T-II) activation. To obtain cells at different stages of the cell cycle, fetal fibroblast (FF) and granulosa cells (GC) were treated with roscovitine, an inhibitor of cyclin-dependent kinases (CDKs) resulting in a large percentage of cells in S/G(2)-phase. In contrast to the M-II group, which did better with confluent cells, embryos reconstructed with T-II cytoplasts resulted in higher rates of blastocyst formation when fused to cells recovered at 16-24 h after passage. Embryos reconstructed with FF treated with roscovitine and T-II cytoplasts (Rosc/T-II) resulted in similar blastocyst rate compared to those produced with confluent cells and M-II cytoplasts (Conf/M-II). Transfer of blastocysts to surrogate heifers resulted pregnancies and birth of healthy calves from Rosc/T-II and Conf/M-II reconstructed embryos. These results indicate that, when combined with nuclear donor cells at specific cell cycle stages, M-II and T-II bovine oocytes are similarly effective in supporting the reprogramming of somatic cell nuclei.

Donor Centrosome Regulation of Initial Spindle Formation in Mouse Somatic Cell Nuclear Transfer: Roles of Gamma-Tubulin and Nuclear Mitotic Apparatus Protein 11

During the process of spindle-chromosome complex depletion in the oocyte, it is unclear whether both gamma-tubulin and nuclear mitotic apparatus protein 1 (NUMA1), which are required for mitotic organization and spindle assembly, are removed. The role of the donor cell centrosome and donor nuclear NUMA1 in the initial spindle morphogenesis and chromosome remodeling also remains unclear. In the present study, we show that in the mouse, the level of gamma-tubulin in the poles and around the metaphase II spindle declines significantly, whereas only approximately 10% of NUMA1 is removed during spindle-chromosome complex depletion in the recipient oocyte. This process does not impede initial spindle morphogenesis and is regulated by the centrosome of the donor cumulus cell. Retaining the donor cell centrosome establishes a monopolar spindle, whereas prior removal of the centrosome by a narrow-bore micropipette leads to bipolar spindle formation. Our data show that the centrosome of the donor cell regulates initial spindle morphogenesis and that the donor cumulus cell NUMA1 compensates for the deficiency in recipient NUMA1 during the formation of metaphase-like structures after nuclear transfer. Full-term offspring of cloned mice were obtained after injection of donor cells only with a pipette having an inner diameter of 7-8 microm, which retained the donor cell centrosome. In contrast, removing the donor cell centrosome with a small pipette impaired preimplantation development and prevented full-term development. In conclusion, the initial spindle assembly of a metaphase-like spindle is regulated by the centrosome from the donor cell in the mouse.

Review: Genetic and Epigenetic Aspects of Cloning and Potential Effects on Offspring of Cloned Mammals

Although the biological mechanisms by which host cytoplasm and donor nuclei interact to produce a developmentally competent reconstructed embryo remain largely unknown, some advances have been made to our understanding of the genetic and epigenetic factors involved in the of reprogramming of the donor nucleus. Genetic alterations, which comprise changes to the genetic information in both the nuclear and cytoplasm compartments, are passed on to subsequent generations at fertilization and are a potential source of variation among cloned animals and their offspring. Apart from the major chromosomal anomalies found in developmentally arrested embryos and fetuses, less detrimental rearrangements and/or mutations are likely to go unnoticed in most donor cell karyotypes, suggesting that such problems could lead to inheritable anomalies among clones and their offspring. Mitochondrial DNA is also relevant to cloning because most animals inherit most or all of their mitochondria from the host oocyte. Epigenetic alterations to the DNA or to the histone packaging proteins are independent of gene sequences. Aberrant epigenetic events may lead to variable gene expression or mitosis and consequent effects on development and phenotype. Although much of the epigenetic marking is reset during embryogenesis and development, the impact of epimutations on progeny remains unexplored.

Factors controlling the loss of immunoreactive somatic histone H1 from blastomere nuclei in oocyte cytoplasm: a potential marker of nuclear reprogramming

Nuclei of differentiated cells can acquire totipotency following transfer into the cytoplasm of oocytes. While the molecular basis of this nuclear reprogramming remains unknown, the developmental potential of nuclear-transfer embryos is influenced by the cell-cycle stage of both donor and recipient. As somatic H1 becomes immunologically undetectable on bovine embryonic nuclei following transfer into ooplasm and reappears during development of the reconstructed embryo, suggesting that it may act as a marker of nuclear reprogramming, we investigated the link between cell-cycle state and depletion of immunoreactive H1 following nuclear transplantation. Blastomere nuclei at M-, G1-, or G2-phase were introduced into ooplasts at metaphase II, telophase II, or interphase, and the reconstructed embryos were processed for immunofluorescent detection of somatic histone H1. Immunoreactivity was lost more quickly from donor nuclei at metaphase than at G1 or G2. Regardless of the stage of the donor nucleus, immunoreactivity was lost most rapidly when the recipient cytoplast was at metaphase and most slowly when the recipient was at interphase. When the recipient oocyte was not enucleated, however, immunoreactive H1 remained in the donor nucleus. The phosphorylation inhibitors 6-DMAP, roscovitine, and H89 inhibited the depletion of immunoreactive H1 from G2, but not G1, donor nuclei. In addition, immunoreactive H1 was depleted from mouse blastomere nuclei following transfer into bovine oocytes. Finally, expression of the developmentally regulated gene, eIF-1A, but not of Gapdh, was extinguished in metaphase recipients but not in interphase recipients. These results indicate that evolutionarily conserved cell-cycle-regulated activities, nuclear elements, and phosphorylation-linked events participate in the depletion of immunoreactive histone H1 from blastomere nuclei transferred in oocyte cytoplasm and that this is linked to changes in gene expression in the transferred nucleus.

Mammalian cloning: advances and limitations

For many years, researchers cloning mammals experienced little success, but recent advances have led to the successful cloning of several mammalian species. However, cloning by the transfer of nuclei from adult cells is still a hit-and-miss procedure, and it is not clear what technical and biological factors underlie this. Our understanding of the molecular basis of reprogramming remains extremely limited and affects experimental approaches towards increasing the success rate of cloning. Given the future practical benefits that cloning can offer, the time has come to address what should be done to resolve this problem.

Mechanisms and control of embryonic genome activation in mammalian embryos

Activation of transcription within the embryonic genome (EGA) after fertilization is a complex process requiring a carefully coordinated series of nuclear and cytoplasmic events, which collectively ensure that the two parental genomes can be faithfully reprogrammed and restructured before transcription occurs. Available data indicate that inappropriate transcription of some genes during the period of nuclear reprogramming can have long-term detrimental effects on the embryo. Therefore, precise control over the time of EGA is essential for normal embryogenesis. In most mammals, genome activation occurs in a stepwise manner. In the mouse, for example, some transcription occurs during the second half of the one-cell stage, and then a much greater phase of genome activation occurs in two waves during the two-cell stage, with the second wave producing the largest onset of de novo gene expression. Changes in nuclear structure, chromatin structure, and cytoplasmic macromolecular content appear to regulate these periods of transcriptional activation. A model is presented in which a combination of cell cycle-dependent events and both translational and posttranslational regulatory mechanisms within the cytoplasm play key roles in mediating and regulating EGA.

Influence of the timing of blastomere isolation after the removal of nocodazole in bovine nuclear transfer

This study was conducted to investigate the influence of the timing of blastomere isolation after the removal of nocodazole on the subsequent division of blastomeres and developmental ability of reconstituted bovine embryos. The division rate of isolated blastomeres was examined at 3, 5 and 24 h of culture after nocodazole removal. Furthermore, isolated blastomeres and those of whole embryos were used as donors in nuclear transfer to determine the development of reconstituted embryos. The division rate of isolated blastomeres at 3 h was significantly lower than the presumptive division rate of blastomeres from whole embryos (P<0.05). When these blastomeres were used as donor nuclei, the dividing blastomeres yielded a significantly higher development rate than blastomeres from whole embryos (P<0.05). These results confirm that the timing of blastomere isolation influences the subsequent division of blastomeres and the developmental ability of the reconstituted embryos.

Nuclear transfer in mammals: recent developments and future perspectives

A clone can be defined as a set of genetically identical animals. Small clones of two or occasionally up to four identical animals can be obtained by embryo splitting or blastomere separation. Embryo cloning by nuclear transfer involves the transfer of genetic material from a donor cell (karyoplast) to the cytoplasm of an oocyte or zygote from which the genetic material has been removed (cytoplast). In farm animals, metaphase II oocytes are most widely used as cytoplasts. There are now many factors known to influence the efficiency of embryo cloning by nuclear transfer. These include stage of development and cell cycle of donor cells, the choice of the recipient cell, the methods for activation of oocytes, the cell cycle coordination between donor cell and recipient cytoplast, and the method for fusion between nuclear donor and recipient cytoplast. Recent progress in cloning embryos and animals from cultured cells of embryonic, fetal, or adult origin offers a wide spectrum of potential applications of nuclear transfer, such as the unlimited multiplication of elite embryos or animals from selected matings and the potential for precise genetic modification of farm animals for gene farming or xenotransplantation.

Telophase enucleation: an improved method to prepare recipient cytoplasts for use in bovine nuclear transfer

The enucleation of oocytes to be used as host cytoplasts for embryo reconstruction by nuclear transfer is an important limiting step when cloning mammals. We propose an enucleation technique based on the removal of chromatin after oocyte activation, at the telophase stage, by aspirating the second polar body and surrounding cytoplasm. In a preliminary experiment to determine an optimal activation protocol, oocytes were matured for 26 and 30 hr and exposed for 5 min to 7% ethanol and/or for 3 hr at either 25 or 4 degrees C. Relative to most activation treatments tested, oocytes matured for 30 hr and exposed to ethanol alone showed highest activation rates, as determined by low levels of H1 kinase activity within 90 min from exposure and high pronuclear formation (82%) after 12 hr of culture. No synergistic effect on activation rates was observed when oocytes also were exposed to reduced temperature after ethanol treatment. Microsurgical removal of the telophase-stage chromatin in a small volume of cytoplasm adjacent to the second polar body was significantly more effective in enucleating than aspiration of a larger cytoplasm volume surrounding the first polar body of metaphase-arrested oocytes (98% versus 59%; P < 0.01). Moreover, compared with a nuclear transfer protocol based on enucleation of metaphase-arrested oocytes followed by aging and cooling, more (38% versus 16%; P < 0.001) and better-quality blastocytes (126 versus 84 nuclei per blastocyst; P < 0.02) were obtained from embryos reconstructed using the telophase procedure. Higher development potential of embryos reconstructed by the telophase procedure may be attributed to (1) the selection of oocytes that activate and respond by extruding the second polar body, (2) avoiding the use of DNA dyes and ultraviolet irradiation, and (3) the limited removal of cytoplasm during enucleation. The ease with which telophase enucleation can be performed is likely to render this technique widely useful for research and practice on mammalian cloning.

Influence of combined activation treatments on the success of bovine nuclear transfer using young or aged oocytes

This study was determined if the combined activation of young or aged oocytes influence their development. The 16-cell stage in vitro maturation/fertilization embryos were exposed to 10 microL nocodazole for 18-20 h, blastomeres that divided within 3 h after the release from nocodazole were used as synchronized donor blastomeres. Metaphase II oocytes were enucleated at 20-22 h post onset of maturation. Enucleated oocytes were divided into 2 groups: oocytes activated at 24 h (young) and oocytes activated at 38 h (aged). In both groups (young and aged), one group of oocytes was activated in 7% ethanol alone for 5 min (alone) and the other group (combination) was activated in ethanol and subsequently incubated in 5 micrograms/ml cycloheximide in TCM199 for 6 h (combination). Electrofusion was carried out at 30 h (young) and 44 h (aged). The nuclear morphology of the blastomere-oocyte complexes at 1 h post-fusion and their development to the blastocyst stage after 6 days of culture in modified synthetic oviduct fluid were examined. Interphase and swollen nuclei were observed at 1 h post-fusion following nuclear transfer to the cytoplasm from young oocytes of combined activation and aged oocytes of combined and ethanol alone activation. When young oocytes were treated with the combined activation method, the reconstituted embryos had a significantly higher developmental rate to the blastocyst stage than the aged oocyte groups (P < 0.05). We conclude that the combined activation of young oocytes leads to a more efficient development of bovine nuclear transfer embryos.

Synchronization of cell division in eight-cell bovine embryos produced in vitro: Effects of aphidicolin

To date, methods for synchronizing the cell division of ungulate embryos without reducing their developmental potential have not been reliable or simple. The overall objective of this study was to determine the reliability of aphidicolin, a powerful inhibitor of eukaryotic DNA synthesis, to arrest and synchronize blastomere division in cleavage-stage bovine embryos and to assess its reversibility and toxicity in vitro. Eight-cell stage embryos obtained at 58 h post insemination were treated with several concentrations of aphidicolin for 12 h. Treated embryos were assessed for cleavage arrest, chromatin morphology and DNA synthesis; scored for blastocyst formation and hatching rate; and fixed for determination of the number of nuclei. Complete arrest of cell division was observed at aphidicolin concentrations of 1.4 microM and above. At these concentrations, no morphological alteration to interphase chromatin was observed in treated embryos compared with the controls. Removal of aphidicolin led to at least a 4-h delay before resumption of DNA synthesis and cleavage. The ability of treated embryos to reach the blastocyst stage in vitro, the hatching rate and the number of cells per blastocyst were significantly reduced compared with the control group. Since the ability of treated embryos to develop to the blastocyst stage was significantly reduced even at the minimal effective dosage, it is concluded that aphidicolin is unlikely to provide suitable cell cycle synchronization without damage to the embryos.

Nuclear transfer in sheep embryos: the effect of cell-cycle coordination between nucleus and cytoplasm and the use of in vitro matured oocytes

The developmental ability of nuclear transplant sheep embryos derived from in vitro matured oocytes was studied by controlling cell-cycle coordination of donor embryonic nuclei and recipient cytoplasts. Oocytes were recovered from nonatretic antral follicles of adult sheep ovaries and cocultured with follicle shells in M199-based medium supplemented with gonadotrophins in a nonstatic system. Effective activation if IVM oocytes was obtained by applying two pulses of 1.0 kv/cm 22 min apart in inositol-based electroporation medium to oocytes matured in vitro for 27 hr. Synthesis of DNA (S-phase) was assessed by BrdU incorporation and was found to initiate around 5 hpa (hours postactivation) and to persist until 18 hpa. Mitotic blastomeres were induced by treating embryos with 6.6 microM nocodazole for 14-17 hr. Three types of transfers were compared directly: "S-->S," early embryonic nuclei (mostly in S-phase) were transferred to presumptive S-phase cytoplasts; "M-->MII," nocodazole-treated embryonic nuclei (most in M-phase) were transferred to MII-phase cytoplasts; and control (S-->MII), conventional nuclear transfer of fusion and activation simultaneously. The results showed that fusion and recovery rates did not differ among the three groups. However, after 6 days of in vivo culture, the morula and blastocyst formation rate was significantly higher for the M-->MII combination than for the control (28.3% vs. 8.1%, P < 0.05), while no significant differences in developmental rate were observed between S-->S and M-->MII, and between S-->S and control, though developmental rate was also increased for S-->S compared to control (20.9% vs. 8.1%, P > 0.05). Transfer of blastocysts derived from M-->MII or S-->S nuclear cytoplasmic reconstitution to synchronized recipient ewes resulted in the birth of lambs. These data suggest that in vitro matured oocytes can support full-term development of nuclear transplant sheep embryos when the cell cycle of nucleus and cytoplasm is coordinated, and that M-->MII nuclear transfer might be an efficient and simple way to improve the developmental competence of the reconstituted embryos.

Embryo cloning in sheep: Work in progress

We summarize here the procedures for nuclear transfer using S-phase cytoplasts and describe a new method for avoiding loss of reconstructed embryos from the oviducts during in vivo culture. We obtained 2 clones of 5 genetically identical animals following the transfer of blastomeres from 16-cell embryos into enucleated preactivated cytoplasts. Metaphase II oocytes and embryos were surgically collected from superovulated Sarda breed ewes 54 and 120 h after sponge removal, respectively. Oocytes were exposed for 15 min to 5 mug/ml of Hoechst 33342 and were micromanipulated at room temperature. Efficiency in embryo reconstruction was 100% for enucleation and 98% for fusion. Embryos were embedded in agar as separate clones and transferred into the oviducts of temporary recipients. The fimbriae were closed with glass-nylon made filters. Embryo recovery from the temporary recipients was 97.3%, with a cleavage rate of 81.4%; development to morula-blastocyst stage was 70.6%. A total of 29 Grade 1 blastocysts corresponding to 5 clones were transferred into 13 naturally synchronous ewes, and scanning was performed at 30 and 90 d. Ten ewes were pregnant at the first scanning and nine at the second for a final pregnancy rate of 71.4%; the survival rate at term was 48%. Overall, we obtained 4 clones of identical lambs: two sets of 5 (one male set and one female set) and two sets of twins (both sets male). Pregnancy length in recipients carrying clones was longer than the standard period in Sarda breed (153 vs 150 d, respectively). Weight at birth was higher for male lambs obtained from nuclear transfer than for normal males (4.1 vs 3.6 kg), while the weight for females was normal.

Effects of cell-cycle-arrest agents on cleavage and development of mouse embryos

In mammals, there are no reliable methods for synchronizing cell division of early embryos without reducing their ability to develop into blastocysts and fetuses. The present study was undertaken to examine the in vitro inhibition of cell division of four-cell mouse embryos by cell cycle arrest agents. The reversibility of the agents was also tested by examining the developmental ability of treated embryos. Four-cell mouse embryos obtained at 54 hr post-human chorionic gonadotrophin (post-hCG) were cultured for 4, 8, 12, or 16 hr in media supplemented with either nocodazole, an inhibitor of tubulin polymerization, 6-dimethylaminopurine (6-DMAP), an inhibitor of maturation promoting factor (MPF) activation, or aphidicolin, a specific inhibitor of DNA polymerase alpha. Reversibility and toxicity of these agents were both dose and time dependent. For all three agents, prolonging cleavage arrest for 8 or 16 hr (at the effective concentrations) caused embryo lethality. Although nocodazole treatment was least cytotoxic, 6-DMAP and aphidicolin concentrations which induce cleavage arrest were detrimental to development beyond the blastocyst stage. The results of this study show that the development of embryos treated with these three cell-cycle-arrest agents is dose and incubation time dependent. Toxic effects beyond the blastocyst stage could only be minimized for nocodazole by reducing the exposure time of treatment and concentration of the mitotic inhibitor. However, these results render doubt on the usefulness of 6-DMAP and aphidicolin for synchronization studies leading to embryo transfer procedures.

Transfer of nuclei from 8-cell stage mouse embryos following use of nocodazole to control the cell cycle

Mouse 2-, 4-, 8-, and 16-cell embryos were exposed to nocodazole in M16 culture medium. The effect of different concentrations and exposure times on the efficiency of cell cycle synchronization and the development of the treated embryos after release from the drug was determined. The minimum effective concentration (> 95% of arrested nuclei) for 4-, 8-, and 16-cell embryos was 5 microM nocodazole. The effect upon subsequent development of mouse embryos depended upon both the stage of development of the embryo at treatment (P < 0.001) and the length of exposure to nocodazole (P < 0.001). Exposure to any concentration of nocodazole within the range 2.5-10 microM for 12 hr caused a reduction in the proportion of embryos that formed blastocysts. As the period of exposure to 5 microM nocodazole increased from 12 to 24 hr, the proportion of embryos developing to the blastocyst stage decreased. The lower proportion of embryos developing to the blastocyst stage and to term (P < 0.01) suggests that the more advanced stages were more susceptible to damage as a result of exposure to nocodazole. The rate of development of 4-cell embryos to blastocysts was not affected when an exposure time of 9 hr was used. Together these results show that it is possible to use nocodazole to arrest mouse embryonic cells in mitosis but that it is not appropriate to culture the embryos in the presence of this drug for prolonged periods. Individual blastomeres completed mitosis at 60-90 min and started DNA synthesis at 120-150 min after release from nocodazole.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of rat x mouse hybrid embryos produced by microsurgery

Experimental production of hybrid embryos between Mus musculus L. and Rattus norvegicus L. was achieved by nuclear transplantation using both ovulated oocytes in metaphase II and pronuclear zygotes. Recipient egg-cells were of mouse origin in all cases. The developmental potential of hybrids was examined in vivo. Nucleo-cytoplasmic hybrids resulting from the introduction of rat metaphase II chromosomes into enucleated mouse oocytes, which were subsequently activated, were regularly blocked at the 1- or 2-cell stage. Nuclear (genetic) hybrids produced by transfer of a rat nucleus (in the form of metaphase II chromosomes or a pronucleus) into a nucleated mouse recipient (oocyte or zygote) were capable of development to the 5- to 8-cell stage. Transplantation of rat cytoplasm alone to intact metaphase II oocytes, followed by oocyte activation, generated cytoplasmic hybrids which developed to the morula stage. In control experiments (nuclear transfer between mouse oocytes or zygotes), a high proportion of embryos formed morulae and blastocysts. These results demonstrate that the rat nucleus is incapable of functioning in mouse cytoplasm, that introduction of the rat genome into intact mouse egg-cells impairs normal development, and that transfer of foreign (rat) cytoplasm into mouse egg-cells affects preimplantation development of manipulated embryos.

Bovine nuclear transfer embryos: oocyte activation prior to blastomere fusion

Successful bovine nuclear transfer (NT) embryo production requires proper oocyte activation and transfer of a nucleus into this oocyte. However, the temporal relationship between these two events is unclear. The current study examined whether activation of the oocyte prior to fusion would induce nuclear swelling while also affecting development to morula and blastocyst stage and finally development to offspring. Aged oocytes can be activated by a number of techniques including exposure to room temperature. In this study oocyte activation was induced through three different means: reduced temperature culture alone, reduced temperature culture and calcium ionophore, and naturally, through the fertilization process. Electrofusion was carried out after the activation stimulus. When used in the NT procedure, activation of oocytes prior to fusion resulted in NT embryos that underwent nuclear swelling and had a high developmental rate to morula and blastocyst stages. Also, these NT embryos developed to normal offspring when transferred to recipient animals. The addition of a calcium ionophore treatment to the reduced temperature culture was not beneficial and resulted in less nuclear swelling. The use of enucleated fertilized oocytes as recipient cytoplasm for the new nucleus resulted in NT embryos developing to morula and blastocyst stages at the same rate as room temperature activated NT embryos. Therefore, improved embryo development can be obtained from NT embryos if the aged recipient oocyte is activated prior to the time of fusion. Also, offspring were obtained from these pre-activated NT embryos.

Influence of recipient oocyte cell cycle stage on DNA synthesis, nuclear envelope breakdown, chromosome constitution, and development in nuclear transplant bovine embryos

Nuclear transplantations into metaphase II (MII) and S phase oocyte cytoplasm were performed to investigate the influence of recipient cell cycle stage on nuclear function and development of bovine nuclear transplant (NT) embryos. Rate of inactivation of histone H1 kinase and duration of DNA synthesis in activated oocytes were determined. The proportion of S phase blastomeres in in vivo produced day 5.5 bovine embryos was measured. DNA synthesis was also assessed in NT embryos after transfer into MII and S phase cytoplasm. MII NT embryos were produced by fusing a blastomere into a MII oocyte; the fusion pulse served to activate the oocyte. S NT embryos were produced by fusing a blastomere into an early S phase oocyte electrically activated 4 h prior to fusion. Nuclear envelope structure, chromosome constitution, and extent of development were examined in MII and S NT embryos. Histone H1 kinase activity dropped to baseline within 2 h of electrical activation. A second electrical pulse did not alter H1 kinase activity when delivered 4 h after the first pulse. The frequency of S phase blastomeres in day 5.5 bovine embryos ranged from 79% to 100%, depending on the duration of culture in 3H-thymidine. Nuclear transplantation into MII cytoplasm resulted in a transient drop in DNA synthesis over 3.5 h. DNA synthesis resumed at 4.5 h post activation (hpa), concomittantly with initiation of DNA replication in activated oocytes. In contrast, DNA synthesis was not interrupted after transfer into S phase cytoplasm. DNA synthesis persisted until 13.5 hpa, as in activated oocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Stage-specific gene expression in asynchronous tetraploid mouse embryos formed by fusion of blastomeres and fertilized eggs

Asynchronous tetraploid mouse embryos were generated by electrofusion of fertilized eggs with blastomeres from different cleavage stages. The majority of the cytoplasm was always contributed by the egg. The best development was observed when eggs were fused with 2-cell blastomeres. Both genomes became active in fusion embryos (at least the genes for glucose phosphate isomerase did). Stage-specific protein synthesis seemed to be more adjusted to the developmental stage of the egg's than of the blastomere's genome, but at the 2-cell stage both contributed slightly differently to the protein patterns. Also, the time range of the first appearance of the stage-specific embryonic antigen SSEA-1 was wider in fusion embryos than in controls. It seems that the two genomes are not completely synchronized in these tetraploid embryos, a further indication that, in the mouse, the cytoplasm of fertilized eggs might not be compatible with older embryonic nuclei.

Development of reconstituted mouse embryos produced from the cytoplast of bisected oocytes or pronuclear-stage embryos and single blastomeres of 2-cell stage embryos

The present study investigated the in vitro developmental potential of reconstituted mouse embryos produced from the cytoplast of pronuclear-stage embryos or oocytes and single blastomeres of 2-cell stage embryos by electrofusion. The cytoplast of pronuclear-stage embryos and oocytes were obtained by manual bisection with a fine glass needle under a dissecting microscope. The fusion rates of the reconstituted embryos produced from the cytoplast of oocytes and single blastomeres of 2-cell stage embryos (O-SB2: 38.1 and 41.5%) were significantly lower than those produced from the cytoplast of pronuclear-stage embryos and single blastomeres of 2-cell stage embryos (P-SB2: 91.2 and 97.6%; P<0.001). Reconstituted embryos were encapsulated in alginate gel and were cultured for 96 hours. Similarly, the cleavage and development rates to the blastocyst stage of O-SB2 (56.3, 61.2 and 2.0, 3.1%, respectively) were significantly lower than those of the P-SB2 (91.0, 91.2 and 18.6, 20.7%; respectively, P<0.05). The cleavage and development rates to the blastocyst stage (61.2 and 2.0%) of reconstituted embryos produced from single blastomeres of late 2-cell stage embryos and oocyte cytoplast improved after activation by ethanol treatment (76.1 and 21.7%). However, the use of single blastomeres of early 2-cell stage embryos as nuclear donors did not enhance the cleavage and development rates of the reconstituted embryos to the blastocyst stage.

Development of reconstituted mouse embryos produced from bisected and electrofused pronuclear-stage embryos

The present study was designed to investigate the in vitro and in vivo development potential of reconstituted mouse embryos produced by bisection and electrofusion of pronuclear stage embryos (PN-E). Pronuclear-stage ICR and F1 (C57BL x CBA) strain mouse embryos were bisected manually with a fine glass needle under the dissecting microscope to produce karyoplasts (KP) and cytoplasts (CP). The KP of ICR PN-E and CP of F1 PN-E (KP: ICR + CP:F1) or the KP of F1 PN-E and CP of F1 PN-E (KP:F1 + CP:ICR) were attached using phytohemagglutinin-P (PHA-P) and then electrofused. High fusion rates of the KP and CP of PN-E were obtained (93.5%). The fused embryos were encapsulated in alginate gel and cultured for 72 or 96 hours. The cleavage rates of reconstituted embryos were also high (98.8%). Developmental rates to the blastocyst stage in vitro for the 96-hour culture of reconstituted embryos were 68.9% (KP:ICR + CP:F1) and 78.4% (KP:F1 + CP:ICR). Furthermore, the developmental ability of reconstituted embryos in vivo was investigated, and some live young were obtained (KP:ICR + CP:F1, 7.5% and KP:F1 + CP:ICR, 10.8%). In this study, it was confirmed that reconstituted embryos produced by bisection and electrofusion of pronuclear stage embryos were able to develop into blastocysts in vitro and into live young in vivo.

Genomic potential in mammals

Embryos of amphibians, fish, sheep, cattle, swine and rabbits have been multiplied by nuclear transfer. Successful nuclear transfer in these species has been accomplished by transfer of a blastomere from a late stage embryo into an enucleated oocyte or egg with large scale multiplication achieved by serial repetition of the procedure using blastomeres from nuclear transfer embryos. This allows the production of clonal lines, which when appropriately selected for performance in a given trait, can be reproduced to capture in the offspring expression of both additive and nonadditive inheritance. The efficiency of producing offspring from nuclear transfer is low in mammals in both frequency of morula or blastocyst produced and maintenance of pregnancy after embryo transfer. In domestic animals the largest number of offspring from one embryo has been eight calves. Embryos as late as the 64-cell stage in cattle and 120-cell blastocyst in sheep have been used successfully as donors of blastomeres. Recloning has also been done in cattle. Potentially, nuclear transfer provides a mechanism for multiplication and production testing of clonal lines, a method for rapid genetic improvement and a means for rapid propagation of a selected genotype.

Preimplantation development of rabbit embryos after transfer of embryonic nuclei into different cytoplasmic environment

The development of nuclear-transfer oocytes and zygotes was tested in the rabbit. Metaphase II oocytes and zygotes in the early pronuclear stage were treated with a cytoskeletal inhibitor (cytochalasin D), enucleated, and subsequently fused either with single blastomeres from eight- and 16-cell stages (oocytes and zygotes) or with pronuclei-containing karyoplasts (zygotes only). Also, nonenucleated zygotes were fused with 1/8 blastomeres. Fusion was performed by means of an electric field. Development of reconstituted embryos was monitored mainly in vitro, but a certain number of embryos developed from oocytes and zygotes receiving nuclei from eight-cell stages were also transferred into pseudopregnant does. Development of nuclear-transfer oocytes was distinctly better than that of nuclear-transfer zygotes, since 16.9% and 9.5% oocytes vs. 8.1% and 3.7% zygotes carrying eight- and 16-cell nuclei, respectively, developed to the blastocyst stage. Two advanced but already dead fetuses were found after transfer of 27 four-cell embryos obtained after fusion of oocytes with 1/8 blastomeres. No implantations were observed after transfer of 25 four-cell embryos developed from enucleated zygotes receiving eight-cell nuclei. These findings indicate that, in the rabbit, some nuclei from 16-cell embryos are still capable of promoting at least preimplantation development. Comparison between the developmental abilities of oocyte- and zygote-derived nuclear-transfer embryos also suggests that the cytoplasmic environment of recipient cell is more crucial for the development of reconstituted embryos than the stage of introduced nuclei (at least up to the 16-cell stage). The majority of pronuclear exchange embryos (69.9%) and 40% of nonenucleated zygotes receiving eight-cell nuclei were able to develop to the blastocyst stage. This latter observation indicates, similarly as with mouse, a supporting role of residual pronuclei for participation of an eight-cell nucleus in the development of reconstituted zygotes.