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Epigenetic Modifications Necessary for Normal Development Are Established During Oocyte Growth in Mice¹

^a Department of Animal Science, Tokyo University of Agriculture, Setagaya-ku, Tokyo 156–8502, Japan ^b Departments of Anatomy and Developmental Biology and Physiology, University College London, London,WC1E 6BT, United Kingdom

	ABSTRACT

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The ability of maternal chromatin to support full-term development is attained during oocyte growth. The aim of this study was to identify when during the growth phase the maternal chromatin developed the capacity to support term development. Mature metaphase II-arrested oocytes that contained chromatin from oocytes at different stages of oocyte growth were constructed by micromanipulation. The oocytes were fertilized in vitro, developed to the blastocyst stage in vitro, and transferred to recipients to assay developmental potential. The results demonstrate, firstly, that the origin of the maternal chromatin has no effect on the rate of oocyte maturation, fertilization, or development to the blastocyst in vitro. Secondly we demonstrate that maternal chromatin is first competent to support development to term during the latter half of oocyte growth when oocytes are 60–69 µm in diameter in juvenile mice or 50–59 µm in diameter in adult mice. These data show that epigenetic modifications necessary for postimplantation development occur during a specific phase of oocyte growth.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
In female mice, fetal primordial germ cells enter meiosis around Day 13.5 of gestation and arrest in diplotene of the first meiotic prophase around the time of birth [1]. At this stage the oocyte is 15–20 µm in diameter and is surrounded by cells that will form the follicular envelope. During the growth of the oocyte there is an impressive 200-fold increase in volume as the diameter reaches 75–80 µm. Accompanying the growth of the oocyte is an increase in the size of the nucleus (germinal vesicle) to a diameter of about 20 µm, as well as ultrastructural changes in the nucleolus and extranucleolar bodies. Chromatin exists in the form of highly diffuse bivalent chromosomes [2, 3] that condense around the nucleolus about the time the oocyte reaches 60 µm in diameter. At the same time, the oocyte becomes capable of reentering the cell cycle but is maintained in meiotic arrest by the follicular environment. The final period of oogenesis is oocyte maturation, in which, under hormonal control, the oocyte is stimulated to resume the first meiotic cell cycle and undergo the first meiotic division before arresting at metaphase of meiosis II.

Reaching metaphase II indicates that the cell cycle requirements of meiotic maturation have been completed. However, full developmental competence requires additional nuclear and cytoplasmic modifications during oogenesis. Studies thus far have revealed that mouse oocytes that have attained 80% (60–65 µm) of their full size (75–80 µm) complete the maturation process [4, 5] but fail to develop after fertilization. The ability to support development occurs in a stepwise manner as oocyte diameter increases from 65 to 75 µm. For example, oocytes 65–70 µm in diameter cleave to the 2-cell stage at normal rates, but subsequent development to term is not maximal until the oocytes are 75–80 µm [6]. These results demonstrate that developmental changes occurring in the oocyte during the final stages of the growth phase are critical for full developmental competence. Whether such developmental changes are occurring in the cytoplasm, nucleus, or both is unclear.

Recently, we demonstrated, using a nuclear transfer technique, that a nucleus from a nongrowing oocyte (15–20 µm) at the diplotene stage of the first meiotic division is able to complete meiotic maturation when transferred into enucleated fully grown germinal vesicle (GV)-stage oocytes [7]. Furthermore, oocytes containing nuclei from nongrowing oocytes could achieve high rates of fertilization, developed to the blastocyst stage in culture, and implanted after embryo transfer, but failed to develop to term. This suggests that modifications in the chromatin during oocyte growth are not required for preimplantation development but are necessary for postimplantation development.

We have proposed that the failure of oocytes with a nucleus from a nongrowing oocyte to develop to term is due to incomplete epigenetic modification of the genome, that is, a disruption of maternal primary imprinting. The process of genomic imprinting is responsible for the expression or repression of alleles depending on their parental origin [8–12]. The imprints responsible for parental-specific expression are established during gametogenesis [8, 9, 11, 13, 14], and it has been shown that at least some of these necessary for postimplantation development are established during the period of oocyte growth.

In the present study we addressed the question of when during the growth phase maternal chromatin becomes competent to support development to term. Further, we investigated whether the timing is influenced by the maternal environment using oocytes obtained from juvenile and adult cycling mice. Here we show that the ability of the maternal genome to support term development first occurs in oocytes that are 60–69 µm and 50–59 µm in diameter in immature and adult female mice, respectively.

	MATERIALS AND METHODS

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Collection and Classification of Oocytes

B6CB F1 (C57BL/6NCrj x CBA/JNCrj) hybrid mice were used for all experiments. Ovaries of 1-, 10-, 13-, 15-, 16-, 17-, 18-, and 20-day-old and adult females (6–8 wk) were immersed in 3.5 ml of M2 medium [15] containing 1.5 mg/ml crude collagenase (Wako Pure Chemicals, Tokyo, Japan) [16]. Oocyte-granulosa cell complexes from the preantral follicles were transferred to M2 containing 1.5 mg/ml trypsin (Sigma, Tokyo, Japan) and 1.5 mg/ml crude collagenase. After 15 min, the complexes were washed, and the granulosa cells were removed by pipetting. The granulosa cell-free oocytes were placed in M2 containing 0.5% polyvinyl alcohol (PVA; Sigma) and 0.5% pronase (Kaken Ltd., Tokyo, Japan) to remove the zona pellucida. Oocytes were washed and placed in M2 until required as nuclear donors. GV-stage oocytes were collected from ovaries 48 h after i.p. injection of 5 units of eCG (Peamex; Sankyo Ltd., Tokyo, Japan). GV-stage oocytes were released from antral follicles using a sterile needle, and the cumulus-intact oocytes were collected. The cumulus cells were removed from the oocytes by pipetting through a fine-bore pipette. To prevent germinal vesicle breakdown (GVBD), all oocytes were collected and manipulated in M2 medium containing 240 µM dibutyryl cAMP (dbcAMP; Sigma) and 5% fetal bovine serum (FBS; Gibco BRL, Tokyo, Japan).

Nuclear Transfer

Nuclear transfer was carried out by standard micromanipulation techniques [7, 17, 18]. Before nuclear transfer, the zona pellucida of the GV-stage oocyte was slit with a glass needle along 10–20% of the circumference. The GV was removed and the remaining cytoplast was placed into a small drop of M2 medium containing cytochalasin B (5 µg/ml; Sigma) and colcemid (0.1 µg/ml; Sigma). Either a nongrowing oocyte or a nucleus removed with minimal cytoplasm from a growing oocyte was introduced with Sendai virus (HVJ) into the perivitelline space of an enucleated fully grown GV-stage oocyte. The manipulated oocytes were cultured in Waymouth's medium (Gibco BRL) containing dbcAMP. Fusion occurred within 30 min. The reconstituted oocytes were washed and cultured in Waymouth's medium for 17 h in 5% CO₂, 5% O₂, 90% N₂ at 37°C. At the end of the maturation period, oocytes that had extruded the first polar body were pooled, and the metaphase plate was transferred to enucleated ovulated oocytes obtained from superovulated mice [7]. This additional nuclear transfer step was found to be necessary to obtain normal pronuclear development [7].

In Vitro Fertilization (IVF)

Sperm were collected from fertile mice and capacitated for 1 h in T6 medium at a concentration of 0.5–1.0 x 10⁶ sperm/ml [19]. For IVF, the reconstituted oocytes were incubated with capacitated sperm in T6 medium for up to 3 h as previously described [20]. After fertilization, the reconstituted oocytes were cultured in M16 medium [21] for 5 h.

In Vitro Culture and Embryo Transfer

To assess the developmental capacity of the constructed oocyte after fertilization in vitro, 1-cell embryos with single male and female pronuclei were cultured for 4 days in a drop of M16 medium supplemented with 0.4% BSA and 100 µM EDTA in 5% CO₂, 5% O₂, 90% N₂ at 37°C. The blastocysts derived from constructed oocytes were transferred to the uterine horns of CD-1 females on Day 3 of pseudopregnancy (2.5 days postcoitus).

Statistical Analysis

Data were analyzed by chi-square test.

	RESULTS

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Oocytes constructed by nuclear transfer contained chromatin derived from a growing oocyte that had been obtained from either a juvenile or an adult mouse (Fig. 1). Oocytes from the ovaries of juvenile mice at a given age are similar in size and were grouped accordingly (Tables 1 and 3). Adult mice yielded oocytes of all sizes, which were classified into 4 groups based on oocyte diameter: 20–39, 40–49, 50–59, and 60–69 µm (Tables 2 and 4). As control, a fully grown oocyte, 75–80 µm, from an adult ovary was enucleated and received a GV from another oocyte of the same size.

View larger version (18K): [in this window] [in a new window]   FIG. 1. Construction of oocytes containing genomes from growing-stage oocytes

Fertilization In Vitro and Preimplantation Development

After in vitro culture, a high proportion of oocytes extruded the first polar body and formed a metaphase II plate (Tables 1 and 2). The origin of the nucleus did not influence the capacity to complete maturation. Similarly, after fertilization in vitro, similar proportions of oocytes from all groups extruded a second polar body and formed pronuclei (79–97%; data not shown). Polyspermic fertilization occurred in a proportion of the embryos due to the zona-cutting technique employed during nuclear transfer [22]. Of the diploid monospermic embryos, as indicated by a single male and a female pronucleus, 64–86% developed to the morula and blastocyst stage after in vitro culture for 4 days, showing morphology similar to that of the normal fertilized embryos. The rates were not significantly different from those for oocytes constructed with fully grown GV oocytes of 75–80 µm (86%). Thus, the origin of the donor chromatin had no effect on the ability of these embryos to develop in vitro, suggesting that nuclei of all stages tested supported preimplantation development to the level found for nuclei from fully grown oocytes.

Postimplantation Development

To determine when oocyte chromatin became competent to support development to term, blastocysts derived from constructed oocytes were transferred to pseudopregnant females (Tables 3 and 4). Laparotomy was performed at a range of times (9.5–19.5 days of gestation) to determine approximately when fetal development was failing. Embryos were able to implant irrespective of the origin and size of the donor oocytes; however, the extent of fetal development was strictly related to the size of the oocyte and the age of oocyte donor from which the chromatin had been obtained. Two grossly abnormal fetuses were recovered after transferral of embryos containing chromatin from oocytes between 15 and 65 µm from 1- and 13-day-old juvenile mice (Table 3), but no normal fetuses were recovered. Embryos containing chromatin from oocytes from 15-day-old juvenile mice (60–69 µm) developed to midgestation, but live pups were not successfully obtained. In contrast, fetuses were recovered at 9.5 and 11.5 days of gestation after the transfer of embryos that contained chromatin from 40- to 49-µm oocytes of adult mice, and no further development was observed at 12.5 and 14.5 days of gestation (Table 4).

Live pups (7%) were first produced from embryos containing chromatin from 60- to 69-µm oocytes from 16-day-old juvenile mice (Table 3), while in adult oocytes, chromatin originating from smaller-sized oocytes (50–59 µm) supported development to term (5%) (Table 4). The rates of constituted embryos that developed to full term were significantly lower than in controls (30%). The development of embryos containing chromatin from larger-sized oocytes, 65–75 µm from 17- to 20-day-old juvenile mice (10–14%) and 60–69 µm from adult mice (15%), was gradually enhanced (Table 3 and 4). The results suggest that the epigenetic changes necessary for full-term development are completed in the later half of oocyte growth period.

	DISCUSSION

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The factors that contribute to the developmental competence of mammalian gametes remain a fundamental question in developmental biology. One of the requirements of gametogenesis is to perform the necessary epigenetic modifications to imprint a set of genes that will ensure their expression or repression from a maternal or paternal allele. Although it is clear that epigenetic modifications are established during gametogenesis [7–9, 11, 13, 14], the timing of these epigenetic modifications has not been established. By using nuclear transfer techniques to vary the stage of oocytefrom which maternal chromatin was obtained and by controlling other developmental parameters, we addressed the question of when maternal chromatin becomes competent to support full-term development. We show that the maternal genome achieves developmental competence gradually from about 50 µm in diameter. The first embryos capable of term development contained maternal chromatin from oocytes in the range of 50–59 µm in adult mice and 60–69 µm in juvenile mice. Thus it appears that epigenetic modifications of maternal chromatin necessary for development to term are established earlier in the oocyte growth phase in the adult ovary.

Our previous study [7] and the present study show that chromatin from nongrowing oocytes can support normal rates of development up until implantation. This indicates that preimplantation development is relatively unaffected by the epigenetic status of the maternal genome. This is reflected in the ability of parthenogenetic embryos to develop at significant rates up to implantation [23–25]. The apparent lack of influence of the state of the maternal chromatin on development through the preimplantation period is also suggested by the fact that monoallelic expression of many imprinted genes is not tightly regulated during the preimplantation period and becomes established only after implantation [26, 27]. In contrast, development beyond implantation does require epigenetic modifications to the maternal genome during the oocyte growth phase [7]. The present study reveals that these changes are completed late in the process of oocyte growth.

Growing oocytes are surrounded by follicular cells and exist in an environment where steroids, gonadotropins, and growth factors are constantly changing [28]. We used a juvenile model, in which the levels of steroids and gonadotropins are different from those of the adult, to examine whether the environment had any major effects on the ability of oocytes to undergo the changes necessary for developmental competence. The environment in which the oocytes were developing (juvenile or adult mouse) did not have a major effect on the ability of chromatin from growing oocytes to support development, although there is some indication that the precise timing of the changes necessary for full development may be delayed in the juvenile mice. The first live offspring were seen after the transfer of embryos containing chromatin from 50- to 59-µm oocytes in the adult group and 60- to 69-µm oocytes in the juvenile group. Further studies using nuclei from more restricted size ranges are required to confirm this possibility.

The precise nature of the epigenetic modifications established during oocyte growth is not known. The changes may represent the epigenetic changes, thought largely to be regulated by DNA methylation, that are responsible for determining the pattern of gene expression in a given cell type. Alternatively, the epigenetic modifications may be responsible for sex-specific monoallelic expression of imprinted genes. This latter possibility is most likely for a number of reasons. The changes established during oocyte growth have no effect on preimplantation development and are not apparently effective until after implantation. Therefore, gene expression necessary for development beyond the activation of the embryonic genome is apparently normal in embryos with chromatin from small oocytes. This suggests that general patterns of gene expression are not limiting. In further support of the concept that the changes during oocyte growth involve the establishment of maternal imprints, the timing of epigenetic changes established during oocyte growth correlate with epigenetic modifications of known imprinted genes. The sex-specific patterns of methylation associated with maternal expression are established during the growth phase of the oocyte in at least one maternally expressed transgene [29] and one endogenous gene, Igf2r [7, 30]. Both are unmethylated on the sites associated with maternal expression in nongrowing oocytes and have the mature pattern of methylation in fully grown oocytes. Furthermore, an altered pattern of expression of imprinted genes has been revealed in developing embryos containing genomes from nongrowing oocytes. These embryos express Peg1/Mest, Snrpn, and Peg3, which are normally expressed from paternal alleles, and do not express maternally expressed genes p57kip2 and Igf2r [31]. Thus the imprints appear to be established during oocyte growth, and manipulations that interfere with the normal pattern of these imprints influence embryonic development in a manner predicted by the new balance of maternally and paternally imprinted genes [7, 31]. Thus the evidence strongly supports the idea that the epigenetic changes established during oocyte growth that are necessary for full-term development are the same as those that establish maternal primary imprinting.

The acquisition of epigenetic maturity develops as the oocyte becomes competent to enter metaphase of the first meiotic division. This is accompanied by shortening of the microtubules and condensation of the chromatin around the nucleolus. These changes take place when the oocyte is 60–65 µm in diameter [32], around the time that oocyte chromatin first becomes competent to support full-term development. The mechanism required to establish the maternal primary imprints is likely to involve factors gaining access to and modifying the chromatin. Imprinting is therefore most likely to be established when chromatin is diffuse, prior to the cell cycle-induced changes in chromatin condensation. Localization of DNA methyltransferase 1 (Dnmt1) during oocyte growth, known as the major de novo and maintenance methyltransferase in mammals [33], also supports this idea. The Dnmt1 is localized to the nucleus of growing oocytes but not in fully grown oocytes [34].

The recent success of mammalian cloning [35–37] suggests that oocytes possess the capacity to extensively reprogram heterologous chromatin. The results of our study indicate that this is only possible provided that the primary (rather than maintenance) imprints governing sex-specific gene expression have been established. This is the case in the majority of somatic cells; but in cases in which these imprints are lost, so will be the ability to be reprogrammed after nuclear transfer. Our data also indicate that primary maternal imprinting must be established during a specific stage of oogenesis. Nuclei that had not been exposed to the ooplasm during a particular phase of oocyte growth were not endowed with the necessary epigenetic modifications during the maturation phase [7, 31]. As described above, this may be due to condensation of oocyte chromatin and the absence of nuclear Dnmt1 during oocyte maturation.

The identification of the stage in the development of the female germ line in which epigenetic changes are completed is important for further studies on the mechanism and regulation of these changes. In addition, the acquisition of epigenetic changes late in the growth phase has implications for the development of reproductive strategies for manipulating fertility involving the growth of mammalian oocytes in vitro.

	FOOTNOTES

 
First decision: 1 June 1999.

¹ This work was supported in part by grants-in-aid from Ministry of Education, Science and Culture of Japan, Japanese Society for the Promotion of Science (JSPS RFTF9700905), the Association of Livestock Technology (JAPAN). J.C. is supported by an MRC Career Development Award.

² Correspondence: Tomohiro Kono, Department of Animal Science, College of Agriculture, Tokyo University of Agriculture, 1737 Funako, Atsugi-shi, Kanagawa 243-0034, Japan. FAX: 81 46 270 6575; tomohiro{at}nodai.ac.jp

Accepted: September 24, 1999.

Received: April 13, 1999.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

1. Hogan B, Beddington R, Costantini F, Lacy E. Manipulating the Mouse Embryo. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press; 1994.
2. Chouinard L. A light and electron-microscope study of the oocyte nucleus during development of the antral follicle in the prepubertal mouse. J Cell Sci 1975; 17:589–615.[Abstract]
3. Wassarman P, Albertini D. The mammalian ovum. In: Knobil E, Neill JD, (eds.), The Physiology of Reproduction, vol 1. New York, NY: Raven Press; 1994:79–122.
4. Sorensen R, Wasserman P. Relationship between growth and meiotic maturation of the mouse oocytes. Dev Biol 1976; 50:531–536.[CrossRef][Medline]
5. Eppig JJ, Schultz RM, O'Brien M, Chesnel F. Relationship between the developmental programs controlling nuclear and cytoplasmic maturation of mouse oocytes. Dev Biol 1994; 164:1–9.[CrossRef][Medline]
6. Eppig JJ, Schroeder AC. Capacity of mouse oocytes from preantral follicles to undergo embryogenesis and development to live young after growth, maturation, and fertilization in vitro. Biol Reprod 1989; 41:268–276.[Abstract]
7. Kono T, Obata Y, Yoshimzu T, Nakahara T, Carroll J. Epigenetic modifications during oocyte growth correlates with extended parthenogenetic development in the mouse. Nat Genet 1996; 13:91–94.[CrossRef][Medline]
8. Monk M. Genome imprinting. Genes Dev 1988; 2:921–925.[Free Full Text]
9. Moore T, Reik W. Genetic conflict in early development: parental imprinting in normal and abnormal growth. Rev Reprod 1995; 2:73–77.
10. Reik W, Sasaki H, Ferguson-Smith A, Feil R, Bowden L, Penberth J, Surani A, Gurtmann I, Klose J. Parental imprinting and epigenetic programming of the mouse genome: long lasting consequences for development and phenotype. Chromosomes Today 1993; 11:367–376.
11. Surani MA, Kothary R, Allen ND, Singh RB, Fundele R, Ferguson-Smith AC, Barton SC. Genome imprinting and development in the mouse. Development Supplement 1990; 89–98.
12. Solter D, Gilligan A. The role of imprinting in early mammalian development. In: Genomic Imprinting. Cambridge: Cambridge University Press; 1995: 3–16.
13. Chaillet JR, Knoll JHM, Horsthemke B, Lalande M. The syntenic relationship between the critical deletion region for the Prader-Willi/Angelman syndromes and proximal mouse chromosome 7. Genomics 1991; 11:773–776.[CrossRef][Medline]
14. Brandeis M, Kafri T, Ariel M, Chaillet JR, McCarrey J, Razin A, Cedar H. The ontogeny of allele-specific methylation associated with imprinted genes in the mouse. EMBO J 1993; 12:3669–3677.[Medline]
15. Fulton B, Whittingham D. Activation of mammalian oocytes by intracellular injection of calcium. Nature 1978; 273:149–151.[CrossRef][Medline]
16. Eppig JJ, Telfer EE. Guide to Techniques in Mouse Development. New York: Academic Press; 1993: 77–83.
17. McGrath J, Solter D. Nuclear transplantation in the mouse embryo by microsurgery and cell fusion. Science 1983; 220:1300–1302.[Abstract/Free Full Text]
18. Kwon OY, Kono T. Production of identical sextuplet mice by transferring nuclei from four-cell embryos. Proc Natl Acad Sci USA 1996; 93:13010–13013.[Abstract/Free Full Text]
19. Quinn P, Barros C, Whittingham D. Preservation of hamster oocytes to assay the fertilizing capacity of human spermatozoa. J Reprod Fertil 1982; 66:161–168.[Abstract/Free Full Text]
20. Glenister PH, Wood MJ, Kirby C, Whittingham DG. Incidence of chromosome anomalies in first-cleavage mouse embryos obtained from frozen-thawed oocytes fertilized in vitro. Gamete Res 1987; 16:205–216.[CrossRef][Medline]
21. Whittingham D. Culture of mouse ova. J Reprod Fertil 1971; 14:7–12.
22. Kono T, Sotomaru Y, Sato Y, Nakahara T. Development of androgenetic mouse embryos produced by in vitro fertilization of enucleated oocytes. Mol Reprod Dev 1993; 34:43–46.[CrossRef][Medline]
23. Barton SC, Surani MA, Norris ML. Role of paternal and maternal genomes in mouse development. Nature 1984; 311:374–376.[CrossRef][Medline]
24. O'Neill GT, Rolfe LR, Kaufman MH. Developmental potential and chromosome constitution of strontium-induced mouse parthenoge-nones. Mol Reprod Dev 1991; 30:214–219.[CrossRef][Medline]
25. Yoshimizu T, Obata Y, Carroll J, Kono T. Parthenogenetic activation of mouse oocyte by strontium. J Mamm Ova Res 1998; 15:146–152.
26. Szabo PE, Mann JR. Allele-specific expression and total expression levels of imprinted genes during early mouse development: implications for imprinting mechanisms. Genes Dev 1995; 9:3097–3108.[Abstract/Free Full Text]
27. Latham KE, Rambhatla L. Expression of X-linked genes in androgenetic, gynogenetic, and normal mouse preimplantation embryos. Dev Genet 1995; 17:212–222.[CrossRef][Medline]
28. Gilbert SG, Shyamal KR. Follicular development and its control. In: Knobil E, Neill JD (eds.), The Physiology of Reproduction, vol 1. New York, NY: Raven Press; 1994: 629–724.
29. Chaillet JR. Genomic imprinting: lessons from mouse transgenes. Mutat Res 1994; 307:441–449.[Medline]
30. Stoger R, Kubiccka P, Liu CG, Kafri T, Razin A, Cedar H, Barlow DP. Maternal-specific methylation of the imprinted mouse Igf2r locus identifies the expressed locus as carrying the imprinting signal. Cell 1993; 73:61–71.[CrossRef][Medline]
31. Obata Y, Kaneko-Ishino T, Koide T, Takai Y, Ueda T, Domeki I, Shiroishi T, Ishino F, Kono T. Disruption of primary imprinting during oocyte growth leads to the modified expression of imprinted genes during embryogenesis. Development 1998; 125:1553–1560.[Abstract]
32. Albertini D. Cytoplasmic microtubular dynamics and chromatin organization during mammalian oogenesis and oocyte maturation. Mutat Res 1992; 296:57–68.[Medline]
33. Yoder JA, Soman NS, Verdine GL, Bestor TH. DNA (cytosine-5)-methyltransferases in mouse cells and tissues. Studies with a mechanism-based probe. J Mol Biol 1997; 270:385–395.[CrossRef][Medline]
34. Mertineit C, Yoder J, Taketo T, Laird D, Trasler J, Bestor T. Sex-specific exons control DNA methyltransferase in mammalian germ cells. Development 1998; 125:889–897.[Abstract]
35. Wilmut I, Schnieke AE, McWhir J, Kind AJ, Campbell KHS. Viable offspring derived from fetal and adult mammalian cells. Nature 1997; 385:810–813.[CrossRef][Medline]
36. Wakayama T, Perry AC, Zuccotti M, Johnson KR, Yanagimachi R. Full-term development of mice from enucleated oocytes injected with cumulus cell nuclei. Nature 1998; 394:369–374.[CrossRef][Medline]
37. Kato Y, Tani T, Sotomaru Y, Kurokawa K, Kato J, Doguchi H, Yasue H, Tsunoda Y. Eight calves cloned from somatic cells of a single adult. Science 1998; 282:2095–2098.[Abstract/Free Full Text]

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				L. J. Mullins, I. Wilmut, and J. J. Mullins Nuclear transfer in rodents J. Physiol., January 1, 2004; 554(1): 4 - 12. [Abstract] [Full Text] [PDF]

				Y. Hirao, T. Itoh, M. Shimizu, K. Iga, K. Aoyagi, M. Kobayashi, M. Kacchi, H. Hoshi, and N. Takenouchi In Vitro Growth and Development of Bovine Oocyte-Granulosa Cell Complexes on the Flat Substratum: Effects of High Polyvinylpyrrolidone Concentration in Culture Medium Biol Reprod, January 1, 2004; 70(1): 83 - 91. [Abstract] [Full Text] [PDF]

				A. Sanfins, G. Y. Lee, C. E. Plancha, E. W. Overstrom, and D. F. Albertini Distinctions in Meiotic Spindle Structure and Assembly During In Vitro and In Vivo Maturation of Mouse Oocytes Biol Reprod, December 1, 2003; 69(6): 2059 - 2067. [Abstract] [Full Text] [PDF]

				H. Liu, H. C. Chang, J. Zhang, J. Grifo, and L. C. Krey Metaphase II nuclei generated by germinal vesicle transfer in mouse oocytes support embryonic development to term Hum. Reprod., September 1, 2003; 18(9): 1903 - 1907. [Abstract] [Full Text] [PDF]

				L. M. Mitchell, C. R. Kennedy, and G. M. Hartshorne Effects of varying gonadotrophin dose and timing on antrum formation and ovulation efficiency of mouse follicles in vitro Hum. Reprod., May 1, 2002; 17(5): 1181 - 1188. [Abstract] [Full Text] [PDF]

				S. Bao, Y. Obata, Y. Ono, N. Futatsumata, S. Niimura, and T. Kono Nuclear competence for maturation and pronuclear formation in mouse oocytes Hum. Reprod., May 1, 2002; 17(5): 1311 - 1316. [Abstract] [Full Text] [PDF]

				Y. Obata and T. Kono Maternal Primary Imprinting Is Established at a Specific Time for Each Gene throughout Oocyte Growth J. Biol. Chem., February 8, 2002; 277(7): 5285 - 5289. [Abstract] [Full Text] [PDF]

				F. Moffa, F. Comoglio, L. C. Krey, J. A. Grifo, A. Revelli, M. Massobrio, and J. Zhang Germinal vesicle transfer between fresh and cryopreserved immature mouse oocytes Hum. Reprod., January 1, 2002; 17(1): 178 - 183. [Abstract] [Full Text] [PDF]

				H. Liu, L. C. Krey, J. Zhang, and J. A. Grifo Ooplasmic Influence on Nuclear Function During the Metaphase II-Interphase Transition in Mouse Oocytes Biol Reprod, December 1, 2001; 65(6): 1794 - 1799. [Abstract] [Full Text] [PDF]

				Y. Hu, I. Betzendahl, R. Cortvrindt, J. Smitz, and U. Eichenlaub-Ritter Effects of low O2 and ageing on spindles and chromosomes in mouse oocytes from pre-antral follicle culture Hum. Reprod., April 1, 2001; 16(4): 737 - 748. [Abstract] [Full Text] [PDF]

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