Behavior of M-phase synchronized blastomeres after nuclear transfer in cattle

M-phase synchronized bovine blastomeres were used to study the effect of nuclear-cytoplasmic synchronization on the developmental potential after nuclear transfer (NT). The capacity of nocodazole and benomyl to reversibly synchronize blastomeres from embryos in M-phase was evaluated. Nocodazole reversibly arrested bovine embryos at the studied stages and induced high rates of M-phases in morulae and compact morulae. In contrast, benomyl was less efficient than nocodazole to synchronize in M-phase. After transfer of an M-phase blastomere, premature chromatin condensation was the prevalent finding 1 hr post-fusion (hpf). Condensed chromosomes non-arranged in the equatorial plate (1-3 hpf) that acquired an organized structure over time (3-7 hpf) were subsequently observed. Anaphase-telophase structures were predominantly recorded at 4-9 hpf. About 50% of the embryos activated at both 3-4 and 6-7 hpf extruded a polar body-like structure 5 hr after activation, but this was not observed in embryos activated immediately after fusion. A significantly lower activation rate was observed for oocytes activated 3-4 hpf compared to those activated 6-7 hpf. However, the ability to undergo first cleavage was significantly lower in the latter group. Reconstructed embryos activated immediately after fusion showed no difference in the rate of activation compared to those activated 6-7 hpf, although the cleavage rate was higher. DNA synthesis was observed at a significantly higher rate in embryos activated both immediately and 3-4 hpf that did not extrude a PB-like structure than in those activated 3-4 hpf that extruded a polar body-like structure. Under the conditions tested M-phase donor cells cannot be properly remodeled after NT in cattle to trigger normal embryonic development. Our observations of chromatin structures together with DNA synthesis suggest that the failure in the development may be due to improper chromatin remodeling of mitotic nuclei after NT, which may result in chromosomal abnormalities incompatible with normal embryo development.     

Developmental potential of bovine nuclear transfer embryos and postnatal survival rate of cloned calves produced by two different timings of fusion and activation

We compared developmental potential of somatic cell nuclear transfer (NT) embryos and postnatal survivability of cloned calves produced by two different fusion and activation protocols. As donor cells for NT, bovine cumulus cell-derived cultured cells of passage 5 were used following culture in serum-starved medium for 5-7 days. Enucleated oocytes were fused with donor cells at 21 or 24 hr post maturation. NT embryos fused at 21 hr were activated chemically 3 hr after fusion (DA group) and embryos fused at 24 hr were activated chemically immediately after fusion (FA group). Chemical activation was accomplished by calcium ionophore for 5 min and cytochalasin D + cycloheximide for 1 hr then cycloheximide alone for 4 hr. After in vitro culture in IVD101 medium for 7 days, embryo transfer was performed. Fusion rates were 86 and 84% in the DA and FA groups, respectively. Developmental rate to the blastocyst stage of NT embryos in the DA group was higher than in the FA group (42% vs. 28%). Pregnancy rate did not differ significantly between the DA and FA groups (11/13 and 5/7 at day 35), and 13 cloned calves (including 1 set of twins from a single embryo transfer) were born. High rates of postnatal mortality were observed in both groups. These results suggest that the DA method improves in vitro developmental potential of NT embryos, but the timing of fusion and chemical activation does not affect the pregnancy rate and the survivability of cloned calves.     

Remodeling of donor nuclei, DNA-synthesis, and ploidy of bovine cumulus cell nuclear transfer embryos: effect of activation protocol

The purpose of this study was to investigate the effects of two activation protocols on nuclear remodeling, DNA synthesis during the first cell cycle, chromosome segregation after first mitosis and development to blastocyst of embryos produced by somatic nuclear transfer. Pronuclear formation was significantly higher when activation lasted 5 hr compared to 3 hr for both ethanol-cycloheximide and ionomycin-bohemine treatment. However, the presence of a single nucleus was significantly higher in embryos activated for 3 hr in bohemine. Initiation of DNA synthesis was delayed in ethanol-cycloheximide group, however, after 12 hr labeling 100% of embryos synthesized DNA in both groups. Embryos activated with ethanol-cycloheximide developed to blastocysts at a significantly higher rate than those activated with ionomycin-bohemine. Analysis of 2-cell embryos with DNA probes for chromosome 6, 7, and 15 by fluorescence in situ hybridization showed that at least 50% of NT embryos were of normal ploidy independent of the activation stimulus. The results presented in this study show differences between the protocols compared on the nuclear events during the first cell cycle and on the development to blastocyst. Mol. Reprod. Dev. 59: 371-379, 2001.     

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

This study examined the chromatin morphology, in vitro development, and expression of selected genes in cloned embryos produced by transfer of mouse embryonic fibroblasts (MEF) into the bovine ooplasm. After 6 hr of activation, inter-species nuclear transfer (NT) embryos (MEF-NT) had one (70%) or two pronuclei (20%), respectively. After 72 hr of culture in vitro, 62.6% of the MEF-NTs were arrested at the 8-cell stage, 31.2% reached the 2- to 4-cell stage, and only 6.2% had more than eight blastomeres, but none of these developed to the blastocyst stage. Whereas, 20% of NT embryos derived from bovine embryonic fibroblast fused with bovine ooplasm (BEF-NT) reached the blastocyst stage. Donor MEF nuclei expressing an Enhanced Green Fluorescent Protein (EGFP) transgene resulted in 1- to 8-cell stage MEF-NT that expressed EGFP. The expression of selected genes was examined in 8-cell MEF-NTs, 8-cell mouse embryos, enucleated bovine oocytes, and MEFs using RT-PCR. The mRNA for heat shock protein 70.1 (Hsp 70.1) gene was detected in MEF-NTs and MEF, but not in mouse embryos. The hydroxy-phosphoribosyl transferase (HPRT) mRNA was found in normal mouse embryos and MEF but not in MEF-NTs. Expression of Oct-4 and embryonic alkaline phospatase (eAP) genes was only detected in normal mouse embryos and not in the inter-species NT embryos. Abnormal gene expression profiles were associated with an arrest in the development at the 8-cell stage, but MEF-NT embryos appeared to have progressed through gross chromatin remodeling, typical of intra-species NT embryos. Therefore, molecular reprogramming rather than chromatin remodeling may be a better indicator of nuclear reprogramming in inter-species NT embryos.     

Transgenic pig expressing the enhanced green fluorescent protein produced by nuclear transfer using colchicine-treated fibroblasts as donor cells

Fetal-derived fibroblast cells were transduced with replication defective vectors containing the enhanced green fluorescent protein (EGFP). The transgenic cells were treated with colchicine, which theoretically would synchronize the cells into G2/M stage, and then used as donor nuclei for nuclear transfer. The donor cells were transferred into the perivitalline space of enucleated in vitro matured porcine oocytes, and fused and activated with electrical pulses. A total of 8.3% and 28.6% of reconstructed oocytes showed nuclear envelope breakdown and premature chromosome condensation 0.5 and 2 hr after activation, respectively. Percentage of pronuclear formation was 62.5, 12 hr after activation. Most (91.4%) of the 1-cell embryos with pronuclei did not extrude a polar body. Most (77.2%) embryos on day 5 were diploid. Within 2 hr after fusion, strong fluorescence was detectable in most reconstructed oocytes (92.3%). The fluorescence in all NT embryos became weak 15 hr after fusion and disappeared when culture to 48 hr. But from day 3, cleaved embryos at the 2- to 4-cell stage started to express EGFP again. On day 7, 85.8% of cleaved embryos expressed EGFP. A total of 9.4% of reconstructed embryos developed to blastocyst stage and 71.5% of the blastoctysts expressed EGFP. After 200 reconstructed 1-cell stage embryos were transferred into four surrogate gilts, three recipients were found to be pregnant. One of them maintained to term and delivered a healthy transgenic piglet expressing EGFP. Our data suggest that the combination of transduction of somatic cells by a replication defective vector with the nuclear transfer of colchicine-treated donors is an alternative to produce transgenic pigs. Furthermore, the tissues expressing EGFP from descendents of this pig may be very useful in future studies using pigs that require genetically marked cells.     

Relationship between nuclear remodeling and subsequent development of mouse embryonic nuclei transferred to enucleated oocytes

The present study was conducted to examine the relationship between nuclear remodeling and subsequent embryonic development in nuclear transplant mouse embryos. Metaphase II oocytes were enucleated without staining and fused with transferred donor nuclei from two-, four-, or eight-cell embryos. Fusion and oocyte activation were performed by means of electric fields. High rates of enucleation (89.1%), fusion (88.0–91.6%), and activation (95.2–96.9%) were obtained using this system. Nuclear remodeling was characterized by premature chromosome condensation (PCC), followed by various pronuclear-like formations upon oocyte activation. Development to blastocysts was obtained from both PCC (17.9%) and non-PCC (NPCC; 52.9%) embryos fused with the two-cell nuclei. However, development to term was obtained only in PCC embryos with a single pronucleus-like structure and a polar body (12.5%). In vitro development of nuclear transplant embryos with four- and eight-cell nuclei was limited. All the NPCC embryos examined had tetraploid chromosome constitutions, but chromosome constitutions of PCC embryos varied. Only 37.5% of the PCC embryos had diploid chromosome constitutions. The results indicated that the development of nuclear transplant embryos is affected by the types of nuclear remodeling and that oocyte activation in relation to their chromosome constitutions. The results also indicated that the PCC of the donor nucleus in nonactivated cytoplasm is important for the development of the nuclear transplant embryos. © 1994 Wiley-Liss, Inc.     

Relationship between nuclear remodeling and development in nuclear transplant rabbit embryos.

The present study characterized the profile of nuclear remodeling in nuclear transplant rabbit embryos and investigated the relationship between chromatin behavior after transfer and embryo development. The developmental potential and pattern of remodeling of donor nuclei from cleavage-, morula-, and blastocyst- (inner cell mass ICM, and trophectoderm, TE) stage donors were evaluated. In addition, we determined whether a modification in the synchrony between blastomere fusion and oocyte activation altered the profile of nuclear remodeling and affected development of reconstituted embryos. Development to blastocysts was similar with 8- and 32-cell-stage donor nuclei (42% and 33%, respectively, p greater than 0.1). However, it was reduced with ICM transplants (17%, p less than 0.05), and development of TE transplants did not progress beyond the 8-cell stage. Upon blastomere fusion into nonactivated oocyte cytoplasm, nuclear remodeling was characterized by premature chromosome condensation (PCC), followed by pronuclear (PN) formation and swelling. PCC occurred synchronously within 1.2-1.5 h post-fusion with all stages of donor nuclei (p greater than 0.1). PN formation in 8- and 32-cell transplants occurred approximately 4 h after fusion, and was synchronous to that of female pronuclei in activated oocytes; however, it was delayed in ICM and TE transplants (p less than 0.01). With all stages of donor nuclei, final nuclear diameter was similar to, or larger than, that of female pronuclei. Fusion to activated oocyte cytoplasm, as opposed to nonactivated cytoplasm, prevented PCC and extensive nuclear swelling (16.0 +/- 0.7 vs. 30 +/- 0.7 microns, respectively, p less than 0.01). Nuclear diameter in early embryos was smaller (p less than 0.01), and development to blastocysts was reduced (p less than 0.05). The results indicate that remodeling of the donor nucleus is not essential for development to blastocysts; however, it is beneficial. Furthermore, complete reprogramming seems possible only after remodeling of the donor nucleus, i.e., PCC in nonactivated cytoplasm, followed by nuclear swelling upon activation of the oocyte.     

Preparation of young preactivated oocytes with high enucleation efficiency for bovine nuclear transfer

To improve the enucleation rate in newly matured bovine oocytes, we investigated the position of cytoplasmic chromatin in relation to the polar body and the consequent enucleation efficiency before and after sequential activation with calcium ionophore A23187 and cycloheximide. Oocytes aspirated from the follicles of slaughterhouse-collected ovaries were cultured for 18 to 20 h. With Hoechst staining, only 40.7% of the chromatin material was found adjacent to the first polar body in metaphase II oocytes, while 100% was located adjacent to the second polar body in oocytes after the activation. Enucleation trials after activation showed a higher enucleation rate (91.5%) than that before activation (59.9%). The following experiment determined the effect of using both kinds of cytoplast on the in vitro development of nuclear transfer embryos. Blastomeres of the 32-cell-stage in vitro-produced embryos were transferred, fused to the activated cytoplasts and cultured in vitro. No significant difference was detected in fusion, cleavage or development to blastocysts obtained 7 d (174 h) post fusion. In conclusion, this study showed that young in vitro-matured bovine oocytes sequentially activated with calcium ionophore and cycloheximide have cytoplasmic chromatin material adjacent to the second polar body, leading to a high enucleation rate.     

Heterogeneous nuclear transfer embryos reconstructed by bovine oocytes and camel (Camelus bactrianus) skin fibroblasts and their subsequent development

Summary This study reconstructed heterogeneous embryos using camel skin fibroblast cells as donor karyoplasts and the bovine oocytes as recipient cytoplasts to investigate the reprogramming of camel somatic cell nuclei in bovine oocyte cytoplasm and the developmental potential of the reconstructed embryos. Serum-starved skin fibroblast cells, obtained from adult camel, were electrically fused into enucleated bovine metaphase II (MII) oocytes that were matured in vitro. The fused eggs were activated by Inomycin with 2 mM/ml 6-dimethylaminopurine. The activated reconstructed embryos were cocultured with bovine cumulus cells in synthetic oviduct fluid supplemented with amino acid (SOFaa) and 10% fetal calf serum for 168 h. Results showed that 53% of the injected oocytes were successfully fused, 34% of the fused eggs underwent the first egg cleavage, and 100% of them developed to four- or 16-cell embryo stages. The first completed cleavage of xenonuclear transfer camel embryos occurred between 22 and 48 h following activation. This study demonstrated that the reconstructed embryos underwent the first embryonic division and that the reprogramming of camel fibroblast nuclei can be initiated in enucleated bovine MII oocytes.     

Effect of elevated Ca(2+) concentration in fusion/activation medium on the fusion and development of porcine fetal fibroblast nuclear transfer embryos

The present study examined the effect of elevated Ca(2+) concentration in fusion/activation medium on the fusion and development of fetal fibroblast nuclear transfer (NT) porcine embryos. Frozen-thawed and serum starved fetal fibroblasts were transferred into the perivitelline space of enucleated oocytes. Cell fusion and activation were induced simultaneously with electric pulses in 0.3 M mannitol-based medium containing 0.1 or 1.0 mM CaCl(2). Some fused embryos were further activated 1 hr after the fusion treatment by exposure to an electric pulse. The NT embryos were cultured in vitro for 6 days. Fusion and blastocyst formation rates were significantly (P<0.05) increased by increasing the Ca(2+) concentration from 0.1 mM (67.1 and 6.3%) to 1.0 mM (84.7 and 15.8%). However, no difference in the number of cells in blastocysts was observed between the two groups. A higher percentage of blastocyst was also observed when control oocytes were parthenogenetically activated in the presence of elevated Ca(2+) (19.3% vs. 32.4%, P<0.05). When the reconstituted oocytes were fused in the medium containing 1.0 mM CaCl(2), increasing the number of pulses from 2 to 3 or an additional activation treatment did not enhance the blastocyst formation rate or cell number in blastocysts. These results demonstrate that increasing the Ca(2+) concentration in the fusion/activation medium can enhance the fusion and blastocyst formation rates of fetal fibroblast NT porcine embryos without an additional activation treatment.     

Electrofusion for the pronuclear transplantation of mouse eggs

The present study was undertaken to determine the efficiency of HVJ treatment and electrofusion for pronuclear transplantation in the mouse. The output voltage and duration of the pulses were fixed to 200 μsec at 10 V or to 150 μsec at 15 V for electrofusion, because the maximum rates of blastomere fusion of 2-cell embryos and development of fused embryos in vitro were obtained under these conditions. Although the proportion of eggs with fused karyoplast (78%) and the fused eggs developed to morulae or blastocysts (67%) was significantly lower than those obtained after HVJ treatment (94% and 94%), the proportion of pregnant recipients and young obtained after treatment of fused eggs was not significantly different between these two procedures. It is advised that electrofusion can be used as a fusogenic procedure for pronuclear transplantation in the mouse in some cases where HVJ cannot be applied.     

Effects of maturational age of recipient oocytes and activation conditions on the development of porcine fetal fibroblast nuclear transfer embryos

This study was conducted to evaluate the nuclear remodeling patterns and the developmental potential of porcine fetal fibroblast nuclear transfer embryos (NTs) following the maturational age of recipient oocytes and activation conditions. Donor cells were transferred into the enucleated oocytes that were matured for 36 or 44h. Electrofused embryos were cultured in PZM-3 for 6 days without activation treatment (EF group). Some of these embryos were additionally activated by electric stimulus (ES; EF+ES group) or a combination of ES and DMAP (EF+ES+D group) before culture. The reconstituted embryos were fixed 2.5h after fusion to evaluate the nuclear remodeling patterns. The nuclear remodeling pattern of NTs reconstituted with 44 h-matured recipients showed a tendency to form a pronucleus-like structure, while that of NTs reconstituted with 36 h-matured recipients showed a tendency to undergo a premature chromosome condensation (PCC) and form one set of chromatin clump. In EF+ES+D group, blastocyst development was significantly increased regardless of maturational age of recipient oocytes (P<0.05). The result indicates that additional activation treatment is necessary to induce the activation of embryos reconstituted with 36 h-matured recipients, and treatment with the combination of electrical stimuli and DMAP could enhance the blastocyst formation rate of porcine NTs reconstituted with both 36 h- and 44 h-matured recipient oocytes.     

Parthenogenetic development of bovine oocytes treated with ethanol and cytochalasin B after in vitro maturation.

The present study was conducted to investigate the effects of different culture durations (24-36 hr) on bovine oocyte maturation in vitro and the effect of the presence or absence of cumulus cells at the time of treatment to induce parthenogenetic activation (exposure to ethanol and cytochalasin B; CB) (experiment I). The effects of dosage (2.5 or 5.0 micrograms/ml) and incubation time (2.5, 5, or 10 hr) in CB (experiment II) on the subsequent development to the blastocyst stage in vitro was also investigated. In experiment I, cleavage and development to the blastocyst stage were not affected by the presence or absence of cumulus cells at the time of parthenogenetic activation. However, the 24-hr culture duration for in vitro maturation had a significantly lower rate of development to the blastocyst stage than the longer culture durations (27-36 hr). In experiment II, treatment with 5 micrograms/ml CB for 5 hr showed the highest percentage of development to blastocyst in the oocytes matured for both 27 and 30 hr. To determine the viability of the parthenogenetic embryos (morulae and blastocysts), four recipient heifers received two embryos each, and one heifer was found to be pregnant on day 35 following transfer. Although fetal heartbeat was not observed, the subsequent estrus was prolonged in all heifers. The present results demonstrate development of in vitro-matured, parthenogenetically activated bovine embryos up to the preimplantation stage.     

In vitro development of reconstructed porcine oocytes after somatic cell nuclear transfer

This study was designed to examine the developmental ability of porcine embryos after somatic cell nuclear transfer. Porcine fibroblasts were isolated from fetuses at Day 40 of gestation. In vitro-matured porcine oocytes were enucleated and electrically fused with somatic cells. The reconstructed eggs were activated using electrical stimulus and cultured in vitro for 6 days. Nuclear-transferred (NT) embryos activated at a field strength of 120 V/mm (11.6 +/- 1.6%) showed a higher developmental rate as compared to the 150-V/mm group (6.5 +/- 2.3%) (P: < 0.05), but the mean cell numbers of blastocysts were similar between the two groups. Rates of blastocyst development from NT embryos electrically pulsed at different times (2, 4, and 6 h) after electrofusion were 11.6 +/- 2.9, 6.6 +/- 2.3, and 8.1 +/- 3.3%, respectively. The mean cell numbers of blastocysts developed from NT embryos were gradually decreased (30.4 +/- 10.4 > 24.6 +/- 10.1 > 16.5 +/- 7.4 per blastocyst) as exposure time (2, 4, and 6 h) of nuclei to oocyte cytoplast before activation was prolonged. There was a significant difference in the cell number between the 2- and 6-h groups (P: < 0. 05). Nuclear-transferred embryos (9.4 +/- 0.9%) had a lower developmental rate than in vitro fertilization (IVF)-derived (21.4 +/- 1.9%) or parthenogenetic embryos (22.4 +/- 7.2%) (P: < 0.01). The mean cell number (28.9 +/- 11.4) of NT-derived blastocysts was smaller than that (38.6 +/- 10.4) of IVF-derived blastocysts (P: < 0. 05) and was similar to that (29.9 +/- 12.1) of parthenogenetic embryos. Our results suggest that porcine NT eggs using somatic cells after electrical activation have developmental potential to the blastocyst stage, although with smaller cell numbers compared to IVF embryos.     

Nucleolar ultrastructure in bovine nuclear transfer embryos

Nuclear transfer experiments in mammals have attempted to reprogram a donor nucleus to a state equivalent to the zygotic one. Reprogramming of the donor nucleus is, among other features, indicated by a synthesis of ribosomal RNA (rRNA). The initiation of rRNA synthesis is simultaneously reflected in nuclear morphology as a transformation of the nucleolus precursor body into a functional rRNA synthesising nucleolus with a characteristic ultrastructure. We examined nucleolar ultrastructure in bovine in vitro produced (control) embryos and in nuclear transfer embryos reconstructed from a MII phase (nonactivated) or S phase (activated) cytoplasts. Control embryos were fixed at the two-, four-, early eight- and late eight-cell stages; nuclear transfer embryos were fixed at 1 and 3 hr post fusion and at the two-, four-, and eight-cell stages. Control embryos possessed a nucleolar precursor body throughout all three cell cycles. In the eight-cell stage embryo, a primary vacuole appeared as an electron lucid area originating in the centre of the nucleolar precursor body. In nuclear transfer embryos reconstructed from nonactivated cytoplasts, the nuclear envelope was fragmented or completely broken down at 1 hr after fusion and, by 3 hr after fusion, it was restored again. At this time, the reticulated fibrillo-granular nucleolus had an almost round shape. The nucleolar precursor body seen in the two-cell stage nuclear transfer embryos consisted of intermingled filamentous components and secondary vacuoles. A nucleolar precursor body typical for the two-cell stage control embryos was never observed. None of the reconstructed embryos of this group reached the eight-cell stage. Nuclear transfer embryos reconstructed from activated cytoplasts, in contrast, exhibited a complete nuclear envelope at all time intervals after fusion. In the two-cell stage nuclear transfer embryo, the originally reticulated nucleolus of the donor blastomere had changed into a typical nucleolar precursor body consisting of a homogeneous fibrillar structure. A primary vacuole appeared in the four-cell stage nuclear transfer embryos, which was one cell cycle earlier than in control embryos. Only nuclear transfer embryos reconstructed from activated cytoplasts underwent complete remodelling of the nucleolus. The reorganisation of the donor nucleolar architecture into a functionally active nucleolus was observed as early as in the four-cell stage nuclear transfer embryo. These ultrastructural observations were correlated with our autoradiographic data on the initiation of RNA synthesis in nuclear transfer embryos.     

Effects of duration, concentration, and timing of ionomycin and 6-dimethylaminopurine (6-DMAP) treatment on activation of goat oocytes

The protocol of ionomycin followed by 6-dimethylaminopurine (6-DMAP) is commonly used for activation of oocytes and reconstituted embryos. Since numerous abnormalities and impaired development were observed when oocytes were activated with 6-DMAP, this protocol needs optimization. Effects of concentration and treatment duration of both drugs on activation and development of goat oocytes were examined in this study. The best oocyte activation (87-95%), assessed by pronuclear formation, was obtained when oocytes matured in vitro for 27 hr were treated with 0.625-20 microM ionomycin for 1 min before 6-hr incubation in 2 mM 6-DMAP. Progressional reduction of time for 6-DMAP-exposure showed that the duration of 6-DMAP treatment can be reduced to 1 hr from the second up to the fourth hour after ionomycin, to produce activation rates greater than 85%. Activation rates of oocytes in vitro matured for 27, 30, and 33 hr were higher (P < 0.05) than that of oocytes matured for 24 hr when treated with ionomycin plus 1-hr (the third hour) 6-DMAP, but a 4-hr incubation in 6-DMAP enhanced activation of the 24-hr oocytes. Goat activated oocytes began pronuclear formation at 3 hr and completed it by 5-hr post ionomycin. An extended incubation in 6-DMAP (a) impaired the development of goat parthenotes, (b) quickened both the release from metaphase arrest and the pronuclear formation, and (c) inhibited the chromosome movement at anaphase II (A-II) and telophase II (T-II), leading to the formation of one pronucleus without extrusion of PB2. In conclusion, duration, concentration, and timing of ionomycin and 6-DMAP treatment had marked effects on goat oocyte activation, and to obtain better activation and development, goat oocytes matured in vitro for 27 hr should be activated by 1 min exposure to 2.5 microM ionomycin followed by 2 mM 6-DMAP treatment for the third hour.     

Cytoplasmic modification of the nuclear lamina during pronuclear-like transformation of mouse blastomere nuclei.

During the successive interphases of cleaving mouse embryos the nuclear periphery diminishes its reactivity to anti-lamin A and C antibodies. This developmentally regulated characteristic can be modified by exposure of the blastomere nuclei to metaphase II (M II) oocyte cytoplasm followed by activation. In the current study we define the cytoplasmic conditions necessary for this modification of 8-cell and 16-cell stage nuclei in hybrids obtained by fusion with metaphase II arrested oocytes, oocytes at various time points after parthenogenetic activation, naturally fertilized eggs (zygotes) and interphase 2-cell embryo blastomeres. The intensity of fluorescence obtained with anti-lamins A/C in the blastomere nuclei increases as a result of fusion with freshly activated oocytes or early zygotes (first 3.0-5.5 h in the case of parthenogenetic activation), and not when eggs or 2-cell blastomeres advanced in interphase are used as partners for fusion. This transformation of the A/C lamin pattern is correlated with the ability to promote pronucleus-like growth of blastomere nuclei in hybrids. Blastomere nuclei introduced into M II-arrested oocytes undergo premature chromatin condensation and dissolution of the nuclear lamina. The results are discussed with regard to certain particularities of the first embryonic interphase of the mouse and the potential involvement of nuclear lamins in pronuclear growth.     

Nuclear lamin antigen and messenger RNA expression in bovine in vitro produced and nuclear transfer embryos

The nuclear lamina is a complex meshwork of nuclear lamin filaments that lies on the interface of the nuclear envelope and chromatin and is important for cell maintenance, nucleoskeleton support, chromatin remodeling, and protein recruitment to the inner nucleolus. Protein and mRNA patterns for the major nuclear lamins were investigated in bovine in vitro fertilized (IVF) and nuclear transfer embryos. Expression of lamins A/C and B were examined in IVF bovine germinal vesicle (GV) oocytes, metaphase II oocytes, zygotes, 2-cell, 8-cell, 16-32-cell embryos, morulae, and blastocysts (n = 10). Lamin A/C was detected in 9/10 immature oocytes, 10/10 zygotes, 8/10 2-cell embryos, 4/10 morulae, 10/10 blastocysts but absent during the maternal embryonic transition. Lamin B was ubiquitously expressed during IVF preimplantation development but was only detected in 4/10 GV oocytes. Messenger RNA expression confirms that the major lamins, A/C and B1 are expressed throughout preimplantation development and transcribed by the embryo proper. Lamin A/C and B expression were observed (15 min, 30 min, 60 min, 120 min) following somatic cell nuclear transfer using adult fibroblasts and at the 2-cell, 8-cell, 16-32-cell, morula and blastocyst stage (n = 5). Altered expression levels and localization of nuclear lamins A/C and B was determined in nuclear transfer embryos during the first 2 hr post fusion, coincidental with only partial nuclear envelope breakdown as well as during the initial cleavage divisions, but was restored by the morula stage. This mechanical and molecular disruption of the nuclear lamina provides key evidence for incomplete nuclear remodeling and reprogramming following somatic cell nuclear transfer.     

In vivo aging of oocytes influences the behavior of nuclei transferred to enucleated rabbit oocytes

The present study examined nuclear remodeling in rabbit nuclear transfer (NT) embryos formed from metaphase II (MII) oocytes aged in vivo until 19 hr postcoitum (hpc), enucleated, and fused at 22–26 hpc with 32-cell morula blastomeres by means of electric fields, which also induced recipient oocyte activation. Post-activation events observed during the first hour following the fusion/activation pulse were studied in terms of chromatin, lamins, and micro-tubules, and revealed that transferred nuclei underwent premature chromosomes condensation (PCC) in only one-third of NT embryos and remained in interphase in others. Recipient oocytes were mostly not activated by manipulations performed before the fusion/activation pulse. The persistance of transferred nuclei in interphase resulted from the rapid progression of recipient oocytes to interphase after activation, suggesting that the cytoplasmic state of MII oocytes aged in vivo was poised for the approach to interphase. Studying micro-tubular organization in MII oocytes before nuclear transfer manipulations, we found that 19 hpc MII oocytes aged in vivo differed from 14 hpc MII oocytes (freshly ovulated) and from 19-hpc MII oocytes aged in vitro (collected at 14 hpc and cultured for 5 hr), notably by the presence of microtubule asters and tubulin foci or only tubulin foci dispersed throughout the cytoplasm. When PCC was avoided, remodeling of the transferred nucleus was well advanced 1 hr after nuclear transfer, and NT embryos developed better to the blastocyst stage. Mol. Reprod. Dev. 46:325–336, 1997. © 1997 Wiley-Liss, Inc.