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.     

Differential sensitivity of mouse pronuclei and zygote cytoplasm to Hoechst staining and ultraviolet irradiation

The exposure of mouse zygotes pre-stained with Hoechst 33342 to u.v. irradiation for 20-30 sec significantly or completely inhibited development to blastocysts in vitro. However, development to the blastocyst stage of enucleated eggs receiving pronuclei from untreated eggs was as good as that of control reconstituted eggs when the cytoplasm originated from eggs exposed to u.v. irradiation for 20-30 sec, but was significantly lower when the cytoplasm was from eggs exposed for 40 sec. The chromosomes at the second metaphase stage could be removed with 15 sec of exposure to u.v. irradiation under a fluorescence microscope. Most eggs enucleated at the second metaphase that received a single inner cell mass nucleus (75%) showed pronuclear formation 6 h after activation; 23% of them developed to morphologically normal 2-cell eggs and 5% developed to blastocysts. These results demonstrate that the cytoplasm of mouse zygotes is more resistant to u.v. irradiation after Hoechst staining. Eggs at the second metaphase, from which chromosomes have been removed under a fluorescence microscope, can therefore be used as cytoplasm recipients for nuclear transplantation of inner cell mass nuclei.     

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.     

Effect of aging of recipient oocytes on the development of bovine nuclear transfer embryos in vitro

The present study was undertaken to examine the effect of the length of in vitro maturation of oocytes on the efficiency of enucleation, parthenogenetic activation and blastomere fusion by electric stimulus. In vitro development of reconstituted oocytes receiving a blastomere from 8 to 16-cell bovine embryos fertilized in vitro was investigated to assess the effect of aging of the oocytes. The proportion of oocytes with a first polar body at 22 to 24 hours after maturation was high (80%) compared with those obtained at 16 to 18, 28 to 30 or 42 to 44 hours (50 to 75%). The success rate of enucleation significantly decreased with aging (88, 85, 74 and 55%). The activation rate significantly increased with the length of maturation in vitro (P<0.01) (1 to 4, 24 to 41, 57 to 70 and 80 to 87%). The proportion of oocytes fused with a blastomere from 8- to 16-cell embryos was not dependent on the age of the oocytes (54 approximately 59%). The ability of the reconstituted oocytes to develop to the 2-cell and the 8- to 16-cell stage increased with the length of maturation of recipient oocytes. When oocytes enucleated and a blastomere at 22 to 24 hours were incubated further for 22 to 23 hours until electrofusion. The proportions of oocytes which developed to the 2-cell and the 8- to 16-cell stages (74 and 17%) were similar to those obtained at 42 to 44 hours after maturation. However, only 1 to 6% of reconstituted eggs receiving a blastomere from 8- to 16-cell embryos fertilized in vitro developed into a blastocyst in vitro.     

Development of enucleated mouse oocytes reconstituted with embryonic nuclei

The chromosomes of mouse oocytes at telophase of the first meiotic division were removed using micromanipulation and differential interference microscopy. The enucleated oocytes were used as recipients for nuclear transplantation, after culture for 4-6 h. The newly synthesized proteins of the enucleated oocytes showed the same pattern as those of secondary oocytes matured in vivo. When the enucleated oocytes received a nucleus from late 2- and 8-cell embryos, or a cell from the inner cell mass (ICM) of blastocysts, 23, 4 and 10%, respectively, of reconstituted embryos developed to blastocysts. After transfer to recipient females, live young were produced from the reconstituted eggs that received a nucleus from late 2-cell embryos.     

Effect of egg composition on the developmental capacity of androgenetic mouse embryos.

Analysis of the developmental capacities of androgenetic and gynogenetic mouse embryos (bearing two paternal or two maternal pronuclei, respectively) revealed a defect in blastocyst formation of androgenetic, but not gynogenetic, embryos that was a function of the maternal genotype. Androgenetic embryos constructed using fertilized eggs from C57BL/6 or (B6D2)F1 mice developed to the blastocyst stage at frequencies similar to those previously reported, whereas androgenetic embryos constructed with fertilized eggs from DBA/2 mice developed poorly, the majority failing to progress beyond the 16-cell stage and unable to form a blastocoel-like cavity, regardless of whether the male pronuclei were of C57BL6 or DBA/2 origin. This impaired development was observed even in androgenetic embryos constructed by transplanting two male pronuclei from fertilized DBA/2 eggs to enucleated C57BL/6 eggs, indicating that the defect cannot be explained as the lack of some essential component in the DBA/2 cytoplasm that might otherwise compensate for androgeny. Rather, the DBA/2 egg cytoplasm apparently modifies the incoming male pronuclei differently than does C57BL/6 egg cytoplasm. Several specific alterations in the protein synthesis pattern of DBA/2 androgenones were observed that reflect a defect in the regulatory mechanisms that normally modulate the synthesis of these proteins between the 8-cell and blastocyst stages. These results are consistent with a model in which cytoplasmic factors present in the egg direct a strain-dependent modification of paternal genome function in response to epigenetic modifications (genomic imprinting) established during gametogenesis and indicate that preimplantation development can be affected by these modifications at both the morphological and biochemical levels.     

Cytogenetic analysis of reconstituted one-cell mouse embryos derived from nuclear transfer of fetal male germ cells.

Micromanipulation techniques were used to produce reconstituted one-cell mouse embryos after the fusion of fetal male germ cells 15.5 day post coitum with enucleated secondary oocytes. At this stage of development, male fetal germ cells are arrested at G1 of mitotic interphase. Two distinct populations of germ cells, differing in size and ploidy, were isolated from the genital ridge of a mid-term fetus. Oocytes that had received male germ cells from the population of smaller (mononuclear) germ cells developed as diploid one-cell reconstituted embryos. When the same procedures were used to produce reconstituted one-cell embryos using male fetal germ cells from a population of larger (multinucleate) cells, they exhibited ploidy of either 4x, 6x or 8x at metaphase of the first cell division. Although most reconstituted embryos (90 and 96%) developed to the two-cell stage, the proportion of embryos receiving small germ cells developed to blastocysts was much higher (62%) than that receiving large germ cells (4%). These studies indicate that not all fetal germ cells are diploid before the onset of meiosis and have identified procedures to produce reconstituted embryos from fetal germ cells that do not carry genome or chromosome anomalies.     

体细胞来源及培养代数对核移植重构胚发育的影响

为探讨体细胞来源及培养代数对核移植重构胚发育的影响,实验采用电融合法将小鼠2—细胞胚胎卵裂球、胚胎干细胞(ES)、胎儿成纤维细胞、耳成纤维细胞、尾尖成纤维细胞、睾丸支持细胞和精原细胞以及不同培养代次的胎儿成纤维细胞进行了核移植。结果显示：2—细胞胚胎卵裂球供核重构胚发育最好,囊胚率为7．4％;ES细胞重构胚虽然发育率低,但仍有囊胚出现,比例为0．7％;胎儿成纤维细胞重构胚最高发育阶段为桑椹胚,比例为0．2％;精原细胞重构胚只能发育到8-细胞阶段,比例为0．3％;其他几类细胞重构胚则仅能发育至4-细胞阶段。不同培养代数的胎儿成纤维细胞重构胚除第3代外都可发育到8-细胞阶段,且发育率差异不显著,但第一代细胞重构胚2-细胞发育率(40．7％)显著低于2、3和4代细胞重构胚。结果表明：不同分化程度的细胞核移植后,重新编程的难易程度是不一样的,分化程度越高则重新编程越难;未调整细胞周期的ES细胞由于多数处于S期,所以重构胚发育率很低;体外培养传代有利于体细胞核移植后重新编程。     

Developmental rate and allocation of transgenic cells in rabbit chimeric embryos

The objective of this study was to compare developmental capacity of rabbit chimeric embryos and the allocation of the EGFP gene expression to the embryoblast (ICM) or embryonic shield. We produced chimeric embryos (TR< >N) by synchronous transfer of two or three blastomeres at the 16-cell stage from transgenic (TR) into normal host embryos (N) at the same stage. In the control group, two to three non-transgenic blastomeres were used to produce chimeric embryos. The TR embryos were produced by microinjection of EGFP into both pronuclei of fertilized rabbit eggs. The developmental rate and allocation of EGFP-positive cells of the reconstructed chimeric embryos was controlled at blastocyst (96 h PC) and embryonic shield (day 6) stage. All chimeric embryos (120/120, 100%) developed up to blastocyst stage. Using fluorescent microscope, we detected green signal (EGFP expression). In 90 chimeric (TR< >N) embryos (75%). Average total number of cells in chimeric embryos at blastocyst stage was 175+/-13.10, of which 58+/-2.76 cells were found in the ICM area. The number of EGFP-positive cells in the ICM area was 24+/-5.02 (35%). After the transfer of 50 chimeric rabbit embryos at the 16-cell stage, 20 embryos (40%) were flushed from five recipients on day 6 of pregnancy, of which five embryos (25%) were EGFP positive at the embryonic shield stage. Our results demonstrate that transgenic blastomeres in synchronous chimeric embryos reconstructed from TR embryos have an ability to develop and colonize ICM and embryonic shield area.     

Assessment of nuclear totipotency of fetal bovine diploid germ cells by nuclear transfer

Nuclear transfer was used to study nuclear reprogramming of fetal diploid bovine germ cells collected at two stages of the fetal development. In the first case, germ cells of both sexes were collected during their period of intragonadal mitotic multiplication at 48 days post co?tum (d.p.c.). In the second case, only male germ cells were collected after this period, between 105 and 185 d.p.c. Isolated germ cells were fused with enucleated oocytes. Reconstituted embryos were cultured in vitro and those reaching the compacted morula or blastocyst stage were transferred into synchronous recipient heifers. Of 511 reconstituted embryos with 48 d.p.c. germ cells (309 males and 202 females), 48% (247/511 ) cleaved; 2.7% (14/511 ) reached the compacted morula stage and 8 of them the blastocyst stage (1.6%). No difference was observed between sexes. All 14 compacted morulae/blastocysts were transferred into 6 recipients and one pregnancy was initiated. This recipient was slaughtered at Day 35 and an abnormal conceptus (extended trophectoderm and degenerated embryo) was collected. Its male sex, genetically determined, corresponded to that of donor fetus. Of 380 reconstituted embryos with male 105 to 185 d.p.c. germ cells, 72.1% (274/380 ) cleaved, 2.1% (8 380 ) reached the compact morula stage and 7 of these the blastocyst stage (1.8%). Three blastocysts and one morula were transferred into 4 recipients. Two became pregnant at Day 21 but only one at Day 35 which aborted around Day 40. Our results show that the nucleus of diploid bovine germ cells of both sexes can be reprogrammed. However, in the absence of further development of these reconstituted embryos, nuclear totipotency of bovine diploid germ cells remains to be evidenced.     

Nuclear transplantation in early pig embryos

Nuclear transfer was evaluated in early porcine embryos. Pronuclear stage embryos were centrifuged, treated with cytoskeletal inhibitors, and subsequently enucleated. Pronuclei containing karyoplasts were placed in the perivitelline space of the enucleated zygote and fused to the enucleated zygote with electrofusion. The resulting pronuclear exchange embryos were either monitored for cleavage in vitro (9/13 cleaved and contained 2 nuclei after 24 h, 69%) or for in vivo development. In vivo development after 3 days resulted in 14/15 (93%) of the embryos transferred cleaving to the greater than or equal to 4-cell stage and after 7 days 6/16 (38%) reaching the expanded blastocyst stage. A total of 56 pronuclear exchange embryos were allowed to go to term, and 7 piglets were born. A similar manipulation procedure was used to transfer 2-, 4- or 8-cell nuclei to enucleated, activated meiotic metaphase II oocytes. Enucleation was effective in 74% (36/49) of the contemporary oocytes. Activation was successful in 81% (37/46) of nonmanipulated but pulsed oocytes versus 13% (4/31) of control oocytes (p less than 0.01). After 6 days in vivo, 9% (1/11) of the 2-cell nuclei, 8% (7/83) of the 4-cell nuclei, and 19% (11/57) of the 8-cell nuclei transferred to enucleated, activated meiotic metaphase II oocytes resulted in development to the compact morula or blastocyst stage (p less than 0.01). A total of 88 nuclear transfer embryos were transferred to recipient gilts for continued development. A single piglet was born after the transfer of a 4-cell nucleus to an enucleated, activated metaphase II oocyte and subsequent in vivo development.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cloning of bovine embryos by multiple nuclear transfer

The in vitro development of multiple generation bovine nuclear transferred embryos to blastocysts and their survival ability after freezing and thawing were examined. Parent donor embryos which had 20 to 50 cells were recovered from superovulated cows. Follicular oocytes matured in vitro were used as recipient oocytes. The recipient oocytes enucleated at 22 to 24 h after the onset of maturation were preactivated at 33 h. Enucleated oocytes with a donor blastomere were fused 9 h after activation by an electric stimulus and the fused oocytes were cultured in vitro (first generation). Reconstituted oocytes that had developed to the 8- to 16-cell stage 3 to 4 d after fusion were used as donor embryos for the next generation. Recloning procedures were performed twice (second and third generations). The proportion of recipient oocytes successfully fused with a blastomere increased with the cycle of nuclear transfer. Eighty to 86% of fused oocytes developed to the 2-cell stage and there was no significant difference with the generation. The proportion of reconstituted embryos receiving blastomeres derived from first generation embryos had higher developmental ability in vitro, than those derived from other generations (43 vs 31% for 8 to 16-cell stage, 37 vs 20 and 21% for blastocyst stage). The number of cloned blastocysts increased with repeated nuclear transfer (once: 6.2 +/- 4.3, twice: 19.8 +/- 9.2 and three times: 30.0 +/- 14.7) but varied greatly with each parent donor embryo. The in vitro viability of cloned blastocysts after freezing and thawing (59%) was low but not significantly different from that obtained for in vitro fertilized blastocysts (72%). After transfer of either fresh or frozen-thawed cloned blastocysts to 21 recipients, 10 of them were pregnant on Day 60. Four and 3 offspring were produced from 20 fresh and 14 frozen-thawed blastocysts,respectively.     

Direct exposure of chromosomes to nonactivated ovum cytoplasm is effective for bovine somatic cell nucleus reprogramming

We examined the in vitro developmental potential of nonactivated and activated enucleated ova receiving cumulus cells at various stages of the cell cycle. Eleven to 29% of activated ova receiving donor cells stopped developing at the 8-cell stage but 21% to 50% of nonactivated ova receiving donor cells at either the G(0), G(1), G(2), or M phase, or cycling cells developed into blastocysts. One normal calf was born after transferring five blastocysts that had developed from ova receiving donor cells at the M phase. The present study demonstrated that direct exposure of donor chromosomes to nonactivated ovum cytoplasm is effective for somatic cell nucleus reprogramming, and activated ovum cytoplasm does not reprogram the nucleus.     

Development of mouse enucleated oocytes receiving a nucleus from different stages of the second cell cycle.

The influence of the stage of the cell cycle of donor nuclei on the development of mouse oocytes enucleated at telophase I was examined. After nuclear transplantation and activation, a high proportion of the oocytes remodelled a nucleus, emitted a polar body and formed a pronuclear-like nucleus. Most of the reconstituted embryos that received an interphase nucleus 30-32 h or 34-36 h after treatment with human chorionic gonadotrophin (hCG) arrested at the 2-cell stage. The reconstituted embryos were able to develop to blastocysts when nuclei from late 2-cell embryos (44-46 and 48-50 h after hCG) were transferred to the oocytes. The resulting blastocysts were transferred to recipients and ten live young were obtained from the embryos that formed a pronuclear-like nucleus after extrusion of a polar body. Thus, the developmental ability of the reconstituted embryos was critically influenced by the stage of the cell cycle of the donor nuclei.     

Establishment of a porcine cell line from in vitro-produced blastocysts and transfer of the cells into enucleated oocytes

The present study was conducted to establish a porcine cell line from blastocysts produced in vitro and to examine the developmental ability of nuclear transfer embryos reconstituted with the cells and enucleated mature oocytes. When hatched blastocysts were cultured in Dulbecco's modified Eagle's medium with supplements, no colonies of embryo-derived cells were observed. In contrast, 56% of embryos that were attached to feeder layers of STO cells formed colonies in NCSU-23 with supplements. When the colonies were subcultured in the absence of feeder cells, a cell line with an epithelial-like cell morphology was obtained. This cell morphology was stable up to at least passage 30. Although no fused embryos were observed when a pulse of 100 V/mm was applied, the fusion rate increased significantly at 150 V/mm (28%) and 200 V/mm (64%). At 200 V/mm, 39% of fused embryos cleaved, but no embryos developed beyond the 3-cell stage. When cocultured with electro-activated oocytes, percentages of reconstructed embryos cleaved (65%) and developed to the 4-cell stage (23%) were significantly higher than percentages for those (cleavage: 38%; 4-cell stage: 3%) in the absence of activated oocytes. At 7 days after culture, one reconstructed embryo successfully developed to the blastocyst stage in the presence of activated oocytes. When green fluorescent protein-expressing cells and enucleated oocytes were fused and the fused embryos were cultured with electro-activated oocytes, 3 of 102 reconstructed embryos developed to the blastocyst stage. All of the blastocysts were positive for fluorescent green under ultraviolet light. The results of the present study indicate that a porcine cell line can be established from the hatched blastocyst and maintained in vitro for a long period, and that reconstructed embryos obtained by transferring the blastocyst-derived cells into enucleated oocytes have the ability to develop to the blastocyst stage in vitro.     

山羊(Capra hircus)重构胚与再重构胚早期发育的研究

选择山羊8细胞期到桑椹胚期的正常胚胎或重构胚的卵裂球,或选择胚泡期胚胎的内细胞团细胞作为供核,并以LRH注射后26—28h的去核成熟卵球作为受体,制备重构胚或再重构胚,琼脂糖包埋,植入山羊输卵管内,体内培养4—6天发育结果表明:不同发育时期(8细胞期至胚泡期)的正常胚胎细胞核或重构胚细胞核中,至少有部分细胞核均保留着发育的全能性,这些细胞核在母系基因表达产物的调控下,实现重新编程,启动并完成正常发育的全过程。     

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.     

Injection of frozen-thawed porcine first polar bodies into enucleated oocytes results in fertilization and embryonic development

The objective was to determine whether enucleated oocytes injected with frozen porcine first polar bodies (pPB1s) could be fertilized and developed into viable embryos in vitro. Metaphase II (MII) oocytes with pPB1s were frozen (vitrified) and stored for 2 mo. The pPB1s were isolated from thawed MII oocytes and injected into enucleated recipient oocytes by micromanipulation. All recipients injected with thawed pPB1s were fertilized by intracytoplasmic sperm injection (ICSI), and the resulting recombinant zygotes were incubated to assess their developmental competence in vitro. Furthermore, double-antibody immunohistochemistry was used to verify that the nucleus of the pPB1 participated in fertilization and supported embryonic development. Porcine embryos (2- to 8-cell stage) were obtained from the recombinants. The average in vitro cleavage rate of 2-, 4-, and 8-cell stage recombinant embryos was 25.3, 17.7, and 9.3% (P < 0.05), respectively. Chromosomes in the labeled pPB1 participated in the formation of the two blastomere nuclei of 2-cell stage embryos derived from recombinant oocytes. In conclusion, nuclear materials of frozen-thawed pPB1 supported oocyte fertilization and subsequent embryonic development, thereby providing a new way to use frozen PB1s for preservation and reproduction of mammals.